Question: What is the focal length of a concave mirror if an object is placed at a distance of 30 cm from the mirror and the image is formed at a distance of 15 cm from the mirror?

Options:

1. 10 cm
2. 15 cm
3. 20 cm
4. 25 cm

Correct Answer: 10 cm

Solution:

Using the mirror formula, 1/f = 1/v + 1/u. Here, v = -15 cm (real image) and u = -30 cm (object distance). Thus, 1/f = 1/(-15) + 1/(-30) = -1/10, so f = -10 cm.