Question 324: Ray Optics

Question

The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is __________ cm.

Accepted Answer

$${1 \over v} - {1 \over u} = {1 \over f}$$ .... (1)
m = $${v \over u}$$ ..... (2)
from (1) and (2) we get
m = $${f \over {f + u}}$$
given conditions
m
1
= -m
2
$${f \over {f - 10}} = {{ - f} \over {f - 20}}$$
$$ \Rightarrow $$ f Ã¢ÂÂ 20 = -f + 10
$$ \Rightarrow $$ 2f = 30
$$ \Rightarrow $$ f = 15 cm

Ray Optics

The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is __________ cm.

Choose the correct answer: