The radius of curvature of a spherical mirror is 40 cm. Its focal length is _______ cm.

Correct option is B

The correct answer is (b) 20 cm.

Explanation:

The focal length (f) of a spherical mirror (either concave or convex) is related to its radius of curvature (R) by the following formula:

f= $$\frac{R}{2}$$

Where:

• f is the focal length of the mirror,
• R is the radius of curvature of the mirror.​​

Given: The radius of curvature R=40cm.

So, the focal length of the mirror, f = $$\frac{R}{2}$$= 20 cm.

Information Booster:

• The radius of curvature is the distance from the mirror's surface to its center of curvature. It essentially represents the radius of the sphere from which the mirror is a part.
• The focal length is half of the radius of curvature because it is defined as the distance from the mirror's surface to the focal point (where parallel rays of light converge or appear to diverge from in the case of a convex mirror).

For spherical mirrors:

• A concave mirror (which is a converging mirror) has a positive focal length.
• A convex mirror (which is a diverging mirror) has a negative focal length.