Question

Two convex lenses, of focal lengths 20 cm and 10 cm,
respectively are separated by a distance of x cm. If a
parallel beam of light remain parallel after refraction
through both the lenses find x.​

Answer 1

Answer:

For the final image to be at infinity, the image formed by the first lens (focal length = 20 cm) should be at the focus of the second lens (focal length = 10 cm).

For first lens

Image distance, v = (x−​10) cm

Object distance, u = ∞

Focal length, f1 = 20 cm

Applying lens maker's formula, we get

1v−1u=1f11x−10−1∞=120⇒1x−10=120⇒x=30 cm