Answer The Following Question.
1. What is meant by power of accommodation of eye?
Ans: The power of accommodation refers to the ability of the eye to adjust its focal length to focus on objects at different distances.
When the ciliary muscles are relaxed, the eye lens becomes thin, increasing its focal length, allowing the eye to focus on distant objects.
When the ciliary muscles contract, the lens becomes thicker, decreasing its focal length to focus on nearby objects.
This ability to change the focal length enables the eye to clearly see both distant and near objects.
2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the corrective lens used to restore proper vision?
Ans: A person with myopia (nearsightedness) can see nearby objects clearly but struggles to see distant objects. In this case, the person’s far point (the farthest distance at which the person can see clearly) is 1.2 m.
To correct myopia, a concave lens is used because:
A concave lens diverges light rays, making the image of distant objects appear at the far point of the eye (1.2 m in this case).
The focal length of the concave lens should be -1.2 m (negative because concave lenses diverge light).
Thus, a concave lens with a focal length of 1.2 m will help restore proper vision by bringing the far point to infinity, allowing the person to see distant objects clearly.
3. What is the far point and near point of the human eye with normal vision?
Ans: Far Point and Near Point of the Human Eye with Normal Vision:
The far point is at infinity, meaning a person with normal vision can see objects at any distance clearly, as long as they are far enough.
The near point is at 25 cm from the eye, which is the closest distance at which the human eye can focus on an object clearly.
4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
Ans: Defect and Correction:
The student is suffering from myopia (nearsightedness), which makes it difficult to see distant objects clearly, such as the blackboard from the last row.
This defect can be corrected using a concave lens with an appropriate focal length, which will diverge light rays and bring the image of distant objects into focus on the retina.
5. 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
Ans : (b) The human eye can adjust the focal length of its lens to focus on objects at different distances. This ability of the eye lens to change its focal length is called the power of accommodation. It allows us to see both nearby and distant objects
6. The human eye forms the image of an object at its
(a) Cornea
(b) Iris
(c) Pupil
(d) Retina
Ans : (d) retina
7. The least distance of distinct vision for an eye lens is caused by the action of the
(a) 25 m
(b) 2.5 cm
(c) 25 cm
(d) 2.5 m
Ans : (c) 25 cm
8. The change in focal length of an eye lens is caused by the action of the
(a) Pupil
(b) Retina
(c) Cilliary muscles
(d) Iris
Ans : (c) Cilliary muscles
9. A person needs a lens of power -5.5 dioptre for correcting his distinct vision. For correcting his near vision he needs a lens +1.5 dioptre. What is the focal length of the lens required for correcting (i) distinct vision, and (ii) near vision?
Ans : (i) Power of lens needed for correction distant vision of the person (P) = -5.5 D
Focal length of lens required for correcting distant vision (f)
= 1/P = 1/-5.5 m = 0.18 m = 18 cm.
(ii) For correcting near vision the power of lens required (P) = +1.5 D
Focal length of lens required for correcting near vision (f)
= 1/P = 1/1.5 m = 0.67 m = 66.7 cm.
10. 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?
Ans : To correct the myopia the person concerned should use concave lens of focal length (f) = -80 cm = -0.80 m
Power of lens (P) = 1/f(m) = 1/-0.80 = 100/-80 = -1.25 D.
11. 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 equired to correct this defect? Assume that near point of the normal eye is 25 cm.
Ans : Diagram representing the correction of hypermetropia is a follows:
Near point of defective eye is 1 m and that of normal eye is 25 cm.
Here, u = -25 cm, v = -1m = 100 cm.
Using lens formula
1/f = 1/v – 1/u
1/f = 1/-100 + 1/25 = 3/100
f = 100/3 cm = 1/3m.
P = 1/f(m) = 1/0.33 = +3.0 D.
12. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Ans: A normal eye cannot see objects closer than 25 cm because the ciliary muscles cannot contract enough to further decrease the focal length of the eye lens. This limit is due to the power of accommodation of the eye, and 25 cm is the near point for a normal human eye.
13. What happen to the image distance in the eye when we increase the distance of an object from the eye?
Ans: The image distance in the eye remains constant because the image is always formed on the retina. As the object moves farther away, the eye lens becomes thinner and its focal length increases to maintain focus on the retina.
14. Why do stars twinkle?
Ans: Stars twinkle due to atmospheric refraction. As starlight passes through the Earth’s atmosphere, it bends unevenly due to varying air densities. Since stars are far away and appear as point sources, the light’s path fluctuates, causing the star’s apparent position and brightness to change slightly. This makes stars appear to twinkle.
15. Explain why the planets do not twinkle.
Ans: Planets are much closer to Earth and appear as extended sources of light, not point sources like stars. The light from different parts of a planet averages out the flickering caused by atmospheric refraction. As a result, the total light entering our eyes remains nearly constant, and planets do not twinkle.
16. Why does the Sun appear reddish early in the morning?
Ans: During sunrise, sunlight travels a longer distance through the Earth’s atmosphere. Shorter wavelengths like blue are scattered out, while longer wavelengths like red reach our eyes. As a result, the Sun appears reddish early in the morning.
17. Why does the sky appear dark instead of blue to astronaut?
Ans: The blue color of the sky is due to the scattering of light by Earth’s atmosphere. In space, there is no atmosphere to scatter sunlight, so light travels straight without scattering. As a result, the sky appears dark to astronauts.