A parallel beam of light is made to fall on a concave mirror.An image is formed at a distance of 7.5 cm from the mirror.What are the focal length and radius of curvqature of the mirror?

Here as the ray of light is parallel we assume that the object is kept very far away or is at infinity.

so,

u = -∞

v = -7.5cm

now, as

1/f = 1/v + 1/u

or

1/f = -1/7.5 - 1/∞ = -1/7.5

thus, we have

the focal length

f = -7.5cm

so, the radius of curvature will be

R = 2f = -15cm