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Light - Reflection and Refraction - Q&A

1. Which one of the following materials cannot be used to make a lens?
(a) Water
(b) Glass
(c) Plastic
(d) Clay

Answer: (d) Clay
Explanation: A lens must be made of a transparent material so that light can pass through it (refraction). Water, glass, and plastic are transparent, but clay is opaque. Therefore, light cannot pass through clay, and it cannot be used to make a lens.

2. The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?
(a) Between the principal focus and the centre of curvature
(b) At the centre of curvature
(c) Beyond the centre of curvature
(d) Between the pole of the mirror and its principal focus.

Answer: (d) Between the pole of the mirror and its principal focus.
Explanation: A concave mirror forms a real and inverted image for all positions of the object except when the object is placed very close to it (between the Pole 'P' and Focus 'F'). Only in this case, the image formed is virtual, erect, and magnified.

[Image of ray diagram of concave mirror object between p and f]

3. Where should an object be placed in front of a convex lens to get a real image of the size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus.

Answer: (b) At twice the focal length
Explanation: For a convex lens, if the object is placed at 2F (twice the focal length), the image is also formed at 2F on the other side. This image is real, inverted, and of the same size as the object.

4. A spherical mirror and a thin spherical lens have each a focal length of –15 cm. The mirror and the lens are likely to be
(a) both concave.
(b) both convex.
(c) the mirror is concave and the lens is convex.
(d) the mirror is convex and the lens is concave.

Answer: (a) both concave.
Explanation: According to the sign convention:
- Focal length of a concave mirror is negative (-).
- Focal length of a concave lens is negative (-).
- Focal length of a convex mirror/lens is positive (+).
Since the given focal length is -15 cm, both must be concave.

5. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be
(a) only plane.
(b) only concave.
(c) only convex.
(d) either plane or convex.

Answer: (d) either plane or convex.
Explanation:
- A plane mirror always forms an erect image (same size).
- A convex mirror also always forms an erect image (smaller size), no matter where the object is.
- A concave mirror forms an inverted image for most positions (except when very close).

6. Which of the following lenses would you prefer to use while reading small letters found in a dictionary?
(a) A convex lens of focal length 50 cm.
(b) A concave lens of focal length 50 cm.
(c) A convex lens of focal length 5 cm.
(d) A concave lens of focal length 5 cm.

Answer: (c) A convex lens of focal length 5 cm.
Explanation:
1. To read small letters, we need a magnifying glass, which is a convex lens.
2. The magnifying power of a lens is inversely proportional to its focal length (Power P = 1/f). This means a lens with a smaller focal length has higher power (more magnification).
Therefore, a convex lens with a smaller focal length (5 cm) is better than one with 50 cm.

7. We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case.

Answer:
Range of object distance: The object must be placed between the Pole (P) and the Focus (F). Since the focal length is 15 cm, the object distance should be less than 15 cm (0 cm to <15 cm).
Nature of the image: Virtual and Erect.
Size of the image: Larger than the object (Magnified).

8. Name the type of mirror used in the following situations.
(a) Headlights of a car.
(b) Side/rear-view mirror of a vehicle.
(c) Solar furnace.
Support your answer with reason.

Answer:
(a) Headlights of a car: Concave Mirror.
Reason: The light bulb is placed at the focus of the concave mirror. This allows the light rays to reflect and emerge as a powerful, parallel beam of light that travels a long distance.

(b) Side/rear-view mirror: Convex Mirror.
Reason: Convex mirrors always form an erect (upright) image. They also have a wider field of view because they are curved outwards, allowing the driver to see a much larger area behind the car.

(c) Solar furnace: Concave Mirror.
Reason: Large concave mirrors are used to concentrate sunlight coming from infinity (parallel rays) onto a single point (the focus). This concentration produces a huge amount of heat at the focus.

9. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

Answer: Yes, the lens will produce a complete image of the object.
Explanation: Each part of a lens can form the full image. However, since half the lens is covered, fewer light rays will pass through it to form the image. As a result, the brightness (intensity) of the image will be reduced (it will look dimmer), but the full structure of the object will still be visible.

10. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.

Answer:
Given:
Object height (h) = +5 cm
Object distance (u) = -25 cm (always negative)
Focal length (f) = +10 cm (convex/converging lens is positive)

1. Finding Image Position (v):
Using Lens Formula: 1/f = 1/v - 1/u
1/10 = 1/v - 1/(-25)
1/10 = 1/v + 1/25
1/v = 1/10 - 1/25
1/v = (5 - 2) / 50
1/v = 3 / 50
v = 50 / 3 = +16.67 cm
The image is formed at 16.67 cm on the other side of the lens.

2. Finding Image Size (h'):
Magnification (m) = h'/h = v/u
h'/5 = 16.67 / -25
h' = (16.67 * 5) / -25
h' = -3.33 cm

Nature: Real (since v is positive) and Inverted (since h' is negative).
Size: Diminished (3.33 cm height compared to 5 cm object).

11. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.

