According to mirror formula, the focal length of a spherical mirror is equal to:

Correct option is D

Correct option : (d)

The mirror formula, the relationship between the focal length (f), the object distance (u), and the image distance (v) for a spherical mirror (whether concave or convex) is given by:

$$\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$$​

Where:
- ( f is the focal length of the mirror.
- ( v ) is the image distance (the distance between the mirror and the image formed).
- ( u ) is the object distance (the distance between the mirror and the object).

From this formula, the focal length of a spherical mirror can be expressed as:
f = $$\frac{vu}{v + u}$$​
 

Key Points:-

- For concave mirrors, the focal length is positive .

- For convex mirrors, the focal length is negative.

This formula is crucial in understanding how spherical mirrors form images and is used to determine the position of the image given the object position or vice versa.