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,



1