CBSE Class 10 Human Eye & Colourful World NCERT Solution

 

Human Eye & Colourful World (NCERT solution)

PG logo with course.JPGQ. 1. What is meant by power of accommodation of the eye?PG logo with course.JPG
Ans. The ability of the eye lens to adjust its focal length is called power of accommodation of the eye. This enables us to see distant objects and nearby object clearly.

PG logo with course.JPGQ. 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?PG logo with course.JPG
Ans. A person with a myopic eye should use a concave lens of focal length 1.2 m so as to
restore proper vision.

PG logo with course.JPGQ. 3. What is the far point and near point of the human eye with normal vision?PG logo with course.JPG
Ans : For a human eye with normal vision the far point is at infinity and the near point is at 25 cm from the eye.

PG logo with course.JPGQ. 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?PG logo with course.JPG
Ans. The student is suffering from myopia or short-sightedness.
This defect can be corrected by using concave (diverging ) lens of an appropriate power.

PG logo with course.JPGQ. 5. The human eye can focus objects at different distance by adjusting the focal length of the eye lens. This is due to  PG logo with course.JPG(a) Presbyopia PG logo with course.JPGb) accommodation PG logo with course.JPG(c) near-sightedness PG logo with course.JPG(d) far-sightedness

Ans : (b) accommodation.

PG logo with course.JPGQ 6. The human eye forms the image of an object at its
PG logo with course.JPG(a) Cornea        PG logo with course.JPG (b) iris             PG logo with course.JPG(c) pupil             PG logo with course.JPG(d) retina
Ans: (d) retina.

PG logo with course.JPGQ.7. The least distance of distinct vision for a young adult with normal vision is about
PG logo with course.JPG(a) 25 m         PG logo with course.JPG(b) 2.5 cm         PG logo with course.JPG(c) 25 cm        PG logo with course.JPG(d) 2.5 m

Ans : (c).

PG logo with course.JPGQ. 8. 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) .

PG logo with course.JPGQ. 9. A person needs a lens of power 5.5 D for correcting his distant vision. For correcting his near vision he needs a lens of power + 1.5 D. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision? PG logo with course.JPG
Ans :
(i) Power of lens needed for correction distant vision of the person P1 = 5.5 D
Focal length of lens required for correcting distant vision f1 = 1/p1 = 1/- 5.5 = -18cm

(ii) For correcting near vision power required P2 = + 1.5 D
Focal length of lens required for correcting near vision f2 = 1/p2 = 1/-1.5 = 0.666 m = 66.7 cm

PG logo with course.JPGQ.10. The far point of a myopic the person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem? PG logo with course.JPG
Ans :

To correct the myopia the person concerned should use concave lens (diverging lens) of suitable focal length.

PG logo with course.JPGQ.12. Why is a normal eye not able to see clearly the objects placed closer than 25 cm ?PG logo with course.JPG

Ans: This is because the focal length of the eye lens cannot be decreased below a certain
minimum limit that 25cm during accommodation.

PG logo with course.JPGQ.13. What happens to the image distance in the eye when we increase the distance of an object from the eye? PG logo with course.JPG

Ans : The image is formed on the retina even on increasing the distance of an object from the eye. When we increase the distance of an object from the eye lens becomes thinner and its focal length increases to focus image on retina.

PG logo with course.JPGQ.14. Why do stars twinkle? PG logo with course.JPG
Ans. Stars twinkle due to atmospheric refraction of starlight. The density of air varies place to place therefore, the path of rays of light coming from the star goes on varying slightly, the apparent position of the star formed randomly. Thus, the stars twinkle.

PG logo with course.JPGQ.15. Explain why the planets do not twinkle. PG logo with course.JPG
Ans. Planets are much closer to the earth so not much refraction occurs. Therefore, planets appear equally bright and there is no twinkling of planets.

PG logo with course.JPGQ.16. why does the Sun appear reddish early in the morning? PG logo with course.JPG
Ans : In the early morning, the sun is situated near the horizon. Light from the Sun passes through thicker layers of air and cover larger distance before reaching our eyes. Therefore, blue light scattered the most and red light least. This is why the sun appear reddish early in the morning.

PG logo with course.JPGQ.17. Why does the sky appear dark instead of blue to an astronaut? PG logo with course.JPG
Ans.: In space there is no air therefore light ray cannot scatter thus, sky appears dark to an astronaut.

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