An object at a distance of 30 cm from a concave mirror gets its image at the same point. The focal length of the mirror is?

An object at a distance of

30 cm

from a concave mirror gets its image at the same point. The focal length of the mirror is

15 cm

.
When an object is placed at

2 f

, the image formed by a concave mirror is also at

2 f

So,

2 f = - 30 cm

(as the mirror is concave, the -ve sign is due to the sign convention)

\Rightarrow f = - 15 cm

Also Check:

An object at a distance of $30 cm$ from a concave mirror gets its image at the same point.

The focal length of the mirror is?