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Light Energy - Questions & Answers

TEST YOURSELF

A. Objective Questions :

1. Write true or false for each statement :

(a) Water is optically denser than glass.

False

(b) A ray of light when passes from glass to air, bends towards the normal.

False

(c) The speed of light is more in glass than in water.

False

(d) The depth of a pond when seen from above appears to be less.

True

(e) Light travels at a lower speed in water than in air.

True

(f) Light travels in the same straight line path while passing through different media.

False

(g) The angle formed between the normal and the refracted ray is known as the angle of incidence.

False

(h) At the point of incidence, a line drawn at right angles to the surface, separating the two media, is called the normal.

True

(i) Image is formed by a mirror due to refraction of light.

False

(j) Rays of light incident parallel to the principal axis pass through the focus after reflection from a concave mirror.

True

(k) A convex mirror is used as a shaving mirror.

False

(l) The focal length of a convex mirror is equal to its radius of curvature.

False

(m) A concave mirror converges the light rays, but a convex mirror diverges them.

True

(n) A virtual image formed by a spherical mirror is always erect and situated behind the mirror.

True


2. Fill in the blanks :

(a) Water is optically ............ than air.

Water is optically denser than air.

(b) Air is optically ............ than glass.

Air is optically rarer than glass.

(c) When a ray of light travels from water to air, it bends ............ the normal.

When a ray of light travels from water to air, it bends away from the normal.

(d) When a ray of light travels from air to glass, it bends ............ the normal.

When a ray of light travels from air to glass, it bends towards the normal.

(e) When white light passes through a prism, it ............ .

When white light passes through a prism, it disperses (or splits into its constituent colours).

(f) The splitting of white light into its constituent colours is called ............ .

The splitting of white light into its constituent colours is called dispersion.

(g) A ............ mirror is obtained on silvering the outer surface of a part of a hollow glass sphere.

A concave mirror is obtained on silvering the outer surface of a part of a hollow glass sphere.

(h) Radius of curvature of a spherical mirror is ............ its focal length.

Radius of curvature of a spherical mirror is twice its focal length.

(i) The angle of incidence for a ray of light passing through the centre of curvature of a spherical mirror is ............ .

The angle of incidence for a ray of light passing through the centre of curvature of a spherical mirror is 0° (zero).

(j) A ............ mirror always forms a virtual image.

A convex mirror always forms a virtual image.

(k) A concave mirror forms a virtual image for an object placed ............ .

A concave mirror forms a virtual image for an object placed between the pole and the focus.


3. Match the following :

Column A:
(a) White light
(b) Refraction
(c) Virtual images
(d) Real images
(e) Prism

Column B:
(i) convex mirror
(ii) concave mirror
(iii) refraction
(iv) spectrum
(v) ray of light from glass to air

Answer: (a)-(iv), (b)-(v), (c)-(i), (d)-(ii), (e)-(iii)


4. Select the correct alternative :

(a) The speed of light in air or vacuum is :

    (i) 3 ×108 m s-1
    (ii) 2.25 × 108 m s-1
    (iii) 332 m s-1
    (iv) 2.0 × 108 m s-1

Answer: (i) 3 ×108 m s-1

(b) A ray of light moving from an optically rarer to a denser medium :

    (i) bends away from the normal
    (ii) bends towards the normal
    (iii) remains undeviated
    (iv) none of the above.

Answer: (ii) bends towards the normal

(c) The angle between the normal and refracted ray is called :

    (i) angle of deviation
    (ii) angle of incidence
    (iii) angle of refraction
    (iv) angle of emergence.

Answer: (iii) angle of refraction

(d) The property of splitting of white light into its seven constituent colours is known as :

    (i) rectilinear propagation
    (ii) refraction
    (iii) reflection
    (iv) dispersion.

Answer: (iv) dispersion.

(e) The seven colours in the spectrum of sunlight in order, are represented as :

    (i) VIBGYOR
    (ii) VIGYBOR
    (iii) BIVGYOR
    (iv) RYOBIVG

Answer: (i) VIBGYOR

(f) A ray of light passing through centre of curvature of a spherical mirror, after reflection :

    (i) passes through the focus
    (ii) passes through the pole
    (iii) becomes parallel to the principal axis
    (iv) retraces its own path.

Answer: (iv) retraces its own path.

(g) If the radius of curvature of a concave mirror is 20 cm, its focal length is :

    (i) 10 cm
    (ii) 20 cm
    (iii) 40 cm
    (iv) 80 cm.

Answer: (i) 10 cm

(h) The image formed by a convex mirror is :

    (i) erect and diminished
    (ii) erect and enlarged
    (iii) inverted and diminished
    (iv) inverted and enlarged.

Answer: (i) erect and diminished

(i) The image formed by a concave mirror is of the same size as the object, if the object is placed :

    (i) at the focus
    (ii) between the pole and focus
    (iii) between the focus and centre of curvature
    (iv) at the centre of curvature.

Answer: (iv) at the centre of curvature.

(j) A convex mirror is used :

    (i) as a shaving mirror
    (ii) as a head mirror by a dentist
    (iii) as a rear view mirror by a driver
    (iv) as a reflector in torch.

Answer: (iii) as a rear view mirror by a driver


B. Short/Long Answer Questions :

1. State the speed of light in (a) air, (b) water, and (c) glass.

(a) Speed of light in air: 3 × 108 m s-1
(b) Speed of light in water: 2.25 × 108 m s-1
(c) Speed of light in glass: 2 × 108 m s-1

2. How does the speed of light determine the optical density of a medium ?

A medium in which the speed of light is less is said to be optically denser, whereas a medium in which the speed of light is more is said to be optically rarer. Thus, higher optical density corresponds to lower speed of light.

3. Which is optically denser : water or air ? Give reason.

Water is optically denser than air. The reason is that the speed of light in water (2.25 × 108 m s-1) is less than the speed of light in air (3 × 108 m s-1).

4. Out of air and glass, which is optically rarer ? Give reason.

Air is optically rarer than glass. The reason is that the speed of light in air (3 × 108 m s-1) is greater than the speed of light in glass (2 × 108 m s-1).

5. What do you understand by refraction of light ?

The change in direction of the path of light when it passes from one optically transparent medium to another is called refraction of light.

6. Describe an experiment to show that a light ray bends when it passes from one transparent medium into another transparent medium.

Experiment: Take a coin and place it at the bottom of an empty cup. Position your eye such that the coin is just hidden by the rim of the cup. Now, ask a friend to pour water gently into the cup without disturbing the coin. As water is poured, the coin becomes visible again from the same position.
Explanation: This happens because light rays coming from the coin travel from water (denser) to air (rarer) and bend away from the normal at the surface. This refracted light reaches the eye, making the coin appear raised and thus visible. This proves light bends when changing media.

7. Draw a ray diagram to show that the depth of a vessel containing water when seen from above, appears to be less than its real depth.

(For this answer, draw a diagram similar to Fig 5.6 in the book: A vessel with water, a point O at the bottom. A ray OA travels straight up. Another ray OB strikes the surface obliquely and bends away from the normal. When these rays are traced back, they meet at point I, which is above O. The distance AI is the apparent depth, which is less than the real depth AO.)

8. Define the following terms :
Incident ray, Refracted ray, Angle of incidence, Angle of refraction.

Incident ray: The ray of light falling on the surface separating the two media is called the incident ray.
Refracted ray: The ray of light travelling in the other medium in the changed direction is called the refracted ray.
Angle of incidence: The angle between the incident ray and the normal is called the angle of incidence.
Angle of refraction: The angle between the refracted ray and the normal is called the angle of refraction.

9. A ray of light falls normally on a glass slab. What is the angle of incidence ?

The angle of incidence is 0° (zero).

10. A ray of light travels from a rarer medium to a denser medium. How will it bend ?

It will bend towards the normal.

11. A ray of light travels from a denser medium to a rarer medium. How will it bend ?

It will bend away from the normal.

12. The diagram given below in Fig. 5.38 shows a ray of light AO falling on a surface separating two media. Draw the refracted ray in each case.

