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 20 cm separated from the eyes?
Given data –
Near point of the person = 100 cm.
It is required to read at 20 cm in the following two cases-
(a) When the contact lens is used-
u = – 20 cm = – 0.2 m,
v = – 100 cm = – 1 m
Using lens formula –
Substituting the values-
Hence, power of the contact lens required is,
(b)When the person uses spectacles at 2 cm from the eyes-
u = – (20 – 2) = – 18 cm = – 0.18 m,
v = – 100 cm = –1 m
Using lens formula –
Substituting the values-
Power of the spectacles,