A convex lens of focal length 20 cm and another plano convex lens of focal length 40 cm are placed co-axially (see fig.). The plano convex lens is silvered on plane surface. What should be the 5d (in m) so that final image of the object 'O' is formed on O itself.

Question

G1 Physics · Accepted Answer

Derived solution: Step 1: Use the thin-lens formula 1/f = 1/v - 1/u with m = v/u. For lens material/medium changes use the lens-maker relation with the relative refractive index; powers of thin lenses in contact add. Step 2: Substitute the values given in the question and apply the geometry/sign convention shown. Step 3: On simplification the result is 1. Therefore the correct option is B.