(a) Air to Water: The ray enters a denser medium, so the refracted ray should bend towards the normal.
(b) Water to Air: The ray enters a rarer medium, so the refracted ray should bend away from the normal.
(c) Normal incidence (Air to Water): The ray falls perpendicularly (along the normal), so it passes undeviated (straight line).

13. Draw a diagram showing the refraction of a light ray from water to glass. Label on it the incident ray, the angle of incidence (i), and the angle of refraction (r).

(Draw a horizontal line separating Water (top) and Glass (bottom). Draw a normal line. Draw an incident ray coming from water striking the surface. Since glass is denser than water, draw the refracted ray bending towards the normal inside the glass. Label the angle between the normal and incident ray as 'i' and the angle between the normal and refracted ray as 'r'.)

14. The diagram in Fig. 5.39 shows a ray of light AO falling on a rectangular glass slab PQRS. Complete the diagram till the ray of light emerges out of the slab. Label on the diagram the incident ray, the refracted ray and the emergent ray.

(1. At point O on surface PQ, the ray enters from air to glass. Draw the refracted ray bending towards the normal inside the slab.
2. Let this ray hit the opposite face SR. At this point, draw a normal.
3. As the ray exits from glass to air, draw the emergent ray bending away from the normal. The emergent ray should be parallel to the original direction of the incident ray AO. Label AO as Incident Ray, the ray inside the slab as Refracted Ray, and the ray leaving the slab as Emergent Ray.)

15. Explain the following :
(a) A coin placed at the bottom of a vessel appears to be raised when water is poured in the vessel.
(b) A straight stick partly dipped in water obliquely, appears to be bent at the surface of water.
(c) The sun is seen before the sunrise and after the sunset.

(a) This is due to refraction of light. Light rays from the coin travel from water (denser) to air (rarer) and bend away from the normal. To an observer, these rays appear to come from a point slightly above the actual position of the coin.
(b) The rays of light coming from the part of the stick immersed in water travel from water to air. They refract and bend away from the normal. When produced backwards, they appear to meet at a higher point, making the immersed part appear raised, causing the stick to look bent at the surface.
(c) This is due to atmospheric refraction. The density of air layers decreases with height. Light from the sun below the horizon passes from rarer to denser layers of the atmosphere and bends towards the normal. This continuous bending makes the sun appear visible above the horizon even when it is actually slightly below it.

16. What is mirage ? Give a reason for its formation.

A mirage is an optical illusion seen in deserts where an inverted image of a distant object (like a tree) is seen, giving a false impression of water under the tree.
Reason: It is caused by the refraction of light. On hot days, the air near the ground is hotter and rarer than the air above. Light from the top of a tree travels from denser to rarer layers, bending away from the normal until total internal reflection (or continuous bending) occurs, causing the light to reach the observer's eye from below, creating an inverted image.

17. What is a prism ? Draw a ray diagram to show the refraction of a light ray through a prism.

A prism is a transparent medium bounded by five plane surfaces with a triangular cross-section (two triangular bases and three rectangular lateral surfaces).
(For diagram: Draw a triangle. Incident ray hits one side, bends towards the base (refraction 1). Ray travels inside prism. Ray hits the other side, bends further towards the base (refraction 2) and emerges. Label incident, refracted, and emergent rays.)

18. What do you mean by the term dispersion ?

Dispersion is the phenomenon of splitting of white light into its constituent colours (spectrum) when it passes through a transparent medium like a prism.

19. A ray of white light falls on a prism. Draw a ray diagram to show that the prism disperses the white light.

(Draw a prism. A single beam of white light enters one side. Inside the prism, the beam splits into different rays. Emerging from the other side, show a fan of rays spreading out. Label the top ray Red and the bottom ray Violet to show dispersion.)

20. In Fig. 5.40, AO is the ray of white light falling on a prism PQR. Complete the diagram till the light emerges out from the prism and falls on the screen.

(Draw the ray entering the prism and splitting. The rays should bend towards the base QR. On the screen, draw a band of colours. The ray bending the least (top) should be labeled Red, and the ray bending the most (bottom) should be labeled Violet. The order from bottom up on the screen is Violet, Indigo, Blue, Green, Yellow, Orange, Red.)

21. What do you understand by the term spectrum ? Name the various colours present in the spectrum of sunlight.

The band of colours obtained on a screen on passing white light through a prism is called a spectrum.
The colours are: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).

22. You are given a disc divided into seven sectors with colours violet, indigo, blue, green, yellow, orange and red in them. What would be its colour when it is rotated rapidly ?

When rotated rapidly, the disc appears white. This is because the sensation of all seven colours reaches the eye simultaneously, creating the effect of white light.

23. State the two laws of reflection of light.

1. The angle of incidence is equal to the angle of reflection (i = r).
2. The incident ray, the reflected ray, and the normal at the point of incidence, all lie in the same plane.

24. What is a spherical mirror ?

A spherical mirror is a mirror whose reflecting surface is a part of a hollow sphere of glass.

25. State the two kinds of spherical mirrors and distinguish them with the aid of proper diagrams.

The two kinds are: 1. Concave mirror: The reflecting surface is curved inwards (the hollow side). The outer bulged side is silvered. 2. Convex mirror: The reflecting surface is curved outwards (the bulged side). The inner hollow side is silvered.
(Distinction in diagrams: Draw a curve. For Concave, shade the outer side. For Convex, shade the inner side.)

26. Explain the following terms :
Pole, Centre of curvature, Radius of curvature, Principal axis.

Pole (P): The geometric centre of the spherical surface of the mirror.
Centre of curvature (C): The centre of the hollow sphere of which the mirror is a part.
Radius of curvature (R): The radius of the hollow sphere of which the mirror is a part (distance between P and C).
Principal axis: The straight line joining the pole and the centre of curvature.

Show them on separate diagrams for each of the concave and convex mirrors.

(For Concave: C is in front of the mirror. For Convex: C is behind the mirror. P is on the mirror surface in both.)

27. What do you understand by the focus and focal length of a spherical mirror ? Show them on the separate diagrams for each of a concave mirror and a convex mirror.

Focus (F): A point on the principal axis where rays of light parallel to the principal axis meet (concave) or appear to come from (convex) after reflection.
Focal length (f): The distance between the pole and the focus of the mirror.
(Diagrams: Concave - Parallel rays converge at F in front. Convex - Parallel rays diverge, appearing to originate from F behind.)

28. Draw suitable diagrams to illustrate how a beam of light incident parallel to the principal axis is reflected by :
(a) a concave mirror, and (b) a convex mirror.

(a) Draw a concave mirror. Incoming parallel rays reflect and meet at a single point (Focus) in front of the mirror.
(b) Draw a convex mirror. Incoming parallel rays reflect and spread out (diverge). If traced back, they meet at a point (Focus) behind the mirror.

29. How is a spherical mirror used to converge a beam of light at a point ? Name the type of mirror used.

By using a concave mirror. When a parallel beam of light falls on a concave mirror, the rays reflect and meet (converge) at the focus.

30. How is a spherical mirror used to diverge a beam of light from a point ? Name the type of mirror used.

By using a convex mirror. When a parallel beam of light falls on a convex mirror, the rays reflect and spread out (diverge) as if they are coming from the focus.

31. State the direction of incident ray which after reflection from a spherical mirror gets reflected along its own path. Give a reason.

An incident ray passing through (or directed towards) the centre of curvature reflects along its own path.
Reason: Such a ray strikes the mirror surface normally (at 90°), so the angle of incidence is 0°, and thus the angle of reflection is also 0°.

32. How is the focal length of a spherical mirror related to its radius of curvature ?

The focal length (f) is half of the radius of curvature (R).
Formula: f = R / 2 (or R = 2f).

33. The diagram (Fig. 5.41) given below shows two parallel rays 1 and 2 incident on (a) a concave mirror; (b) a convex mirror. Draw the reflected rays and mark the focus by the symbol F.

(a) Concave: Both rays 1 and 2 should reflect downwards/inwards to cross each other at a single point on the principal axis. Mark this point F.
(b) Convex: Both rays should reflect outwards/away. Draw dotted lines backwards from the reflected rays to meet at a point on the principal axis behind the mirror. Mark this point F.