Answer:
Given:
Focal length (f) = -15 cm (concave lens is always negative)
Image distance (v) = -10 cm (concave lens always forms virtual image on same side)

Finding Object Distance (u):
Lens Formula: 1/f = 1/v - 1/u
1/(-15) = 1/(-10) - 1/u
1/u = 1/(-10) - 1/(-15)
1/u = -1/10 + 1/15
1/u = (-3 + 2) / 30
1/u = -1 / 30
u = -30 cm
The object is placed 30 cm from the lens.

12. An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image.

Answer:
Given:
Object distance (u) = -10 cm
Focal length (f) = +15 cm (convex mirror is positive)

Finding Image Position (v):
Mirror Formula: 1/f = 1/v + 1/u
1/15 = 1/v + 1/(-10)
1/v = 1/15 + 1/10
1/v = (2 + 3) / 30
1/v = 5 / 30
v = 30 / 5 = +6 cm

Nature: Since 'v' is positive, the image is formed behind the mirror. It is Virtual and Erect.

13. The magnification produced by a plane mirror is +1. What does this mean?

Answer:
1. Magnitude 1: It means the size of the image is exactly the same as the size of the object.
2. Plus sign (+): It means the image is virtual and erect (upright).

14. An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.

Answer:
Given:
Height (h) = 5 cm
Object distance (u) = -20 cm
Radius (R) = +30 cm → Focal length (f) = R/2 = +15 cm

1. Image Position (v):
Mirror Formula: 1/v + 1/u = 1/f
1/v - 1/20 = 1/15
1/v = 1/15 + 1/20
1/v = (4 + 3) / 60
v = 60 / 7 = +8.57 cm (Behind the mirror)

2. Image Size (h'):
m = h'/h = -v/u
h'/5 = -(8.57) / (-20)
h' = (8.57 * 5) / 20
h' = +2.14 cm

Nature: Virtual, Erect, and Diminished.

15. An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained? Find the size and the nature of the image.

Answer:
Given:
Object size (h) = +7 cm
Object distance (u) = -27 cm
Focal length (f) = -18 cm (concave mirror)

1. Screen Position (v):
1/v + 1/u = 1/f
1/v - 1/27 = -1/18
1/v = -1/18 + 1/27
1/v = (-3 + 2) / 54
1/v = -1 / 54
v = -54 cm
The screen should be placed 54 cm in front of the mirror.

2. Image Size (h'):
m = h'/h = -v/u
h'/7 = -(-54) / (-27)
h'/7 = -2
h' = -14 cm

Nature: Real (since can be put on screen), Inverted (negative height), and Magnified (14 cm > 7 cm).

16. Find the focal length of a lens of power - 2.0 D. What type of lens is this?

Answer:
Formula: Power (P) = 1 / f (in metres)
-2.0 = 1 / f
f = 1 / -2.0
f = -0.5 metres (or -50 cm)

Since the focal length (and power) is negative, it is a Concave Lens.

17. A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?

Answer:
Power (P) = +1.5 D
f = 1 / P
f = 1 / 1.5
f = 10 / 15 = 2 / 3 metres
f = +0.67 metres (or +66.7 cm)

Since the power is positive, it is a Converging Lens (Convex Lens).

Quick Navigation:

Quick Review Flashcards - Click to flip and test your knowledge!

Question

What determines whether objects are visible in a room?

Answer

The reflection of light off the objects received by our eyes.

Question

Why is it possible to see through a transparent medium?

Answer

Light is transmitted through the medium rather than being reflected or absorbed.

Question

What phenomenon occurs when light bends around the edges of a very small opaque object?

Answer

Diffraction

Question

Which theory of light was developed to reconcile its particle properties with its wave nature?

Answer

Modern quantum theory of light

Question

State the first law of reflection regarding the angle of incidence.

Answer

The angle of incidence is equal to the angle of reflection.

Question

Which three lines lie in the same plane according to the laws of reflection?

Answer

The incident ray, the normal to the mirror at the point of incidence, and the reflected ray.

Question

To which types of reflecting surfaces are the laws of reflection applicable?

Answer

All types of reflecting surfaces, including spherical surfaces.

Question

What are the nature and orientation of an image formed by a plane mirror?

Answer

Virtual and erect

Question

How does the size of an image in a plane mirror compare to the object size?

Answer

The size of the image is equal to that of the object.

Question

What is the term for an image where the left side of the object appears as the right side of the image?

Answer

Lateral inversion

Question

Term: Concave mirror

Answer

Definition: A spherical mirror with a reflecting surface curved inwards, facing the centre of the sphere.

Question

Term: Convex mirror

Answer

Definition: A spherical mirror with a reflecting surface curved outwards.

Question

What is the 'pole' of a spherical mirror?

Answer

The centre of the reflecting surface of the mirror, usually represented by the letter $P$.

Question

Where does the centre of curvature of a concave mirror lie relative to its reflecting surface?

Answer

It lies in front of the reflecting surface.

Question

Where does the centre of curvature of a convex mirror lie relative to its reflecting surface?

Answer

It lies behind the reflecting surface.

