Question

A person has near point at 100 cm. What power of lens is needed to read at 20 cm if he/she uses (a) contact lens, (b) spectacles having glasses 2.0 cm separated from the eyes?

Answer 1

Given: A person has near point of 100 cm.
According to the question, it is needed to read at a distance of 20 cm.
(a) When the contact lens is used:
 u = – 20 cm = – 0.2 m,
 v = – 100 cm = – 1 m
The lens formula is given by

`1/v -1/u =1/f`

On substituting the values in the above formula, we get:

`1/f = 1/((-1)) -1/((-0.2)) = 4`

∴ Power of the lens, P = `1/f` = 4 D
(b) When the person wears spectacles at a distance of 2 cm from the eyes:
u = – (20 – 2) = – 18 cm = – 0.18 m,
v = – 100 cm = –1 m
The lens formula is given by

`1/v-1/u=1/f`

On substituting the values, we get:

`1/f = 1/((-1)) -1/((-0.18)) = 4.53`

∴ Power of the lens, P = `1/f` = 4.53 D