34. Complete the following diagrams in Fig. 5.42 by drawing the reflected rays for the incident rays 1 and 2 if F is the focus and C is the centre of curvature.

(a) Concave mirror diagram: Ray 1 (through F): Reflects parallel to the principal axis. Ray 2 (through C): Reflects back along the same path (retraces itself).
(b) Convex mirror diagram (assuming Ray 1 is directed towards F and Ray 2 towards C): Ray 1 (directed towards F): Reflects parallel to the principal axis. Ray 2 (directed towards C): Reflects back along the same path.

35. Which are the two convenient rays that are chosen to construct the image by a spherical mirror for a given object? Explain with the help of suitable ray diagrams.

Any two of the following are usually chosen: 1. A ray parallel to the principal axis: After reflection, it passes through the Focus (concave) or appears to come from the Focus (convex). 2. A ray passing through the Centre of Curvature: After reflection, it retraces its own path. 3. A ray passing through the Focus: After reflection, it becomes parallel to the principal axis.

36. Draw a ray diagram to show the formation of image of an object placed beyond the centre of curvature of a concave mirror. State the position, size and nature of the image.

Diagram: Object beyond C. Rays intersect between F and C. Position: Between the focus (F) and centre of curvature (C). Size: Diminished (smaller than object). Nature: Real and inverted.

37. Draw a ray diagram to show the formation of image of an object placed at the centre of curvature of a concave mirror. State the position, size and nature of the image.

Diagram: Object at C. Rays intersect exactly at C. Position: At the centre of curvature (C). Size: Same size as the object. Nature: Real and inverted.

38. Draw a ray diagram to show the formation of image of an object placed between the focus and centre of curvature of a concave mirror. State the position, size and nature of the image.

Diagram: Object between F and C. Rays intersect beyond C. Position: Beyond the centre of curvature (C). Size: Enlarged (magnified). Nature: Real and inverted.

39. Draw a ray diagram to show the formation of image of an object placed between the pole and focus of a concave mirror. State the position, size and nature of the image.

Diagram: Object between P and F. Reflected rays diverge. When traced back, they meet behind the mirror. Position: Behind the mirror. Size: Enlarged (magnified). Nature: Virtual and erect.

40. Draw a ray diagram to show the formation of image of an object placed in front of a convex mirror. State the position, size and nature of the image.

Diagram: Object anywhere in front of convex mirror. Reflected rays diverge. Traced back, they meet between Pole (P) and Focus (F) behind the mirror. Position: Between the pole and focus, behind the mirror. Size: Diminished. Nature: Virtual and erect.

41. Draw separate diagrams for the formation of virtual image of an object by
(a) concave mirror and
(b) convex mirror.
State the difference in the two images.

(a) Concave: Object placed between P and F. Image is behind mirror, magnified. (b) Convex: Object placed anywhere. Image is behind mirror, diminished.
Difference: The virtual image formed by the concave mirror is magnified (larger than object), while the virtual image formed by the convex mirror is diminished (smaller than object).

42. Name the mirror which always forms an erect and virtual image. What is the size of the image as compared to that of the object ?

Convex mirror. (A plane mirror also does this, but in the context of spherical mirrors, the answer is convex). The size of the image is diminished (smaller than the object).

43. Name the mirror which forms an erect, virtual and enlarged image of an object. What is the position of object relative to the mirror ?

Concave mirror. The object must be placed between the pole and the focus of the mirror.

44. What is a real image ? Name the mirror which can be used to obtain the real image of an object. What should be the position of the object relative to the mirror ?

A real image is an image that can be obtained on a screen and is formed by the actual intersection of reflected rays. It is always inverted.
Mirror: Concave mirror.
Position: The object should be placed beyond the focus (at F, between F and C, at C, or beyond C).

45. How can a concave mirror be used to obtain a virtual image of an object ? Draw a diagram to illustrate your answer.

By placing the object very close to the mirror, specifically at a distance less than the focal length (between the pole and the focus). The reflected rays diverge and appear to meet behind the mirror forming a virtual image.

46. State two uses of a concave mirror.

1. As a shaving mirror (to see an enlarged face). 2. As a reflector in torches and car headlights (to obtain a parallel beam of light).

47. State two uses of a convex mirror.

1. As a rear-view mirror in vehicles. 2. As a reflector in street lamps to diverge light over a large area.

48. A driver uses a convex mirror as a rear view mirror. Explain the reason with the help of a ray diagram.

Reason: A convex mirror always forms an erect and diminished image. Because the image is smaller, it offers a much wider field of view compared to a plane mirror, allowing the driver to see more traffic behind them. (Diagram: Show a wide angle of rays entering the eye from the convex mirror surface).

49. State the kind of mirror used
(a) by a dentist, and
(b) as a street light reflector.

(a) Concave mirror (to see an enlarged image of teeth).
(b) Convex mirror (to spread light over a wider area).

50. Name the kind of mirror used to obtain
(a) a real and enlarged image,
(b) a virtual and enlarged image,
(c) a real and diminished image, and
(d) a virtual and diminished image.

(a) Concave mirror (Object between F and C)
(b) Concave mirror (Object between P and F)
(c) Concave mirror (Object beyond C)
(d) Convex mirror (Object anywhere)


Project Work

You are given few mirrors labelled A, B, C, D, ...... . Identify them as plane, concave or convex mirror by looking your own face in each mirror one by one and recording the size and nature of image.

Mirror Size of image Kind of mirror Nature of image
A Same size Plane mirror Virtual and erect
B Enlarged Concave mirror (face close) Virtual and erect
C Diminished Convex mirror Virtual and erect
D Inverted (if held far) Concave mirror Real and inverted
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Multiple Choice Questions - Test your understanding with these MCQs. Click "Show Answer" to reveal the correct answer.

Question 1What is the speed of light in air?

Correct Answer: 3 × 10⁸ m/s

Question 2How does the speed of light in water compare to the speed of light in air?

Correct Answer: It is slower in water

Question 3Which medium is considered optically denser?

Correct Answer: The medium where the speed of light decreases

Question 4What is the approximate speed of light in glass?

Correct Answer: 2 × 10⁸ m/s

Question 5What is the phenomenon called when light changes direction passing from one transparent medium to another?

Correct Answer: Refraction

Question 6When a ray of light travels from a rarer to a denser medium, how does it bend?

Correct Answer: Towards the normal

Question 7When a ray of light travels from a denser to a rarer medium, how does it bend?

Correct Answer: Away from the normal

Question 8What happens to a light ray that falls normally (perpendicularly) on the surface separating two media?

Correct Answer: It passes undeviated

Question 9What is the angle of incidence when a ray falls normally on a surface?

Correct Answer: 0 degrees

Question 10Which term describes the ray of light falling on the surface separating two media?

Correct Answer: Incident ray

Question 11The perpendicular drawn on the surface separating two media at the point of incidence is called the:

Correct Answer: Normal

Question 12The angle between the refracted ray and the normal is known as:

Correct Answer: Angle of refraction

Question 13If a ray travels from air to glass, what is the relationship between the angle of incidence (i) and the angle of refraction (r)?

Correct Answer: angle r < angle i

Question 14Which law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant?

Correct Answer: Snell's Law

Question 15What symbol is used to denote the refractive index?

Correct Answer: µ (mu)

Question 16The refractive index of a second medium with respect to a first medium is calculated as:

Correct Answer: Speed of light in first medium / Speed of light in second medium

Question 17What is the refractive index of air?

Correct Answer: 1

Question 18Can a medium have a refractive index less than 1?

Correct Answer: No

Question 19Why does a swimming pool appear shallower than it actually is?

Correct Answer: Refraction of light

Question 20When viewing an object in a denser medium from a rarer medium, the object appears:

Correct Answer: At a shallower (raised) depth

Question 21The refractive index of water is 4/3. What is the relationship between real depth and apparent depth?

Correct Answer: Apparent depth is 3/4 of the real depth

Question 22Why does a pencil partly immersed in water appear bent at the water surface?

