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.
Let u = object distance from convex lens
= -15 cm
v1= image distance from convex lens
= 30 cm
f1= focal length of convex lens.
= 10 cm
Again, let v = image (final) distance from concave lens = + (30 + 30) = + 60 cm
v1= object distance from concave lens
= + 30 cm
f2= focal length of concave lens
or, f2=−60 cm
So, the focal length of convex lens is 10 cm and that of concave lens is 60 cm.