Question

A concave mirror has a radius of curvature of 15 cm. Its focal length is:

Options

• −7.5 cm

• −30 cm

• +7.5 cm

• +15 cm

Answer 1

Using \[f = \frac{R}{2}\] with R = 15 cm gives a magnitude of 7.5 cm. Since the focus of a concave mirror lies in front of it, the focal length is negative: f = −R/2 = −7.5 cm.