Question

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

$\frac{1}{v}$ - $\frac{1}{u}$ = $\frac{1}{f}$

$\frac{1}{v}$ + $\frac{1}{60}$ = + $\frac{1}{20}$

$\frac{1}{v}$ = $\frac{1}{20}$ - $\frac{1}{60}$ = $\frac{1}{30}$ ⇒ v = 30 cm

Since the image is formed at the centre of curvature of the concave mirror, the final image is also formed there.