Kevin Nash
Last Activity: 10 Years ago
Sol. The person has near point 100 cm. It is needed to read at a distance of 20cm.
(a) When contact lens is used,
u = – 20 cm = – 0.2m, v = – 100 cm = –1 m
So, 1/f = 1/v – 1/u = 1/-1 – 1/-0.2 = - 1 + 5 = + 4D
(b) When spectacles are used,
u = – (20 – 2) = – 18 cm = – 0.18m, v = – 100 cm = –1 m
So, 1/f = 1/v – 1/u = 1/-1 – 1/-0.18 = - 1+ 5.55 = + 4.5D