First slide

Ray optics

Question

A person can see clearly only upto a distance of 30 cm. He wants to read a book placed at a distance of 50 cm from his eyes. What is the power of the lens he requires for his spectacles?

Moderate

A

+2.67 D
B

-1.33 D
C

+2.5 D
D

-2.5 D

Solution

object distance $u = - 50 c m$
Image distance $V = - 30 c m$
$∴ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{(- 30)} - \frac{1}{(- 50)} = \frac{20}{1500} = \frac{- 1}{75} \Rightarrow Power = (\frac{100}{- 75}) = - 1.33 D$

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