Question

For a plane mirror, the focal length is ……..

Hint:

A plane mirror can be thought of as a spherical mirror with an infinite radius of curvature. This makes its focal length also infinity (\( f = \infty \)).

Answer 1

Step 1: Understanding Focal Length 
The focal length (\( f \)) of a mirror is related to its radius of curvature (\( R \)) by the formula: \[ f = \frac{R}{2} \] where:
- \( f \) is the focal length,
- \( R \) is the radius of curvature. 
Step 2: Plane Mirror as a Special Case 
- A plane mirror can be considered as a spherical mirror with an infinite radius of curvature.
- That is, for a plane mirror: \[ R = \infty \] 
Step 3: Final Calculation 
Substituting \( R = \infty \) in the focal length formula: \[ f = \frac{\infty}{2} = \infty \] Thus, the focal length of a plane mirror is infinity.