Question

The focal length of a plane mirror is?
A) Positive
B) Negative
C) Zero
D) Infinite

Answer 1

Hint: Understand the nature of the image formed by a plane mirror when object is at u distance from object also about focus, focal length and their relationship with each other. A plane mirror is a mirror with a flat reflective surface. For light rays striking a plane mirror, the angle of reflection equals the angle of incidence.

Complete answer:

You might be familiar that after reflection a set of parallel rays of light from a source kept at a distance far away from the mirror, converge at or seem to diverge from the focus of the mirror. The distance of this point from the pole of the mirror is called the focal length.
The focal length of a plane mirror is infinite as the image can be formed at an infinite distance inside the mirror. A plane mirror can be recognized as a spherical mirror of infinite radius of curvature. Focal length is half of the radius of curvature and hence it is infinity
The focal length of a plane mirror is infinity; its optical power is zero.

Hence, the correct answer is (D).

Note: Focal length is always negative if the focus lies on the left of the mirror and if the focal length is positive then focus lies on the right side of the plane mirror but if the focus lies on the mirror then the focal length will be zero. For Ray Optics, one should always draw the ray diagrams for a better understanding of concepts.