Question

The far point of a myopic person is 150cm in front the eye.calculate the focal length and power of a lensrequiredto enable him to seedistant object clearly

Answer 1

Here,

Far point = 150 cm

For normal eyes far point is infinity thus,

u = -∞

Let normal eye have a focal length f

By lens equation we have

1/v – 1/u = 1/f

=> 1/v – 1/-∞ = 1/f

=> 1/v – 0 = 1/f

=> v  = f  ---1.

Let the myopic eye have a focal length fd

u = -150 cm

1/v – 1/(-150) = 1/fd

=> 1/f + 1/150 = 1/fd  ---2.

Let fc be the focal length of the corrective lens

Thus power of lens 1/fc must be added so that the eye have a

Normal eye lens power 1/f

1/fd + 1/fc = 1/f

Putting value of 1/fd from 2

=> 1/f + 1/150 + 1/fc = 1/f

=> 1/fc = -1/150

Focal length fc  = -1.5 m

Type of lens concave

Now power of the lens = -1/1.5 D = -0.67 D

Rohit is back.....

Answer 2

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