The Human Eye and the Colourful World - Q&A
1. The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to
(a) presbyopia.
(b) accommodation.
(c) near-sightedness.
(d) far-sightedness.
Answer: (b) accommodation.
Explanation: Accommodation is the ability of the eye lens to change its curvature and focal length with the help of ciliary muscles to focus on objects at various distances. Presbyopia, near-sightedness, and far-sightedness are defects of vision.
2. The human eye forms the image of an object at its
(a) cornea.
(b) iris.
(c) pupil.
(d) retina.
Answer: (d) retina.
Explanation: The retina acts as a screen at the back of the eye where the light rays are focused by the eye lens to form a real and inverted image of the object.
3. The least distance of distinct vision for a young adult with normal vision is about
(a) 25 m.
(b) 2.5 cm.
(c) 25 cm.
(d) 2.5 m.
Answer: (c) 25 cm.
Explanation: This is the minimum distance at which an object can be seen most distinctly without strain. If objects are brought closer than 25 cm, the eye muscles (ciliary muscles) have to strain excessively.
4. The change in focal length of an eye lens is caused by the action of the
(a) pupil.
(b) retina.
(c) ciliary muscles.
(d) iris.
Answer: (c) ciliary muscles.
Explanation: Ciliary muscles relax to make the lens thin (increase focal length) for distant objects and contract to make the lens thick (decrease focal length) for nearby objects.
5. A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
Answer:
Formula: Focal length (f) = 1 / Power (P)
(i) For Distant Vision:
Power (P) = -5.5 D
f = 1 / -5.5
f = -0.1818... metres
f = -18.2 cm (approx)
(ii) For Near Vision:
Power (P) = +1.5 D
f = 1 / +1.5
f = 10 / 15 = 2 / 3 metres
f = 0.666... metres
f = +66.7 cm (approx)
6. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Answer:
Explanation: Myopia (near-sightedness) means the person can see near objects clearly but cannot see distant objects. Their far point is closer than infinity.
To correct this, we need a concave lens that takes an object at infinity and forms a virtual image at the person's far point (80 cm).
Given:
Object distance (u) = Infinity (∞)
Image distance (v) = -80 cm (virtual image in front of lens)
Step 1: Find Focal Length (f)
Using Lens Formula: 1/f = 1/v - 1/u
1/f = 1/(-80) - 1/∞
1/f = -1/80 - 0
f = -80 cm = -0.8 metres
Step 2: Find Power (P)
P = 1 / f (in metres)
P = 1 / -0.8
P = -1.25 D
Nature: Concave Lens (since power is negative).
7. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.
Answer:
Explanation: Hypermetropia (far-sightedness) means the person can see distant objects but cannot see near objects clearly. Their near point has moved away (1 m instead of 0.25 m).
To correct this, we use a convex lens. The object is placed at the normal near point (25 cm), and the lens forms a virtual image at the person's defective near point (1 m).
Given:
Object distance (u) = -25 cm (normal near point)
Image distance (v) = -100 cm (person's near point, 1 m)
Step 1: Find Focal Length (f)
1/f = 1/v - 1/u
1/f = 1/(-100) - 1/(-25)
1/f = -1/100 + 1/25
1/f = (-1 + 4) / 100
1/f = 3 / 100
f = 100 / 3 cm = 33.3 cm = 1/3 metre
Step 2: Find Power (P)
P = 1 / f (in metres)
P = 1 / (1/3)
P = +3.0 D
Nature: Convex Lens.
8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer: To focus on closer objects, the ciliary muscles must contract to make the eye lens thicker (more curved). However, there is a limit to how much the ciliary muscles can contract. They cannot squeeze the lens enough to focus on objects closer than 25 cm. Attempting to do so causes strain and blurred vision.
9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer: The image distance in the eye remains constant. The image is always formed on the retina. Instead of changing the image distance (like a camera moving its lens), the eye changes the focal length of its lens (accommodation) to keep the image focused exactly on the retina, regardless of whether the object is near or far.
10. Why do stars twinkle?
Answer: Stars twinkle due to atmospheric refraction.
1. Starlight travels through different layers of the Earth's atmosphere, which have varying densities and temperatures.
2. These layers are constantly moving and changing.
3. As a result, the path of the light rays coming from the star keeps changing slightly.
4. This causes the apparent position of the star to fluctuate and the amount of light entering our eyes to flicker, making it look like the star is twinkling.
11. Explain why the planets do not twinkle.
Answer:
1. Planets are much closer to Earth than stars, so they appear as extended sources of light (tiny discs) rather than point sources.
2. We can consider a planet as a collection of many point-sized light sources.
3. While the light from some points may twinkle (dim), the light from others may brighten at the same time.
4. These changes cancel each other out, so the total amount of light entering our eyes remains fairly steady, and no twinkling is observed.
12. Why does the Sun appear reddish early in the morning?
Answer:
1. At sunrise (and sunset), the Sun is near the horizon.
2. The sunlight has to travel a much longer distance through the Earth's thick atmosphere to reach our eyes compared to when it is overhead at noon.
3. During this long journey, most of the shorter wavelengths (blue and violet light) are scattered away by the air particles (Rayleigh scattering).
4. Only the longer wavelengths (red and orange) reach our eyes efficiently, making the Sun appear reddish.
13. Why does the sky appear dark instead of blue to an astronaut?
Answer: The blue colour of the sky is due to the scattering of sunlight by the particles in the Earth's atmosphere. In space, there is no atmosphere (no air particles). Without particles to scatter the light, no scattered light reaches the astronaut's eyes. Therefore, the sky appears dark or black.