THE HUMAN EYE AND THE COLOURFUL WORLD
1. What is meant by power of accommodation of the eye?
Answer: The ability of the eye to focus the distant objects as well as the nearby objects on the retina by changing the focal length of the eye lens is called power of accommodation.
2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
Answer: The person is able to see nearby objects clearly, but he is unable to see objects beyond 1.2 m. This happens because the image of an object beyond 1.2 m is formed in 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.
3. What is the far point and near point of the human eye with normal vision?
Answer: The near point of the eye is the minimum distance of the object from the eye, which can be seen distinctly without strain. For a normal human eye, this distance is 25 cm.
The far point of the eye is the maximum distance to which the eye can see the objects clearly. The far point of the normal human eye is infinity.
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: The student is suffering from myopia or short-sightedness. The defect can be corrected by the concave lens of an appropriate power.
5. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer: A normal eye is not able to see objects closer than 25 cm because the eye is not able to decrease the focal length beyond a limit since ciliary muscle of eyes are unable to contract beyond a certain limit, then the object appears blurred and produces strain in the eyes.
6. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer: 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.
7. Why do stars twinkle?
Answer: The twinkling of stars is due to atmospheric refraction of star-light. The atmosphere is made of several layers and the refractive indices which keep on changing continuously due to this path of light rays from the star keep on changing their path continuously. As a consequence the number of rays entering, the pupil of the eye goes on changing with time and the stars appear twinkling.
8. 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.
9. 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 reaches the eyes of the astronauts and the sky appears black to them.
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