Correct Answer: Light bends as it travels from water to air

Question 23The phenomenon of advanced sunrise and delayed sunset is caused by:

Correct Answer: Atmospheric refraction

Question 24Due to atmospheric refraction, the sun is seen:

Correct Answer: A few minutes before actual sunrise

Question 25What is a mirage?

Correct Answer: An optical illusion caused by total internal reflection and refraction

Question 26In a desert mirage, the image of a tree appears:

Correct Answer: Inverted, giving an impression of water

Question 27In a rectangular glass block, the emergent ray is parallel to the incident ray but shifted. This shift is called:

Correct Answer: Lateral displacement

Question 28When light passes through a rectangular glass block, refraction occurs at how many surfaces?

Correct Answer: Two

Question 29In a prism, the angle between the two opposite refracting surfaces is called:

Correct Answer: Angle of prism

Question 30When a light ray passes through a prism, it bends towards the:

Correct Answer: Base of the prism

Question 31The splitting of white light into its constituent colors is called:

Correct Answer: Dispersion

Question 32Which acronym helps remember the order of colors in a spectrum?

Correct Answer: VIBGYOR

Question 33In the spectrum of white light formed by a prism, which color deviates the most?

Correct Answer: Violet

Question 34In the spectrum of white light formed by a prism, which color deviates the least?

Correct Answer: Red

Question 35What is the cause of dispersion of white light?

Correct Answer: Different colors have different speeds in a transparent medium

Question 36Which color of light travels the fastest in glass?

Correct Answer: Red

Question 37A rainbow is a natural example of:

Correct Answer: Dispersion of sunlight by raindrops

Question 38If you rotate a Newton's color disc rapidly, what color does it appear?

Correct Answer: White

Question 39A spherical mirror where reflection takes place from the inner hollow surface is called:

Correct Answer: Concave mirror

Question 40A spherical mirror where reflection takes place from the outer bulging surface is called:

Correct Answer: Convex mirror

Question 41The geometric center of the spherical surface of a mirror is called the:

Correct Answer: Pole

Question 42The center of the sphere of which the mirror is a part is known as:

Correct Answer: Centre of curvature

Question 43The straight line joining the pole and the centre of curvature is called the:

Correct Answer: Principal axis

Question 44For a concave mirror, rays parallel to the principal axis converge at a point called the:

Correct Answer: Focus

Question 45The focus of a convex mirror is:

Correct Answer: Virtual and behind the mirror

Question 46What is the relationship between focal length (f) and radius of curvature (R)?

Correct Answer: f = R / 2

Question 47Which type of image can be obtained on a screen?

Correct Answer: Real image

Question 48A virtual image is always:

Correct Answer: Erect (upright)

Question 49A ray passing through the centre of curvature of a spherical mirror is reflected:

Correct Answer: Back along its own path

Question 50A ray incident parallel to the principal axis of a concave mirror will pass through which point after reflection?

Correct Answer: Focus
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True or False Questions - Determine whether each statement is true or false. Click "Show Answer" to reveal the correct answer.

Statement 1Light travels faster in water than in air.

Correct Answer: False

Statement 2The speed of light in air is 3 × 10⁸ m s⁻¹.

Correct Answer: True

Statement 3Glass is optically denser than water.

Correct Answer: True

Statement 4A medium is said to be rarer if the speed of light in it decreases.

Correct Answer: False

Statement 5Refraction is the change in direction of light when it passes from one transparent medium to another.

Correct Answer: True

Statement 6When a ray of light travels from a rarer to a denser medium, it bends away from the normal.

Correct Answer: False

Statement 7When a light ray falls normally on the surface separating two media, it passes undeviated.

Correct Answer: True

Statement 8The angle of incidence is the angle between the incident ray and the surface of the medium.

Correct Answer: False

Statement 9Snell's Law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.

Correct Answer: True

Statement 10The refractive index of air is considered to be zero.

Correct Answer: False

Statement 11No medium can have a refractive index less than 1.

Correct Answer: True

Statement 12The depth of a pond appears deeper than it actually is due to refraction.

Correct Answer: False

Statement 13A pencil partly immersed in water appears bent due to reflection.

Correct Answer: False

Statement 14The sun is seen a few minutes before actual sunrise due to atmospheric refraction.

Correct Answer: True

Statement 15A mirage is an optical illusion caused by total internal reflection and refraction in deserts.

Correct Answer: True

Statement 16In a mirage, the image of a tree is seen inverted.

Correct Answer: True

Statement 17When light passes through a rectangular glass block, the emergent ray is perpendicular to the incident ray.

Correct Answer: False

Statement 18Lateral displacement occurs when light passes through a rectangular glass block.

Correct Answer: True

Statement 19A prism has a circular cross-section.

Correct Answer: False

Statement 20In a prism, the emergent ray bends towards the base.

Correct Answer: True

Statement 21Dispersion is the splitting of white light into its constituent colors.

Correct Answer: True

Statement 22Red light deviates the most when passing through a prism.

Correct Answer: False

Statement 23Violet light travels the slowest in glass compared to other colors.

Correct Answer: True

Statement 24A rainbow is formed due to the dispersion of sunlight by raindrops.

Correct Answer: True

Statement 25Newton's color disc appears black when rotated rapidly.

Correct Answer: False

Statement 26A concave mirror is made by silvering the inner surface of a hollow sphere.

Correct Answer: False

Statement 27The pole is the geometric center of the spherical surface of a mirror.

Correct Answer: True

Statement 28The centre of curvature lies on the surface of the spherical mirror.

Correct Answer: False

Statement 29The radius of curvature is twice the focal length.

Correct Answer: True

Statement 30A real image can be obtained on a screen.

Correct Answer: True

Statement 31A virtual image is always inverted.

Correct Answer: False

Statement 32A ray passing through the centre of curvature retraces its path after reflection.

Correct Answer: True

Statement 33Rays parallel to the principal axis converge at the focus in a convex mirror.

Correct Answer: False

Statement 34A concave mirror always forms a real image.

Correct Answer: False

Statement 35When an object is placed at the focus of a concave mirror, the image is formed at infinity.

Correct Answer: True

Statement 36A convex mirror always forms a virtual, erect, and diminished image.

Correct Answer: True

Statement 37Concave mirrors are used as rear-view mirrors in vehicles.

Correct Answer: False

Statement 38Convex mirrors are used in street lamps to diverge light.

Correct Answer: True

Statement 39Dentists use concave mirrors to examine teeth.

Correct Answer: True

Statement 40The image formed by a plane mirror is laterally inverted.

Correct Answer: True

Statement 41Water is optically rarer than glass.

Correct Answer: True

Statement 42The speed of light is maximum in diamond.

Correct Answer: False

Statement 43Refraction does not occur if the light ray strikes the surface at an angle of 90 degrees.

Correct Answer: True

Statement 44The apparent depth of a coin in water is less than its real depth.

Correct Answer: True

Statement 45A concave mirror diverges parallel beams of light.

Correct Answer: False

Statement 46The focal length is half the radius of curvature.

Correct Answer: True

Statement 47Virtual images are formed by the actual intersection of light rays.

Correct Answer: False

Statement 48A convex mirror has a wider field of view than a plane mirror.

Correct Answer: True

Statement 49In a spectrum, violet is at the top and red is at the bottom.

Correct Answer: False

Statement 50Prisms produce colors rather than separating them.

Correct Answer: False
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Fill in the Blanks - Complete the sentences by filling in the missing words. Click "Show Answer" to check your response.

Question 1The change in direction of path of light when it passes from one transparent medium to another is called ______.

Correct Answer: refraction

Question 2The speed of light in air is approximately ______ m/s.

Correct Answer: 3 × 10⁸

Question 3The speed of light in water is ______ m/s.

Correct Answer: 2.25 × 10⁸

Question 4The speed of light in glass is ______ m/s.

Correct Answer: 2 × 10⁸

Question 5A medium is said to be optically ______ if the speed of light in it decreases compared to another medium.

Correct Answer: denser

Question 6A medium is said to be optically ______ if the speed of light in it increases compared to another medium.

