Question

A convex lens of focal length 30 cm and a concave lens of 10 cm focal length are placed so as to have the same axis. If a parallel beam of light falling on convex lens leaves concave lens as parallel beam, then the distance between two lenses will be

(a)  40 cm

(b)  30 cm

(c)  20 cm

(d)  10 cm

Answer 1

Correct option  20 cm

Explanation :

Acc. to Given condition, Per = 0

Per = P₁ + P₂ ­ dP₁P₂

P₁ = 1/f = + 100/30cm , P₂ = - 100/10

Per = 0 ⇒10/3 - 10 - d(-10)(10/3) = 0

d = 20 cm