NCERT Solutions for Class 10th Science
Chapter 11 – THE HUMAN EYE AND THE COLOURFUL WORLD
(Complete Downloadable Chapter Solution PDF file is at the bottom of the page)
Q.1 What is meant by power of accommodation of the eye?
Ans The ability of the eye lens to adjust its focal length so as to see distant as well as nearby objects clearly is called power of accommodation of the eye.
Q.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?
Ans Concave lens
In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. This defect can be corrected by using a concave lens of suitable power as shown below.
Q.3 What is the far point and near point of the human eye with normal vision?
Ans Near point: The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision or the near point of the eye. For human eye with normal vision, the near point is about 25 cm.
Far point: The farthest point upto which the eye can see objects clearly is called the far point of the eye. For human eye with normal vision, the far point is infinity.
Q.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?
EXERCISES
Q.1 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) accommodation
Q.2 The human eye forms the image of an object at its
(a) cornea. (b) iris. (c) pupil. (d) retina.
Ans (d) retina
Q.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.
Ans (c) 25 cm
Q.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.
Ans (c) ciliary muscles
Q.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?
Q.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?
Q.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.
Q.8 Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Q.9 What happens to the image distance in the eye when we increase the distance of an object from the eye?
Q.10 Why do stars twinkle?
Ans The twinkling of a star is due to atmospheric refraction of starlight. 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 due to atmospheric refraction (because of air density variation at different level of the atmosphere), 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 is the twinkling effect.
Q.11 Explain why the planets do not twinkle.
Q.12 Why does the Sun appear reddish early in the morning?
Q.13 Why does the sky appear dark instead of blue to an astronaut?
Ans It is because of no scattering of light in space due to the absence of atmosphere.