Question

The power of plane mirror is ___________________
A. $$0$$
B. $$\mathrm{\infty }$$
C. $$2\text{D}$$
D. $$\text{4D}$$

Answer 1

- Hint: Here, we will proceed by defining the terms plane mirror and power of a plane. Then we will first prove the focal length of the plane mirror to be infinity then the power of the plane mirror to be zero.

Complete step-by-step solution -

Plane mirror: A plane mirror is a mirror with a flat (planar) reflective surface.
Power of a mirror: Power of a mirror is defined as reciprocal of focal length of a mirror.
Mathematical representation is,
$$\text{P}={\displaystyle \frac{1}{f}}$$ where $$f$$ is the focal length (in metres)
We know that focal is equal to half of the radius of curvature of the mirror i.e., $$f={\displaystyle \frac{R}{2}}$$.
And also, the radius of curvature of a plane mirror is infinity i.e., $$R=\mathrm{\infty }$$.
So, we have $$f={\displaystyle \frac{\mathrm{\infty }}{2}}=\mathrm{\infty }$$
Hence, the focal length of the plane mirror is also infinity
Now, power $$\text{P}={\displaystyle \frac{1}{\mathrm{\infty }}}=0$$.
Hence, the power of the plane mirror is zero.
Thus, the correct answer is A. 0

Note: S.I unit of power is Dioptres. If the focal length is small then the power of the mirror is large and if the focal length is large then the power of the mirror is small. For light striking a plane mirror, the angle of reflection equals the angle of incidence.