Question

A person cannot see distinctly an object placed beyond 40 cm from his eye. what is the power of the lens which will enable him to see distant stars clearly

Answer 1

we know that P=1\f where f measure in meter (m). so now 40cm covert into meter we get 40cm=0.4 m now apply formulae P=1\f . solution, f=0.4 then P=1/0.4 =5/2 . 5/2 divide we get 2. 5 answer in diopter power unit so the ans is 2.5 d for proper vision of person beyond 40cm can't see

Answer 2

Answer:

the person cannot see objects clearly beyond 0.4m so his far point is 0.4m

distance of object is infinity

-1/0.40 - 1/-infinity = 1/f

f= -0.40

Power = 1/f = 1/-0.40m = -2.5D