Correct Answer: rarer

Question 7When a ray of light travels from a rarer to a denser medium, it bends ______ the normal.

Correct Answer: towards

Question 8When a ray of light travels from a denser to a rarer medium, it bends ______ from the normal.

Correct Answer: away

Question 9When a ray of light falls normally on the surface separating two media, it passes ______.

Correct Answer: undeviated

Question 10The angle between the incident ray and the normal is called the angle of ______.

Correct Answer: incidence

Question 11The angle between the refracted ray and the normal is called the angle of ______.

Correct Answer: refraction

Question 12According to Snell's law, the ratio of the sine of angle of incidence to the sine of angle of refraction is a ______.

Correct Answer: constant

Question 13The constant ratio in Snell's law is known as the ______ index of the second medium with respect to the first.

Correct Answer: refractive

Question 14The refractive index of air is ______.

Correct Answer: 1

Question 15The refractive index of water is approximately ______.

Correct Answer: 1.33

Question 16The refractive index of glass is approximately ______.

Correct Answer: 1.5

Question 17Due to refraction, the depth of a pond appears ______ than its real depth.

Correct Answer: less

Question 18A coin placed at the bottom of a vessel filled with water appears to be ______.

Correct Answer: raised

Question 19A stick partly immersed in water appears bent due to the phenomenon of ______.

Correct Answer: refraction

Question 20The sun is seen a few minutes before actual sunrise due to ______ refraction.

Correct Answer: atmospheric

Question 21An optical illusion observed in deserts due to refraction is called a ______.

Correct Answer: mirage

Question 22In a mirage, the image of a tree formed on the sand is ______.

Correct Answer: inverted

Question 23When light passes through a rectangular glass block, the emergent ray is ______ to the incident ray.

Correct Answer: parallel

Question 24The perpendicular distance between the incident ray and the emergent ray in a glass block is called ______ displacement.

Correct Answer: lateral

Question 25A prism is a transparent medium bounded by five plane surfaces with a ______ cross section.

Correct Answer: triangular

Question 26When a light ray passes through a prism, it bends towards the ______ of the prism.

Correct Answer: base

Question 27The splitting of white light into its constituent colors is called ______.

Correct Answer: dispersion

Question 28The colored band obtained on a screen on passing white light through a prism is called a ______.

Correct Answer: spectrum

Question 29The color of light that deviates the most when passing through a prism is ______.

Correct Answer: violet

Question 30The color of light that deviates the least when passing through a prism is ______.

Correct Answer: red

Question 31White light is a mixture of ______ colors.

Correct Answer: seven

Question 32In a transparent medium like glass, the speed of ______ light is minimum.

Correct Answer: violet

Question 33In a transparent medium like glass, the speed of ______ light is maximum.

Correct Answer: red

Question 34A rainbow is formed due to the dispersion of sunlight by ______.

Correct Answer: raindrops

Question 35When a color disc with seven colors is rotated rapidly, it appears ______.

Correct Answer: white

Question 36A spherical mirror made by silvering the outer surface of a hollow sphere is called a ______ mirror.

Correct Answer: concave

Question 37A spherical mirror made by silvering the inner surface of a hollow sphere is called a ______ mirror.

Correct Answer: convex

Question 38The geometric center of the spherical surface of a mirror is called the ______.

Correct Answer: pole

Question 39The center of the sphere of which the mirror is a part is called the ______ of curvature.

Correct Answer: centre

Question 40The straight line joining the pole and the center of curvature is called the ______ axis.

Correct Answer: principal

Question 41The normal at any point of a spherical mirror passes through the ______ of curvature.

Correct Answer: centre

Question 42Parallel rays after reflection from a concave mirror meet at a point called the ______.

Correct Answer: focus

Question 43The focus of a convex mirror is ______ because light rays only appear to meet there.

Correct Answer: virtual

Question 44The distance from the pole to the focus is called the ______ length.

Correct Answer: focal

Question 45The focal length of a spherical mirror is equal to ______ of its radius of curvature.

Correct Answer: half

Question 46If the reflected rays actually meet at a point, the image formed is ______.

Correct Answer: real

Question 47If the reflected rays appear to meet when produced backwards, the image formed is ______.

Correct Answer: virtual

Question 48A real image is always ______.

Correct Answer: inverted

Question 49A virtual image is always ______.

Correct Answer: erect

Question 50A real image can be obtained on a ______.

Correct Answer: screen
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Short Answer Questions - Practice concise answers to these questions. Click "Show Answer" to reveal the response.
Question 1

What is the speed of light in air?

Answer:
3 × 10⁸ m/s
Question 2

What is the speed of light in water?

Answer:
2.25 × 10⁸ m/s
Question 3

What is the speed of light in glass?

Answer:
2 × 10⁸ m/s
Question 4

Which is optically denser: water or glass?

Answer:
Glass
Question 5

What phenomenon causes light to bend when passing between media?

Answer:
Refraction
Question 6

How does light bend when going from a rarer to a denser medium?

Answer:
Towards the normal
Question 7

How does light bend when going from a denser to a rarer medium?

Answer:
Away from the normal
Question 8

What happens to a light ray falling normally on a surface?

Answer:
Passes undeviated
Question 9

What is the angle of incidence for a ray striking a surface normally?

Answer:
Zero
Question 10

What is the perpendicular line drawn at the point of incidence called?

Answer:
Normal
Question 11

What is the ratio of sine of angle of incidence to sine of angle of refraction?

Answer:
Refractive index
Question 12

Which laws govern the refraction of light?

Answer:
Snell's laws
Question 13

What is the symbol for refractive index?

Answer:
µ (mu)
Question 14

What is the refractive index of air?

Answer:
1
Question 15

What is the refractive index of water?

Answer:
1.33 (or 4/3)
Question 16

What is the refractive index of glass?

Answer:
1.5
Question 17

Can the refractive index of a medium be less than 1?

Answer:
No
Question 18

Why does a swimming pool appear shallower than it is?

Answer:
Refraction
Question 19

Does the apparent depth of water appear more or less than real depth?

Answer:
Less
Question 20

Why does a pencil in water appear bent?

Answer:
Refraction
Question 21

What phenomenon causes advanced sunrise and delayed sunset?

Answer:
Atmospheric refraction
Question 22

What is the optical illusion of water seen in deserts?

Answer:
Mirage
Question 23

In a mirage, is the air near the ground denser or rarer?

Answer:
Rarer
Question 24

What is the shift of the emergent ray in a glass block called?

Answer:
Lateral displacement
Question 25

Is the emergent ray in a glass block parallel to the incident ray?

Answer:
Yes
Question 26

How many plane surfaces bound a prism?

Answer:
Five
Question 27

Towards which part of the prism does a light ray bend?

Answer:
Base
Question 28

What is the splitting of white light into colors called?

Answer:
Dispersion
Question 29

What is the band of colored light obtained on a screen called?

Answer:
Spectrum
Question 30

Which acronym represents the colors of the spectrum?

Answer:
VIBGYOR
Question 31

Which color deviates the most in a prism?

Answer:
Violet
Question 32

Which color deviates the least in a prism?

Answer:
Red
Question 33

Which color of light travels fastest in glass?

Answer:
Red
Question 34

Which color of light travels slowest in glass?

Answer:
Violet
Question 35

Does a prism create colors or separate them?

Answer:
Separate them
Question 36

What natural phenomenon is caused by dispersion in raindrops?

Answer:
Rainbow
Question 37

What color does Newton's color disc appear when rotated?

Answer:
White
Question 38

What mirror is made by silvering the outer surface of a hollow sphere?

Answer:
Concave mirror
Question 39

What mirror is made by silvering the inner surface of a hollow sphere?

Answer:
Convex mirror
Question 40

What is the geometric centre of a spherical mirror called?

Answer:
Pole
Question 41

What is the centre of the sphere of which the mirror is a part?

Answer:
Centre of curvature
Question 42

What is the straight line joining the pole and centre of curvature?

Answer:
Principal axis
Question 43

What passes through the centre of curvature to any point on the mirror?

Answer:
Normal
Question 44

Where do parallel rays converge after reflection in a concave mirror?

