A concave mirror of focal length 6 cm is placed at a distance 'd' from the convex lens of focal length 8 cm. A beam of light coming from infinity and falling on this convex lens-concave mirror combination returns to infinity. The distance 'd' must be equal to:

20 cm

The correct answer is Option (2) → 20 cm

The concave mirror is placed at a distanced from lens. Therefore, the distance from the image formed by the lens to mirror is $d-f_2$.

For the light rays to return to infinity after reflecting off the concave mirror, the virtual object must be at the centre of the curvature of the mirror.

$d-f_2=2f_1$

$d=2f_1+f_2$ [$f_1=6cm,f_2=8cm$]

$=2×6+8$

$=20cm$