Question

Explain why the focal length of a plane mirror is infinite, and explain what it means for the focal point to be at infinity.

Short answer

The focal length of a plane mirror is because the parallel rays couldn’t be focused hence f is infinity.

Step-by-step solution

Radius of curvature for a flat mirror

Now, we know that the focal length of any mirror is equal to the half of the radius of curvature of the mirror:

$f=\frac{R}{2}$

Since we know that the flat mirror doesn’t have a curvature meaning that the radius of curvature of such mirror is equal to infinity. Hence if we replace$R=\infty$ , putting this in the above equation we get:

$$\begin{gathered} f=\frac{\infty }{2} \\ =\infty \end{gathered}$$

Conclusion

All light rays are parallel to the main optical axis of the mirror and are reflected from the surface of the mirror through the focal point. Since the focal point of the plane mirror is in infinity, meaning that the rays which are parallel to the main optical axis are reflected back to the parallel to the main optical axis.