Question

The focal length of a plane mirror is:

• A. Zero
• B. Infinity
• C. Greater than zero
• D. Less than zero

Answer 1

The correct option is B Infinity

A plane mirror can be thought of as a part of a very large spherical surface.
For a very large spherical surface, the radius of curvature tends to infinity.
$\Rightarrow R\to \infty$
We know that focal length,
$f=\frac{R}{2}$
$\Rightarrow f\to \infty$