Question 8: Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer: The minimum distance for clear vision in the human eye is 25 cm. Due to this, a person with a normal eye is unable to see clearly the objects placed closer than 25 cm.
Question 9: What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer: In human eyes, the image is always formed at the retina; no matter where the object is. So, if we increase the distance of an object from the eye; the image will be formed at the retina.
Question 10: Why do stars twinkle?
Answer: The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position when viewed near the horizon. Further, this apparent position of the star is not stationary, but keeps on changing slightly, since the physical conditions of the earth’s atmosphere are not stationary. Since the stars are very distant, they approximate point-sized sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers – the star sometimes appears brighter, and at some other time, fainter, which gives the twinkling effect.
Question 11: Explain why the planets do not twinkle.
Answer: Planets are much closer to us than stars. Because of their proximity to us, they don’t act as a point sized source of light. Instead, they act as a collection of various point sized sources of light and consequently, as a bigger source of light. Because of apparent bigger size of the source of light, the fluctuation in its apparent position is negated. Due to this, planets do not twinkle.
Question 12: Why does the Sun appear reddish early in the morning?
Answer: Red colour scatters the least and hence travels the farthest. During early morning, the sunlight has to travel a longer distance to reach us. During this, the blue colour is scattered almost completely and the sun and the sky appear reddish.
Question 13: Why does the sky appear dark instead of blue to an astronaut?
Answer: There is vacuum in the space and hence no particle is available for scattering of light. In the absence of scattering, none of the colours from the visible spectrum reach the viewer’s eye and the sky appear dark to the astronaut.
Question 14: What is meant by power of accommodation of the eye?
Answer: The human eye can clearly see a nearby object as well as an object on infinity. This ability of the human eye is called the power of accommodation of human eye.
Question 15: 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: In this case, the person is unable to see a distant object. Hence, this person is suffering from myopia. A concave lens will help in restoring his proper vision.
Question 16: What is the far point and near point of the human eye with normal vision?
Answer: For the human eye with normal vision, the far point is at infinity and the near point is at 25 cm from the eye.
Question 17: 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: Since the child is unable to see a distant object, he may be suffering from myopia. This defect can be corrected by a concave lens of proper focal length.