A student's near point is 0.5 m and far point is 3m . What are the powers of the spectacle lenses required for (a) reading the books and (b) seeing the distant objects ?

Options

A. `-2D, +0.33D`
B. `+2D, -0.33D`
C. `-2D, +3D`
D. `+2D, -3D`

Answer

B

Explanation

(i) For reading the books
u = -25 cm (Distance of Distinct vision)
and v = -50 cm
`therefore (1)/(f)=(1)/(v)-(1)/(u)= -(1)/(50)+(1)/(25)=(1)/(50)`
`therefore f=50 cm therefore P=(100)/(f)=(100)/(50)=2D`
This is a convex lens.
(ii) For seeing the distance objects, the distance of the far point = f' = -3 m
`therefore P= -(1)/(f') = -(1)/(3)D= -0.33D`
It is a concave lens.