Question

Term: Principal axis

Answer

Definition: A straight line passing through the pole and the centre of curvature of a spherical mirror.

Question

What point on the principal axis do rays parallel to the axis meet after reflection from a concave mirror?

Answer

Principal focus ($F$)

Question

Term: Focal length

Answer

Definition: The distance between the pole and the principal focus of a spherical mirror.

Question

Term: Aperture

Answer

Definition: The diameter of the circular outline of the reflecting surface of a spherical mirror.

Question

What is the relationship between the radius of curvature ($R$) and focal length ($f$) for small aperture mirrors?

Answer

$R = 2f$

Question

Where is the image formed when an object is placed at infinity in front of a concave mirror?

Answer

At the focus $F$

Question

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

Answer

Highly diminished, point-sized, real, and inverted.

Question

What is the position of the image when an object is placed at the centre of curvature ($C$) of a concave mirror?

Answer

At the centre of curvature ($C$)

Question

Describe the size of the image when an object is at $C$ in front of a concave mirror.

Answer

The image is the same size as the object.

Question

Where must an object be placed in front of a concave mirror to produce a virtual and erect image?

Answer

Between the pole ($P$) and the focus ($F$)

Question

How does a ray parallel to the principal axis behave after reflecting off a concave mirror?

Answer

It passes through the principal focus.

Question

How does a ray passing through the centre of curvature behave after reflection from a spherical mirror?

Answer

It is reflected back along the same path.

Question

Which type of mirror is commonly used by dentists to see large images of teeth?

Answer

Concave mirror

Question

Why are convex mirrors preferred as rear-view mirrors in vehicles?

Answer

They provide an erect, diminished image and a wider field of view due to being curved outwards.

Question

According to the New Cartesian Sign Convention, where is the object always placed?

Answer

To the left of the mirror.

Question

In the New Cartesian Sign Convention, what sign is assigned to distances measured to the right of the origin?

Answer

Positive

Question

Formula: Mirror Formula

Answer

$\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$

Question

How is magnification ($m$) defined in terms of image height ($h'$) and object height ($h$)?

Answer

$m = \frac{h'}{h}$

Question

What is the relationship between magnification ($m$), image distance ($v$), and object distance ($u$) for mirrors?

Answer

$m = -\frac{v}{u}$

Question

What does a negative sign in the value of magnification indicate about the image?

Answer

The image is real.

Question

What does a positive sign in the value of magnification indicate about the image?

Answer

The image is virtual.

Question

Term: Refraction of light

Answer

Definition: The change in the direction of propagation of light when it travels obliquely from one transparent medium to another.

Question

What causes the phenomenon of refraction?

Answer

The change in the speed of light as it enters from one transparent medium to another.

Question

State Snell’s law of refraction regarding the ratio of sines.

Answer

The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media.

Question

What is the approximate speed of light in a vacuum?

Answer

$3 \times 10^{8} \text{ m s}^{-1}$

Question

Formula: Absolute refractive index ($n_m$)

Answer

$n_m = \frac{c}{v}$, where $c$ is the speed of light in air and $v$ is the speed in the medium.

Question

How does light bend when travelling from an optically rarer medium to an optically denser medium?

Answer

It slows down and bends towards the normal.

Question

How does light bend when travelling from an optically denser medium to an optically rarer medium?

Answer

It speeds up and bends away from the normal.

Question

Term: Convex lens

Answer

Definition: A lens bound by two spherical surfaces bulging outwards, which converges light rays.

Question

Term: Concave lens

Answer

Definition: A lens bound by two spherical surfaces curved inwards, which diverges light rays.

Question

What is the 'optical centre' of a lens?

Answer

The central point of the lens, usually represented by the letter $O$.

Question

How does a ray of light behave when passing through the optical centre of a lens?

Answer

It passes through without suffering any deviation.

Question

Where is the image formed when an object is placed at infinity for a convex lens?

Answer

At the focus $F_2$

Question

What kind of image does a concave lens always produce?

Answer

A virtual, erect, and diminished image.

Question

Where is the image formed by a convex lens when the object is between $F_1$ and the optical centre $O$?

Answer

On the same side of the lens as the object.

Question

Formula: Lens Formula

Answer

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

Question

What is the relationship between magnification ($m$), image distance ($v$), and object distance ($u$) for lenses?

Answer

$m = \frac{v}{u}$

Question

Term: Power of a lens

Answer

Definition: The reciprocal of the focal length of a lens, representing the degree of convergence or divergence.

Question

What is the SI unit of power for a lens?

Answer

Dioptre ($D$)

Question

What is the power of a lens with a focal length of $1$ metre?

Answer

$1$ dioptre ($1 D$)

Question

Is the power of a convex lens positive or negative?

Answer

Positive

Question

Is the power of a concave lens positive or negative?

Answer

Negative

Question

How is the net power ($P$) calculated for multiple lenses placed in contact?

Answer

The algebraic sum of the individual powers ($P = P_1 + P_2 + P_3 + \dots$).

Question

What is the refractive index of water?

Answer

$1.33$

Question

What is the refractive index of diamond?

Answer

$2.42$