Question

Focal length of convex lens is 20 cm, focal length of concave mirror is 30 cm. And the separation between lens and mirror is 90 cm. If the object is placed 60 cm before lens, find out position of image formed by concave mirror?

• A. 60 cm from point B
• B. 60 cm from point A
• C. 90 cm from point B
• D. 60 cm left of lens

Answer 1

The correct option is A

60 cm from point B

GIven:
object distance $u=-60cm$
foacl length of the lens $f=+20cm$
Using the lens formula
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{v}+\frac{1}{60}=+\frac{1}{20}$
$\frac{1}{v}=\frac{1}{20}-\frac{1}{60}=\frac{1}{30}$
$\Rightarrow v=+30cm$. This means image is formed on the right side of the lens between lens and the mirror as shown in the image below.

For a concave mirror, an object placed at centre of curvature forms an image at centre of curvature. So, the final image is 60 cm from point B.