In the arrangement shown, f1=20 cm and f2=10 cm. A parallel beam of light is incident on the convex lens. Then find the distance ‘d’ for which the final emergent beam after refraction through the concave lens will become parallel again.

After refraction through the convex lens the beam will converge at a point P on the common principal axis at a distance of 20 cm from the convex lens. The final emergent beam will be parallel again if P be the first focal length of the concave lens. So distance of P from the concave lens will be 10 cm. ∴ d=(20−10)cm=10​ cm