Answer to Question #101667 in Optics for Manohar sharma

Question

A convex lens forms a real image of a point object at a distance of 50 cm from the convex lens lens of the image side on placing a plane Mirror on the image side and facing the convex and it is observed that the final image now concepts with objects itself the focal length of the concave lenses

Answer 1

The focal length can be calculated by formula

$\frac{1}{u} + \frac{1}{v} =\frac{1}{f} (1)$

The image coincides with the object only if the ray retraces its path. In this case, the rays are perpendicular to the plane mirror. It means that the images is formed at infinity by the concave lens when there is no mirror.

In our case, we have

v=−∞

u=10cm-50 cm=-40 cm

Using (1) we get

f=40 cm

Answer

40 cm