First slide

The optical instruments

Question

A man who cannot see clearly beyond 5 m wants to see stars clearly. He should use a lens of focal length

Moderate

A

$- 100 m$
B

$+ 5 m$
C

$- 5 m$
D

$Very large$

Solution

$For lens u = want's to see = \infty v = can see = - 5 m ∴ From \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \Rightarrow \frac{1}{f} = \frac{1}{- 5} - \frac{1}{\infty} \Rightarrow f = - 5 m$

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