Question

A converging beam of light forms a sharp image on a screen. A lens is placed 10 cm from the screen in the path of the beam. It was found that the screen has to be moved 8 cm further away to obtain the sharp image. Focal length of the lens is –

• A. 6.43 cm
• B. – 2.5 cm
• C. – 22.5 cm
• D. 2.66 cm

Answer 1

Since the screen has to be moved away, the lens used should be concave.

= –

Ž = $\frac { 1 } { 18 } - \frac { 1 } { 10 }$

f = $\frac { 180 } { 10 - 18 }$= – 22.5 cm