Intext Questions

Q.1       What is meant by power of accommodation of the eye ?
Sol.       The power of the eye by which it can see nearby objects as well as faraway objects by changing its focal length is called its power of accommodation.

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 ?
Sol.         Concave lens

Q.3       What is the far point and near point of the human eye with normal vision ?
Sol.         Far point – infinity

Near Point – 25 cm.

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 ?
Sol.          Myopia, concave lens

Exercise

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
Sol.         (b) Accommodation

Q.2      The human eye forms the image of an object at its
               (a) cornea                             (b) iris
            (c) pupil                                 (d) retina
Sol.         (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.
Sol.          25cm.

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
Sol.          (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 ?
Sol.

(i) f=1P=−15.5=−0.1818m.
or
f=−18.18cm.
(ii) f=1P=11.5=0.666m.
f = 66.6cm.

Q.6       The far point of a myopic person is 80cm in front of the eye. What is the nature and power of the lens required to correct the problem ?
Sol.          Concave lens P=1f=−10080=−1.25D

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.
Sol.

u = –25cm.
v = –100 cm.
1f=1v=1u
=−1100+125=3100
f=1003cm=13m.
P = –3D
Convex lens of power +3D is needed

Q.8      Why is a normal eye not able to see clearly the objects placed closer than 25 cm ?
Sol. 

To see on object placed at a distance of less than 25cm, the ciliary muscles have to make the focal length still smaller, which puts strain on the ciliary muscles and they cannot make the focal length so small. Hence it is not possible to see at distance lens then 25 cm.

Q.9      What happens to the image distance in the eye when we increase the distance of an object from the eye ?
Sol.         Image distance does not change.

Q.10    Why do stars twinkle ?
Sol.

Due to atmospheric refraction, starlight bends from its path. Due to continuously changing atmosphere, starlight varies causing it to twinkle.

Q.11     Explain why the planets do not twinkle.
Sol.

Planets are much closer to the earth and can be considered as a collection of large number of point sources of light. If some light from plant is refracted, the effect is not observable and hence planets do not twinkle.

Q.12     Why does the Sun appear reddish early in the morning ?
Sol.

At sunrise, the sunlight has to pass through a thicker layer of atmosphere. Blue wavelength is scattered the most, hence blue is scattered away, leaving mainly red colour which reaches our eyes. Hence sky appears reddish.

Q.13     Why does the sky appear dark instead of blue to an astronaut ?
Sol.

Outside the earth, there is no atmosphere to scatter sunlight, so the sky appears dark or black to an astronaut in outer space.

Exemplar

Q.1    A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard.
What advice will a doctor give to her? Draw ray diagram for the correction of this defect.
Sol.        If distant objects appear to be blurred or not clearly visible then the patients are said to be near-sighted. So we can say that the student is suffering from myopia. In this case a concave lens is used to correct the problem.

Q.2     How are we able to see nearby as well as the distant objects clearly?
Sol.        We are able to see clearly far away as well as nearby objects by the power of accommodation. The ciliary muscles change the shape of the lens thereby changing its focal length such that in all cases image is formed on the retina.

Q.3     A person needs a lens of power –4.5 D for correction of her vision.
           (a) What kind of defect in vision is the suffering from?
           (b) What is the focal length of the corrective lens?
           (c) What is the nature of the corrective lens?
Sol.
              (a) Myopia
              (b) f=1p=−145=−222m.=−22.2cm
              (c) Concave lens

Q.4     How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw the diagram.
Sol.         We can place one prism straight and the other upside down, so that the light that emerges from the second prism is white light.

Q.5      Is the position of a star as seen by us its true position? Justify your answer.
Sol.        The position of the star as seen by us is slightly higher than its actual position. This is caused by refraction of light.

Q.6     Explain the structure and functioning of Human eye. How are we able to see nearby as well as distant objects?
Sol.         The eye consists the eyeball having diameter approximately 2.3 cm. The front part of it is transparent, bulging portion called the cornea. Behind the lens is the coloured portion of the eye which controls the amount of light entering the eye the through the pupil. Behind the pupil is a lens made of jelly like substance attached is ciliary muscles, which can control the focal length of the eye lens by changing its shape. At the back of the eye is a light sensitive membrane on which images are formed. The front of lens is filled with a fluid called aqueous humous and behind it by a fluid called vitreous humous. 
               The entire function of the eye is to converge rays falling on the eye to form a diminished, inverted image on the retina.This image is converted into electrical signals and sent to the brain which enables us to see.

Q.7     When do we consider a person to be myopic or hypermetropic? Explain using diagrams how the defects associated with myopic. and hypermetropic eye can be corrected?
Sol.         We consider a person myopic if he cannot see faraway objects and hypermetropic if he cannot see nearby objects.

Myopia
In this defect a person is not able to see faraway objects. This may be caused by the elongation of eyeball on excessive curvature of the converging system of eye. The image is formed in front of the retina. The can be rectified by the diverging the rays by using concave lens.

Hypermetropia
In this defect, a person is unable to see nearby objects, where objects can be seen. This may be caused by the eyeball being short or less convergence of the rays entering the eye. Hypermetropia can be corrected by using convex lens of appropriate focal length.

Q.8     Explain the refraction of light through a triangular glass prism using a labelled ray diagram. Hence define the angle of deviation.
Sol.         When a ray of light is incident on a glass prism, it refracts bending towards the normal. When this refracted ray is incident on the second side of the prism, it refracts and moves away from the normal . Thus the emergent ray is not in the same direction as the incident ray. Thus the emergent ray has deviated from its original path. The angle by which the emergent ray has bent away from the incident ray is called the angle of deviation.

AB – incident ray
BC – refracted ray
CD – emergent ray
∠EBC=∠ of deviation (where BE is AB extended).

Q.9      How does refraction take place in the atmosphere? Why do stars twinkle but not the planets ?
Sol.          The atmosphere is a very thick layer of gas having varying temperatures at different heights. Due to this the atmosphere has varying optical density which is continuously changing . The light passing through the atmosphere is refracted . Due to atmospheric turbulence this refraction is continuously changing. Stars can be considered as point sources of light their position keeps on fluctuating due to atmospheric refraction. The amount of starlight entering our eyes keeps on changing giving its twinkling effect . Planets are much larger and can be considered as a collection of a number of point sources of light. The average intensity of light from the planets does not change. Thus planets do not twinkle.