Science Class- 10
Class 10 - Chapter 10: The Human Eye and the Colourful World
Q1. The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to
Answer: (b) Accommodation.
The ability of the eye to change the focal length of its lens so that nearby and distant objects can be seen clearly is called accommodation. This adjustment is brought about by the ciliary muscles attached to the eye lens.
Q2. The human eye forms the image of an object at its
Answer: (d) Retina.
The retina is a light-sensitive screen present at the back of the eye. A real and inverted image of the object is formed on the retina, from where signals are sent to the brain through the optic nerve.
Q3. The least distance of distinct vision for a young adult with normal vision is about
Answer: (c) 25 cm.
The least distance of distinct vision is the minimum distance at which an object can be seen clearly without strain. For a normal eye, this distance is approximately 25 cm.
Q4. The change in focal length of an eye lens is caused by the action of the
Answer: (c) Ciliary muscles.
Ciliary muscles hold the eye lens in position and alter its curvature. By changing the curvature of the lens, they change its focal length and help the eye focus on objects at different distances.
Q5. 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?
(i) For distant vision:
Power, P = -5.5 D
Focal length, f = 1/P
f = 1/(-5.5) = -0.182 m
f = -18.2 cm
(ii) For near vision:
Power, P = +1.5 D
f = 1/1.5 = 0.667 m
f = 66.7 cm
Answer:
- Focal length for distant vision = -18.2 cm
- Focal length for near vision = +66.7 cm
Q6. 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?
A myopic person cannot see distant objects clearly. To correct this defect, a concave lens is used.
Given:
- Far point = 80 cm = 0.8 m
- Object at infinity
- Image should be formed at the far point
Therefore, focal length of the corrective lens = -0.8 m
Power, P = 1/f
P = 1/(-0.8)
P = -1.25 D
Answer:
A concave lens of power -1.25 D is required.
Q7. 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.
Hypermetropia is corrected by using a convex lens.
Calculation:
Near point of defective eye = 1 m = 100 cm
Desired object distance = 25 cm
u = -25 cm
v = -100 cm
Using lens formula:
1/f = 1/v - 1/u
1/f = -1/100 + 1/25
1/f = 3/100
f = 100/3 cm = 33.3 cm
f = 0.333 m
Power = 1/f
P = 1/0.333
P = +3.0 D
Answer:
A convex lens of power +3.0 D is required.
Q8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
When an object is brought very close to the eye, the ciliary muscles have to contract more to increase the curvature of the eye lens. Beyond a certain limit, the muscles cannot contract further and the focal length cannot decrease enough to focus the image on the retina.
Therefore, objects placed closer than 25 cm cannot be seen clearly by a normal eye.
Q9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
The distance between the eye lens and the retina remains practically fixed. Therefore, even when the object distance changes, the image continues to be formed on the retina.
The eye adjusts the focal length of its lens through accommodation, while the image distance remains almost constant.
Q10. Why do stars twinkle?
Stars appear to twinkle because of atmospheric refraction of light. As starlight passes through different layers of the Earth's atmosphere, which have varying densities, the path of light continuously changes.
This causes the apparent position and brightness of stars to fluctuate, making them appear to twinkle.
Q11. Explain why the planets do not twinkle.
Planets are much closer to the Earth than stars and appear as extended sources of light rather than point sources.
The variations in light from different parts of a planet cancel one another. Therefore, the overall brightness remains nearly constant and planets do not appear to twinkle.
Q12. Why does the sky appear dark instead of blue to an astronaut?
On Earth, the sky appears blue due to the scattering of sunlight by particles present in the atmosphere.
In space, there is no atmosphere to scatter sunlight. As a result, no scattered light reaches the eyes of the astronaut and the sky appears dark or black.