Answer:
Focus
Question 45

Is the focus of a concave mirror real or virtual?

Answer:
Real
Question 46

Is the focus of a convex mirror real or virtual?

Answer:
Virtual
Question 47

What is the distance between the pole and the focus?

Answer:
Focal length
Question 48

What is the relationship between focal length (f) and radius of curvature (R)?

Answer:
f = R/2
Question 49

What type of image can be obtained on a screen?

Answer:
Real image
Question 50

What type of image cannot be obtained on a screen?

Answer:
Virtual image
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Long Answer Questions - Practice detailed answers to these comprehensive questions. Click "Show Answer" to reveal the response.
Question 1

Define refraction of light and explain the cause of refraction.

Answer:
Refraction of light is defined as the change in the direction of the path of light when it passes from one transparent medium to another. For example, when light travels from air to water, it changes its path. The primary cause of refraction is the change in the speed of light when it enters from one medium to another. Light travels at different speeds in different optical media; for instance, it travels at 3 x 10^8 m/s in air but slows down to 2.25 x 10^8 m/s in water. This change in speed causes the light ray to bend.
Question 2

State the laws of refraction, also known as Snell's laws.

Answer:
Refraction of light obeys two specific laws, known as Snell's laws: 1. The first law states that the incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane. 2. The second law states that for a given pair of media and a given color of light, the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is a constant. This constant is denoted by the symbol mu (refractive index). Mathematically, it is expressed as sin i / sin r = constant.
Question 3

Explain the terms 'Optically Rarer Medium' and 'Optically Denser Medium' with respect to the speed of light.

Answer:
A transparent medium is described based on the speed of light traveling through it. A medium is said to be optically rarer if the speed of light in it is relatively higher. For example, air is optically rarer than water because light travels faster in air. Conversely, a medium is said to be optically denser if the speed of light in it decreases. For instance, glass is optically denser than water because the speed of light in glass (2 x 10^8 m/s) is lower than in water (2.25 x 10^8 m/s). It is important to note that no medium can have a refractive index less than 1, as the speed of light is maximum in a vacuum.
Question 4

Describe an experiment to demonstrate that a coin placed at the bottom of a vessel appears raised when water is poured into it.

Answer:
To demonstrate this phenomenon: 1. Place a coin in an empty vessel and place the vessel on a table. 2. Step back slowly until the coin is just out of your line of sight, hidden by the edge of the vessel. 3. While maintaining this position, ask a friend to gently pour water into the vessel. 4. You will observe that as the water level rises, the coin becomes visible again. Explanation: When the vessel was empty, light from the coin traveled in a straight line and was blocked by the edge. When water is added, the ray of light traveling from the coin (denser medium) to air (rarer medium) bends away from the normal at the water surface. This refracted ray reaches your eye, creating a virtual image of the coin at a position slightly higher than its actual position, making it visible.
Question 5

Explain why a pencil partly immersed in water appears to be bent at the water surface.

Answer:
A pencil partly immersed in water appears bent due to the refraction of light. Consider the tip of the pencil inside the water. Rays of light coming from the tip travel from water (a denser medium) to air (a rarer medium). As they exit the water surface, they bend away from the normal. When these divergent refracted rays reach the observer's eye, they appear to originate from a point higher than the actual tip. This apparent shift occurs for every point of the pencil submerged in water, causing the submerged part to appear raised. Consequently, the pencil looks bent at the interface where it enters the water.
Question 6

What is a mirage? Explain the reason for its formation in a desert.

Answer:
A mirage is an optical illusion observed in deserts where an inverted image of a distant object, such as a tree, creates the false impression of a pool of water. It is caused by the refraction of light. During the day, the sand becomes very hot, heating the layers of air in contact with it. This makes the air near the ground warmer and optically rarer than the cooler, denser air layers above. As a ray of light from the top of a tree travels down towards the ground, it passes through successively rarer layers of air and bends away from the normal. Eventually, the angle of incidence exceeds the critical limit, and the ray is reflected upwards (total internal reflection). When this ray reaches the observer's eye, it appears to come from the ground, forming an inverted image that resembles a reflection in water.
Question 7

Explain the phenomenon of early sunrise and delayed sunset.

Answer:
The phenomenon of early sunrise and delayed sunset is due to atmospheric refraction. The earth's atmosphere consists of layers of varying densities, generally becoming denser closer to the surface. When the sun is just below the horizon, light rays from it travel from the vacuum of space into the atmosphere. As the light passes from rarer to denser layers, it continuously bends towards the normal. This bending causes the rays to reach the observer even when the sun is geometrically below the horizon. As a result, the sun appears to rise a few minutes before it actually does and remains visible for a few minutes after it has actually set.
Question 8

Describe the refraction of light through a rectangular glass block with a diagram description.

Answer:
When a light ray passes through a rectangular glass block, it undergoes refraction twice. 1. First Refraction: The incident ray traveling in air falls obliquely on the first parallel surface of the glass block. Since it enters from a rarer medium (air) to a denser medium (glass), it bends towards the normal. This is the refracted ray inside the block. 2. Second Refraction: This refracted ray travels through the glass and strikes the second parallel surface. Here, it travels from a denser medium (glass) to a rarer medium (air), so it bends away from the normal. This ray exiting the block is called the emergent ray. The emergent ray is found to be parallel to the original direction of the incident ray but is laterally shifted. This perpendicular distance between the incident ray and the emergent ray is called lateral displacement.
Question 9

What is a prism? Describe the path of a monochromatic light ray passing through it.

Answer:
A prism is a transparent optical element bounded by five plane surfaces, typically having a triangular cross-section. It has two triangular bases and three rectangular lateral surfaces. When a ray of monochromatic (single color) light falls obliquely on one refracting face of the prism, it enters from air (rarer) to glass (denser) and bends towards the normal. The ray travels through the prism and strikes the second refracting face. Here, it exits from glass to air, bending away from the normal. Unlike a rectangular glass block, the refracting surfaces of a prism are not parallel but inclined. Therefore, the emergent ray bends towards the base of the prism and is not parallel to the incident ray. The angle between the incident ray and the emergent ray is called the angle of deviation.
Question 10

Define dispersion of light and describe Newton's experiment that demonstrated it.

Answer:
Dispersion of light is the phenomenon of splitting white light into its constituent colors (spectrum) when it passes through a transparent medium like a prism. Sir Isaac Newton demonstrated this by allowing a narrow beam of sunlight (white light) to enter a dark room through a small hole and pass through a glass prism. He observed that the light emerging from the prism was not white but a band of seven colors on a white screen placed behind it. The colors appeared in the order: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR), with violet bending the most and red the least. This proved that white light is a mixture of these seven colors.
Question 11

What is the cause of dispersion of white light?

Answer:
The cause of dispersion is that different colors of light travel at different speeds in a transparent medium like glass, although they travel at the same speed in a vacuum or air. Since the refractive index of a medium is the ratio of the speed of light in air to the speed of light in the medium, the refractive index is different for different colors. Violet light travels the slowest in glass, so it has the highest refractive index and bends the most. Red light travels the fastest in glass, has the lowest refractive index, and bends the least. This difference in bending separation spreads the colors out into a spectrum.
Question 12

Define the terms: Pole, Centre of Curvature, and Radius of Curvature for a spherical mirror.

Answer:
1. Pole (P): The geometric center of the spherical reflecting surface of the mirror is called the pole. It lies on the surface of the mirror. 2. Centre of Curvature (C): The center of the imaginary hollow sphere of which the spherical mirror is a part is called the centre of curvature. For a concave mirror, it is in front of the mirror; for a convex mirror, it is behind it. 3. Radius of Curvature (R): The radius of the hollow sphere of which the mirror is a part is called the radius of curvature. It is the distance between the pole and the centre of curvature (PC).
Question 13

Differentiate between the principal focus of a concave mirror and a convex mirror.

