A thin convex lens of focal length f is put on a plane mirror as shown in the figure. When an object is kept at a distance a from the lens-mirror combination, its image is formed at a distance a 3 in front of the combination. The value of a is:

Question

A thin convex lens of focal length f is put on a plane mirror as shown in the figure. When an object is kept at a distance a from the lens-mirror combination, its image is formed at a distance a 3 in front of the combination. The value of a is:, 2 f, 3 2 f, 3 f, f

MARKS App · Accepted Answer

Equivalent power of combination is P e f f = 2 P l e n s + P m i r r o r ...(i) ∵ P m i r r o r = 0 for plane mirror and P l e n s = 1 f Also combination behaves as a mirror. So. P e f f = 1 f m i r r o r In equation (i) - 1 f m i r r o r = 2 f ⇒ f m i r r o r = - f 2 By mirror formula 1 f m i r r o r = 1 u + 1 v Here u = - a , v = - a 3 , f m i r r o r = - f 2 ⇒ - 2 f = 1 - a + 3 - a = - 4 a ⇒ - 2 f = - 4 a ⇒ a = 2 f

Search any question & find its solution

Looking for more such questions to practice?