First slide

The lens

Question

An object is placed first at infinity and then at 20 cm from the object side focal plane of the convex lens. The two images thus formed are 5 cm apart. The focal length of the lens is

Moderate

A

5 cm
B

10 cm
C

15 cm
D

20 cm

Solution

$For first case : \frac{1}{f} = \frac{1}{v} - \frac{1}{\infty} \Rightarrow f = v For second case \frac{1}{f} = \frac{1}{(f + 5)} - \frac{1}{- (f + 20)} \Rightarrow f = 10 cm Alternative sol. - f^{2} = x_{1} x_{2} \Rightarrow f = 10 cm$

Get Instant Solutions

When in doubt download our app. Now available Google Play Store- Doubts App

Recieve an sms with download link

Doubts App

OR

SCAN ME

Doubts App

Doubts App