question_answer

In an experiment a convex lens of focal length 15 cm is placed coaxially on an optical bench in front of a convex mirror at a distance of 5 cm from it. It is found that an object and its image coincide, if the object is placed at a distance of 20 cm from the lens. The focal length of the convex mirror is -    [JEE Online 09-04-2017]

A)  20.0 cm                               

B)  30.5 cm

C)  25.0 cm                               

D)  27.5 cm

Correct Answer: D

Solution :

                due to this images will from 55 cm behind convex mirror then reflection takes replace due to mirror image will form at v behind mirror (let) this will here as object for lens and final image will from at a distance of 20 cm from lens (i.e. obext) if focal length in of convex lens \[=\frac{55}{2}cm\]