Question

a convex lens of focal length 40 cm is held co-axially 12 cm above a mirror of the focal length 18 cm an object held x cm above the lens gives rise to an image coincident with it. Then x equal to:

Answer 1

Answer:

For the final image to be coincident with the object, the rays must retrace their path after reflection from the concave mirror. Hence the image obtained after first refraction from the convex lens must lie on the radius of curvature of the lens.

Hence the distance of image from the lens =R−13cm=2(18)cm−12cm=24cm

Using lens equation,

v

1

−

u

1

=

f

1

−24

1

−

−x

1

=

40

1

⟹x=15cm