Answer:
1. Concave Mirror Focus: The principal focus of a concave mirror is a point on the principal axis where rays of light incident parallel to the principal axis actually meet (converge) after reflection. It is a real focus and is located in front of the mirror. 2. Convex Mirror Focus: The principal focus of a convex mirror is a point on the principal axis where rays of light incident parallel to the principal axis appear to meet (diverge from) after reflection. It is a virtual focus and is located behind the mirror.
Question 14

State the relationship between focal length and radius of curvature. Describe an activity to find the approximate focal length of a concave mirror.

Answer:
The focal length (f) of a spherical mirror is half of its radius of curvature (R). The relationship is expressed as f = R / 2 or R = 2f. To find the approximate focal length of a concave mirror: Take a concave mirror and face it towards a distant object, like the sun. Hold a sheet of paper in front of the mirror and adjust the distance between the mirror and the paper until a sharp, bright point (image of the sun) is formed on the paper. The distance between the pole of the mirror and the paper (focus) is the approximate focal length.
Question 15

List the rules for constructing ray diagrams for spherical mirrors.

Answer:
To construct a ray diagram, any two of the following standard rays are used: 1. A ray incident parallel to the principal axis, after reflection, passes through the principal focus (in a concave mirror) or appears to diverge from the principal focus (in a convex mirror). 2. A ray passing through the centre of curvature falls normally on the mirror and is reflected back along the same path. 3. A ray passing through the focus (in a concave mirror) or directed towards the focus (in a convex mirror) becomes parallel to the principal axis after reflection.
Question 16

Distinguish between a real image and a virtual image.

Answer:
1. Formation: A real image is formed when light rays actually intersect at a point after reflection. A virtual image is formed when light rays do not actually intersect but appear to diverge from a point when produced backwards. 2. Screen: A real image can be obtained on a screen. A virtual image cannot be obtained on a screen. 3. Orientation: A real image is always inverted relative to the object. A virtual image is always erect (upright). 4. Location: In mirrors, real images are formed in front of the mirror, while virtual images are formed behind the mirror.
Question 17

Describe the position, nature, and size of the image formed by a concave mirror when the object is placed at infinity.

Answer:
When an object is placed at infinity with respect to a concave mirror, the incident rays are parallel to each other and to the principal axis. After reflection, these rays converge at the principal focus. Therefore: 1. Position: The image is formed exactly at the focus (F). 2. Nature: The image is real and inverted. 3. Size: The image is highly diminished, appearing almost as a point.
Question 18

Describe the image formation by a concave mirror when the object is placed beyond the centre of curvature.

Answer:
When an object is placed beyond the centre of curvature (C) of a concave mirror: 1. A ray parallel to the axis passes through the focus. 2. A ray passing through the centre of curvature reflects back along the same path. The intersection of these reflected rays occurs between the focus (F) and the centre of curvature (C). Resulting Image: 1. Position: Between the focus (F) and centre of curvature (C). 2. Nature: Real and inverted. 3. Size: Diminished (smaller than the object).
Question 19

What happens when an object is placed exactly at the centre of curvature of a concave mirror? Describe the image.

Answer:
When an object is placed at the centre of curvature (C) of a concave mirror, the image is formed at the same position. Rays originating from the object and reflecting off the mirror intersect exactly at C. Image Characteristics: 1. Position: At the centre of curvature (C). 2. Nature: Real and inverted. 3. Size: Same size as the object.
Question 20

Describe the image formed by a concave mirror when the object is placed between the centre of curvature and the focus.

Answer:
When the object is situated between the centre of curvature (C) and the focus (F): 1. Rays reflect and intersect at a point beyond the centre of curvature. Image Characteristics: 1. Position: Beyond the centre of curvature (C). 2. Nature: Real and inverted. 3. Size: Magnified (larger than the object).
Question 21

Where is the image formed if an object is placed at the focus of a concave mirror? What are its characteristics?

Answer:
If an object is placed at the focus (F) of a concave mirror, the reflected rays become parallel to the principal axis and parallel to each other. Parallel rays are assumed to meet at infinity. Image Characteristics: 1. Position: At infinity. 2. Nature: Real and inverted. 3. Size: Highly magnified.
Question 22

Describe the special case where a concave mirror forms a virtual image. State the position and nature of the image.

Answer:
A concave mirror forms a virtual image when the object is placed very close to the mirror, specifically between the pole (P) and the focus (F). In this case, the reflected rays diverge and do not meet in front of the mirror. When produced backwards, they appear to meet behind the mirror. Image Characteristics: 1. Position: Behind the mirror. 2. Nature: Virtual and erect. 3. Size: Magnified.
Question 23

Describe the image formation by a convex mirror. Does the position of the object affect the nature of the image?

Answer:
For a convex mirror, regardless of the position of the object in front of it, the image formed is always virtual, erect, and diminished. The reflected rays diverge and appear to originate from a point behind the mirror. Image Characteristics: 1. Position: Always between the pole (P) and the focus (F) behind the mirror. 2. Nature: Virtual and erect. 3. Size: Diminished (smaller than the object). As the object moves closer to the mirror, the image shifts towards the pole and increases slightly in size but remains smaller than the object.
Question 24

Explain the use of a concave mirror as a shaving mirror.

Answer:
A concave mirror is used as a shaving mirror because of its ability to form a magnified and erect image when the object is placed within its focal length (between the pole and the focus). When a person holds their face close to the mirror, they are positioned between the focus and the pole. This results in an enlarged, upright reflection of the face, making it easier to see fine details for shaving or applying makeup.
Question 25

Why are concave mirrors used in torches and car headlights?

Answer:
Concave mirrors are used as reflectors in torches, searchlights, and car headlights to produce a powerful, parallel beam of light. The light source (bulb) is placed at the principal focus of the concave mirror. According to the rules of reflection, rays of light originating from the focus travel parallel to the principal axis after reflection. This results in a concentrated, parallel beam that can travel long distances without much dispersion.
Question 26

How is a concave mirror utilized by doctors?

Answer:
Doctors, specifically dentists and ENT specialists, use concave mirrors as head mirrors or inspection mirrors. By directing a parallel beam of light onto the concave mirror, the mirror converges the light to a distinct point at its focus. The doctor positions the mirror so that this bright spot illuminates the specific small area to be examined, such as a tooth, throat, or ear canal, providing enhanced visibility.
Question 27

Explain the role of a concave mirror in a solar cooker.

Answer:
In a solar cooker, a large concave mirror is used to converge solar radiation. The mirror is positioned to face the sun, and the cooking vessel is placed at the principal focus of the mirror. Since sun rays are parallel (coming from infinity), the concave mirror reflects and converges them onto the focus. This concentration of sunlight at a single point generates a significant amount of heat, which is used to cook the food in the vessel.
Question 28

Why is a convex mirror used as a rear-view mirror in vehicles?

Answer:
A convex mirror is preferred as a rear-view mirror in vehicles for two main reasons: 1. Always Erect Image: Unlike concave mirrors, which can invert images, a convex mirror always forms an erect (upright) image of the objects behind, regardless of their distance. 2. Wider Field of View: Because the mirror is curved outwards, it has a much wider field of view compared to a plane mirror. This allows the driver to see a larger area of the traffic behind them, which is crucial for safety.
Question 29

Explain the use of convex mirrors in street lamps.

Answer:
Convex mirrors are used as reflectors in street lamps to spread light over a larger area. The light source (bulb) is placed in front of the convex mirror. Since a convex mirror diverges light rays incident upon it, the light reflected from the mirror spreads out (diverges) rather than concentrating in one spot. This enables the street lamp to illuminate a wider section of the road or street.
Question 30

What are vigilance mirrors, and which type of mirror is used for this purpose?

