Q. 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.
Answer: In the problem it is given that the near point of the normal eye is 25 cm, hence u = - 25 cm.
The near point of a hypermetropic eye is 1 m . It is the nearest point up to which the hyper me tropic eyes can see object. So to correct hypermetropic eyes of this defect, person has to use
Spectacles with convex lens of suitable focal length. This lens form virtual image of nearby object at the near point of hypermetropic eye. so image distance is 1m=100cm
v= near point of hypermetropic eye = -1m = -100 cm
u = near point of normal eye = -25cm.
1/f = 1/v -1/u = -1/100 + 1/25 = (-1 + 4)/100 =3/100
f = 100/3 cm = 1/3 m
Power = 1/f = 1/(1/3) =3 dioptre
Q. 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?
Answer: This defect is corrected by concave lens.
For myopic personthe object distance is ¥
since the far point this eyes is 80 cm. So to correct myopic eyes of this defect, person has to use
Spectacles with concave lens of suitable focal length. This lens form virtual image of distant object at the far point of myopic eye. So image distance is 80 cm
so applying lens formula 1/v - 1/u = 1/f
v = - 80 cm u = infinity=0
-1/80 + 1/o = 1/f
1/f = -1/80
f = -80 cm
or -0.8 m power is = -1/0.8 = -10/8 = -1.25 D
So it would be a concave lens as it has a negative power
Why is the colour of the clear Sky Blue?
The blue light has lest velocity. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly. Hence the colour of the clear Sky appears blue.
If there is no atmosphere, there would not have been any scattering. Then, the sky would have looked dark.
Explain reason for the reddish appearance of the Sun at the sunrise or sunset.
At the sunrise or the sunset, Light from the Sun has to cover larger distance in the earth’s atmosphere before reaching our eyes. Since red colour light has longer wave length deviate lesser then other colours light and reach to our eyes.
What is Persistence of vision?
Ans: The impression of image is remains on retina for about 1/16th of second. This is used during cinematography. The sequence of still picture taken by movie camera is projected on the screen at he rate of 24 or more images per second so that images merge in one another on screen .
What is astigmatism?
In astigmatism a person can see only in one plane either horizontal or vertical. When light rays enter the eye they do not focus correctly on the retina, resulting in a blurred image. Astigmatism may also be caused by an irregularly shaped lens, which is located behind the cornea.
10th chapter wise solution
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I am a school teacher and serving CBSE students community since 1995.