First slide

Ray optics

Question

For a long sighted person, least distant of distinct vision is x meter. He wants to read news paper placed at x/2 meter from him. He uses spectacle having lenses of power x D. What is x :

Moderate

A

0.5
B

1.5
C

1
D

2

Solution

$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
$u = - \frac{x}{2}, v = - x$
$\frac{1}{f} = - \frac{1}{x} + \frac{2}{x}$
$\frac{1}{f} = \frac{1}{x}$
$f = 1 m$

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