Answer:
Vigilance mirrors, also known as anti-theft mirrors, are used in large shops, supermarkets, and departmental stores to monitor customers and prevent theft. Large convex mirrors are used for this purpose. Their property of providing a wide field of view and forming erect, though diminished, images allows store security or staff to view a large portion of the store from a single vantage point.
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Quick Review Flashcards - Click to flip and test your knowledge!
Question
What is the definition of refraction of light?
Answer
Refraction is the change in the direction of the path of light when it passes from one optically transparent medium to another.
Question
A medium is said to be optically _____ if the speed of light in it decreases.
Answer
denser
Question
A medium is said to be optically _____ if the speed of light in it increases.
Answer
rarer
Question
What is the speed of light in air or a vacuum?
Answer
The speed of light in air is approximately $3 \times 10^8 \text{ m s}^{-1}$.
Question
Compare the speed of light in water to the speed of light in air.
Answer
Light travels faster in air than in water; the speed of light in water is $2.25 \times 10^8 \text{ m s}^{-1}$.
Question
How does a ray of light bend when it travels from a rarer medium to a denser medium?
Answer
When a ray of light travels from a rarer to a denser medium, it bends towards the normal.
Question
How does a ray of light bend when it travels from a denser medium to a rarer medium?
Answer
When a ray of light travels from a denser to a rarer medium, it bends away from the normal.
Question
What happens when a ray of light falls normally on the surface separating two media?
Answer
It passes undeviated (i.e., along the same path).
Question
Term: Incident Ray
Answer
The ray of light falling on the surface separating the two media.
Question
Term: Refracted Ray
Answer
The ray of light travelling in the other medium in the changed direction.
Question
Term: Normal (in refraction)
Answer
The perpendicular drawn on the surface separating the two media at the point where the incident ray strikes it.
Question
Term: Angle of Incidence (i)
Answer
The angle between the incident ray and the normal.
Question
Term: Angle of Refraction (r)
Answer
The angle between the refracted ray and the normal.
Question
What is the first law of refraction?
Answer
The incident ray, the normal at the point of incidence, and the refracted ray, all lie in the same plane.
Question
What is the second law of refraction, also known as Snell's Law?
Answer
For a given pair of media and for a given colour of light, the ratio of the sine of the angle of incidence ($i$) to the sine of the angle of refraction ($r$) is a constant.
Question
The constant ratio $\frac{\sin i}{\sin r}$ is known as the _____ of the second medium with respect to the first medium.
Answer
refractive index ($\mu$)
Question
How is the refractive index ($\mu$) defined in terms of the speed of light?
Answer
Refractive index is the ratio of the speed of light in the first medium to the speed of light in the second medium.
Question
What is the approximate refractive index of water with respect to air?
Answer
The refractive index of water is approximately $4/3$ or $1.33$.
Question
What is the approximate refractive index of glass with respect to air?
Answer
The refractive index of glass is approximately $1.5$.
Question
Due to refraction, the depth of a vessel filled with water when seen from above appears to be _____ than its real depth.
Answer
less
Question
Why does a straight stick partly dipped in water obliquely appear to be bent at the surface of the water?
Answer
This is due to the refraction of light from water to air as it passes from a denser to a rarer medium.
Question
Why do we see the sun a few minutes before the actual sunrise?
Answer
This phenomenon, known as advanced sunrise, is caused by the continuous refraction of sunlight through the Earth's atmosphere.
Question
How does atmospheric refraction affect the timing of sunset?
Answer
Due to atmospheric refraction, the sun is seen for about 3 to 4 minutes longer above the horizon after the actual sunset, an effect known as delayed sunset.
Question
What is a mirage in a desert?
Answer
A mirage is an inverted image of a tree or other object that gives a false impression of a pool of water under it.
Question
What is the primary cause of a mirage?
Answer
The cause of a mirage is the refraction of light through layers of air at different temperatures and densities near hot ground.
Question
Describe the relationship between the incident ray and the emergent ray when light passes through a rectangular glass block.
Answer
The emergent ray is parallel to the incident ray but is laterally displaced.
Question
Term: Prism
Answer
A transparent medium bounded by five plane surfaces with a triangular cross-section.
Question
What is the phenomenon called when white light splits into its constituent colours after passing through a prism?
Answer
Dispersion of white light.
Question
The coloured band obtained on a screen when white light is passed through a prism is called a _____.
Answer
spectrum
Question
List the colours of the spectrum in order, starting from the base of the prism.
Answer
Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).
Question
What is the fundamental cause of the dispersion of white light in a prism?
Answer
Dispersion is caused because the speed of light for different colours is different in a transparent medium like glass, leading to different refractive indices for each colour.
Question
Which colour of light has the maximum refractive index in glass?
Answer
Violet light.
Question
Which colour of light has the minimum refractive index in glass?
Answer
Red light.
Question
What does rotating a disc painted with the seven colours of the spectrum (a Newton's disc) demonstrate?
Answer
It demonstrates that the seven constituent colours of white light combine to produce the white effect.
Question
Term: Spherical Mirror
Answer
A mirror made by silvering the part AB of a hollow glass sphere.
Question
A _____ mirror is made by silvering on the outer surface so that reflection takes place from the inner hollow (concave) surface.
Answer
concave
Question
A _____ mirror is made by silvering on the inner surface so that reflection takes place from the outer convex (bulged) surface.
Answer
convex
Question
Term: Pole (P)
Answer
The geometric centre of the spherical surface of the mirror.
Question
Term: Centre of Curvature (C)
Answer
The centre of the sphere of which the mirror is a part.
Question
Term: Radius of Curvature (R)
Answer
The radius of the sphere of which the mirror is a part, or the distance from the pole to the centre of curvature.
Question
Term: Principal Axis
Answer
The straight line joining the pole of the mirror to its centre of curvature.
Question
Term: Focus (F) of a Concave Mirror
Answer
A point on the principal axis at which light rays incident parallel to the principal axis meet (converge) after reflection.
Question
Is the focus of a concave mirror real or virtual?
Answer
The focus of a concave mirror is real.
Question
Term: Focus (F) of a Convex Mirror
Answer
A point on the principal axis from which light rays incident parallel to the principal axis appear to diverge after reflection.
Question
Is the focus of a convex mirror real or virtual?
Answer
The focus of a convex mirror is virtual.
Question
Term: Focal Length (f)
Answer
The distance from the pole of the mirror to the focus.
Question
What is the mathematical relationship between focal length ($f$) and radius of curvature ($R$)?
Answer
Focal length is half the radius of curvature ($f = \frac{1}{2} R$).
Question
Describe the path of a ray of light passing through the centre of curvature of a spherical mirror after reflection.
Answer
A ray passing through the centre of curvature is reflected back along its own path.
Question
Describe the path of a ray of light incident parallel to the principal axis of a concave mirror after reflection.
Answer
It passes through the focus after reflection.
Question
Describe the path of a ray of light incident parallel to the principal axis of a convex mirror after reflection.
Answer
It appears to come from the focus after reflection.
Question
Describe the path of a ray passing through the focus of a concave mirror after reflection.
Answer
It gets reflected parallel to the principal axis.
Question
What defines a real image?
Answer
A real image is formed when the reflected rays actually meet at a point; it can be obtained on a screen and is inverted.
Question
What defines a virtual image?
Answer
A virtual image is formed when the reflected rays meet on producing them backwards; it is erect and cannot be obtained on a screen.
Question
Concave Mirror Image: What are the characteristics of the image formed when the object is at infinity?
Answer
The image is formed at the focus (F), and it is real, inverted, and highly diminished.
Question
Concave Mirror Image: What are the characteristics of the image formed when the object is beyond the centre of curvature (C)?
Answer
The image is formed between the centre of curvature (C) and the focus (F), and it is real, inverted, and smaller than the object.
Question
Concave Mirror Image: What are the characteristics of the image formed when the object is at the centre of curvature (C)?
Answer
The image is formed at the centre of curvature (C), and it is real, inverted, and of the same size as the object.
Question
Concave Mirror Image: What are the characteristics of the image formed when the object is between the centre of curvature (C) and the focus (F)?
Answer
The image is formed beyond the centre of curvature (C), and it is real, inverted, and larger than the object.
Question
Concave Mirror Image: What are the characteristics of the image formed when the object is at the focus (F)?
Answer
The image is formed at infinity, and it is real, inverted, and highly magnified.
Question
Concave Mirror Image: What are the characteristics of the image formed when the object is between the focus (F) and the pole (P)?
Answer
The image is formed behind the mirror, and it is virtual, erect, and enlarged.
Question
What are the general characteristics of an image formed by a convex mirror?
Answer
The image is always virtual, erect, diminished, and situated between the pole and the focus.