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