The ability of the eye to focus on both near and distant objects, by adjusting its focal length, is called the accommodation of the eye.
The smallest distance, at which the eye can see objects clearly without strain, is called the point of the eye or the least distance of distinct vision. For a young adult with noumal vision, it is about 25 cm.
The common refractiove defects of vision include myopia, hypermetropia and presbyopia. Myopia (short - sightedness - the image of distant objects is focussed before the retina) is corrected by using a concave lens of suitable power. Hypermetropia (far - sightedness - the image of nearby objects is focussed beyond the retina) is corrected by using a convex lens of suitable poser. The eye loses its power of accommodation at old age.
The splitting of white light into its component colours is called dispersion.
Scattering of light causes the blue colour of sky and the reddening of the Sun at sunrise and sunset.
In this chapter, we shall use these ideas to study some of the optical phenomena in nature. We shall also discuss about rainbow for mation, splitting of white light and blue colour of the sky.
Questions and Answers of NCERT Textbook.
Learning points
The Human eye.
Power of Accommodation.
Defects of Vision and their correction.
Refraction of light through a prism.
Dispersion of white light by a glass prism.
Atmospheric Refraction.
Scattering of Light
Q.No.1
What is mehant by power of accomodation of the eye ?
Answer
To see the near by object clearly, the ciliary muscles contact making the eye lens thicker. Thus, the focal length of the eye lense decreases and nearby objects become visible to the eye. Similarly, to see the far objects clearly, the ciliary muscles contact making the eye lens thinner. Thus, the focal length of the eye lens increases and far objects become visible to the eyes.
Hence, the human eye lens is able to adjust its focal length to view both nearby and far objects on the retina. This ability is called the power of accommodation of the eyes.
Q.No.2.
A person with a myopic eye cannon see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore power vision?
Answer
The person is able to see nearby objects clearly, but he or she unable to see objects beyond 1.2 m. This happens because the image of an object beyond 1.2 m is formed in the front of the retina and not at the retina.
To correct this defect of vision, he must use a concave lens. The concave lens will bring the image back to the retina.
Q.No.3.
What is the far point and near point of the human eye with normal vision?
Answer
For normal eyes, the least distance of distinct vision is 25 cm whereas far distance of distinct vision is infinite.
Q.No. 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?
Answer
Last row it means that distant objects. Student is unable to see distant objects clearly. He is suffering from myopia or far sightedness. This defect can be corrected by using concave lens.
1. The human eye can focus on 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
This is possible due to the power of accommodation of the eye lens.
The correct option will be (b)
2. The human eye forms the image of an object at its
(a) cornea. (b) iris. (c) pupil. (d) retina.
Answer
The human eye forms the image of an object at its retina.
The correct option will be (d)
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
The least distance of distinct vision for a young adult with normal vision is about 25 cm.
The correct option will be (c)
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
The change in focal length of an eye lens is caused by the action of the ciliary muscles.
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
From question it is clear that
P = -5.5 D for distant vision.
We know that
P = 1/f
f = 1/P
Now putting the value of P.
f = 1/-5.5 = -0.181 m = -18.1 cm.
P = +1.5 for near vision.
Similarly,
f = 1/P
f = 1/1.5 = 0.667 m = 66.7 cm.
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
From question it is clear that
Object distance = u = infinity
Image distance = v = -80 cm
Focal length = f = ?
According to lens formula, We know that
1/v - 1/u = 1/f
Now, putting the value of u, v and f.
-1/80 - 1/infinity = 1/f
or, -1/80 = 1/f
f = -80 cm = -0.8 m
Power of lens = P = 1/f
or, P = -1/0.8
P = -1.25 D.
Therefore, A concave lens of power -1.25 is required by the person to correct this defect.
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
The convex lens actually creates a virtual image of a nearby objects (N' in the figure) at the near point of vision (N) of the person suffering from hypermetropia.
The given person will be able to clearly see the object kept at 25 cm (least distance of distinct vision), if the image of the the object is formed at his near point, which is given as 1 m.
Object distance = u = -25 cm
Image distance = v = -1 m = -100 cm.
Focal length = f ?
According to lens formula
We know that
1/v - 1/u = 1/f
-1/100 - 1/-25 = 1/f
or, -1/100 + 1/25 = 1/f
Therefore, f = 100/3 cm = 1/3 m.
We know that
P = 1/f
P = 3 D
A convex lens of power +3 D is required to correct the defect.
8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer
The least distance of distinct vision of a normal eye is 25 cm. Therefore, a normal not able to see clearly the objects placed closer than 25 cm.
9. What happens to the image distance in the eye when we increase the distance of an object from the eye ?
Answer
Since the size of eyes cannot increase or decrease, the image distance remains constant. When we increase the distance of an object from the eye, the image distance in the eye does not change. The increase in the object distance is compensated by the change in the focal length of the eye lens. The focal length of the eyes changes in such a way that the image is always formed at the retina of the eye.
10. Why do stars twinkle ?
Answer
Stars emit their own light and they twinkle due to the atmospheric refraction of light. Stars are very far away from the earth. Hence, they are considered as point sources of light. When the light coming from stars enters the earth’s atmosphere, it gets refracted at different levels because of the variation in the air density at different levels of theatmosphere. When the star light refracted by the atmosphere comes more towards us, it appears brighter than when it comes less towards us. Therefore, it appears as if the stars are twinkling at night.
11. Explain why the planets do not twinkle?
Answer
Planets do not twinkle because they appear larger in size than the stars as they are relatively closer to earth. Planets can be considered as a collection of a large number of point-size sources of light. The different parts of these planets produce either brighter or dimmer effect in such a way that the average of brighter and dimmer effect is zero. Hence, the twinkling effects of the planets are nullified and they do not twinkle.
12.Why does the Sun appear reddish early in the morning?
Answer
During sunrise, the light rays coming from the Sun have to travel a greater distance in the earth’s atmosphere before reaching our eyes. In this journey, the shorter wavelengths of lights are scattered out and only longer wavelengths are able to reach our eyes. Since blue colour has a shorter wavelength and red colour has a longer wavelength, the red colour is able to reach our eyes after the atmospheric scattering of light. Therefore, the Sun appears reddish early in the morning.
13.Why does the sky appear dark instead of blue to an astronaut?
Answer
The sky appears dark instead of blue to an astronaut because there is no atmosphere in the outer space that can scatter the sunlight. As the sunlight is not scattered, no scattered light reach the eyes of the astronauts and the sky appears black to them.
