Class Ten Science Chapter 11 NCERT Solutions
Chapter 11: Human Eye and the Colourful World
Q.1. What is meant by the power of accommodation of the eye?
Ans. The ability of the eye lens to adjust its focal length, so as to clearly focus rays coming from distant as well near objects on the retina, is called the 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. A person with a myopic eye should use a concave lens of focal length 1.2 m so as to restore proper vision.
Q.3. What is the far point and near point of the human eye with normal vision?
Ans. 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.
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?
Ans. The student is suffering from myopia or short-sightedness. The defect can be corrected by the use of the concave (diverging) lens of appropriate power.
Q.5. 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-sighted
Ans. (b) Accommodation.
Q.6. The human eye forms the image of an object at its :
(a) cornea (b) iris (c) pupil (d) retina
Ans. (d) Retina.
Q.7. 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.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) Ciliary muscles
Q.9. 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?
Ans. The power P of a lens of focal length f is given by the relation P = 1/ f (in metres)
(i) Power of the lens used for correcting distant vision = – 5.5 D
Focal length of the required lens, f = 1/P= 1/–5.5 P. = – 0.181 m
The focal length of the lens for correcting distant vision is – 0.181 m.
(ii) Power of the lens used for correcting near vision = + 1.5 D
The focal length of the required lens, f = 1P
f = 1/ 1.5 = + 0.667 m
The focal length of the lens for correcting near vision is 0.667 m.
Q.10. 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?
Ans. The person is suffering from an eye defect called myopia. In this defect, the image is formed in front of the retina. Hence, a concave lens is used to correct this defect of vision.
Object distance, u = infinity = ∞
Image distance, v = –80 cm
Focal length = f
According to the lens formula,
f = – 80 cm = – 0.8 m
We know, that,
Power, (P) = 1/ f in metres()
⇒ P = 1/−08.= – 1.25 D
A concave lens of power –1.25 D is required by the person to correct his defect.