Question

A point object is placed at a distance of 15 cm from a convex lens. The image is formed on the other side at a distance of 30 cm from the lens. When a concave lens is placed in contact with the convex lens, the image shifts away further by 30 cm. Calculate the focal lengths of the two lenses.

Answer 1

Let u = object distance from convex lens = –15 cm
v₁ = image distance from convex lens when alone = 30 cm
f₁ = focal length of convex lens

Again, Let v = image (final) distance from concave lens = +(30 + 30) = 60cm
v₁ = object distance from concave lens = +30 m

So, the focal length of convex lens is 10 cm and that of concave lens is 60 cm.