NCERT Solutions for Class 10th Science Chapter 11 The Human Eye and the Colourful World

Page 190

Question 1. What is meant by power of accommodation of the eye?
Answer: The power Of accommodation of the eye is the ability of the eye to observe the distinct objects clearly which are situated at a large distance from the eye. The ciliary muscles are responsible to change the focal length Of the eye lens. The value of the power of accommodation Of the normal human eye is (d = 25 cm) = 100/f = 100/d = 100/25 = 4 dioptres. The value of power of accommodation Of human eye is about 4D

Question 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 far point for myopic eye is 1.2m.

Question 3. What is the far point and near point of the human eye with normal vision ?
Answer:  For human eye with normal vision, far point is at infinity and near point is at 25 cm from the eye.

Question 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:  As the child has difficulty in reading the blackboard, he is suffering from myopia or short sightedness. To correct this defect, he has to use spectacles with concave lens of suitable focal length.

Page 197 – 198
Question 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
Answer:  (b) Human eye can change the focal length of the eye lens to see the objects situated at various distances from the eye. This is possible due to the power of accommodation of the eye lens.

Question 2. The human eye forms the image of an object at its
(a) cornea (b) iris (c) pupil (d) retina
Answer: (d) The human eye forms the image of an object at its retina.

Question 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:  (c) The least distance of distinct vision is the minimum distance of an object to see clear and distinct image. It is 25 cm for a young adult with normal visions.

Question 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:  (c) The relaxation or contraction of ciliary muscles changes the curvature of the eye lens. The change in curvature of the eye lens changes the focal length of the eyes. Hence, the change in focal length of an eye lens is caused by the action of ciliary muscles.

Question 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?

Question 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?

Question 7. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye ¡s 1 m. What ¡s the power of a lens required to correct this defect? Assume that near point of the normal eye is 25 cm.
Question 10. Why do stars twinkle?
Answer:  The stars twinkle at night, because the star light reaching Our eyes increases and decreases continuously due to atmospheric refraction. When star light reaching our eyes increases, the star looks bright and when the star light reaching our eyes decreases, it appears dim.

Question 11. Explain why the planets do not twinkle ?
Answer:  Planets being close to earth appear larger in size. A planet can be Considered as a collection of large number of small sized objects. Twinkling effect Of these objects cancel each other. so, planets do not appear to twinkle.

Question 12. Why does the sun appear reddish early in the morning?
Answer:  At sunrise, the sun looks almost reddish because only red colour which is least scattered is received by our eye and appears to come from sun. Hence the appearance Of sun at sunrise, near the horizon looks almost reddish.

Question 13. Why does the sky appear dark of blue to an astronaut?
Answer:  At such huge heights due to absence of atmosphere, no scattering of the light takes place. Therefore sky appears dark.