Question

The focal length of a plane mirror is:
A. Zero
B. Infinity
C. Greater than zero
D. Less than zero

Answer 1

Hint: Know the characteristics of the image produced by a plane mirror when the subject is at a distance from the mirror, as well as what focus and focal length are and how they relate to one another. A mirror with a flat reflective surface is known as a plane mirror. The angle of reflection for light rays striking a planar mirror is the same as the angle of incidence.

Complete answer:
We know that, a plane mirror is considered as a large spherical surface, such that the radius of curvature tends to infinity i.e., $R\rightarrow \infty$

The focal length of plane mirror is given by:
$f=\dfrac {R}{2}$
$f= \infty$

Therefore, the plane mirror is $\infty$. Option B is correct.

Additional Information:
A light ray can experience one of the three following occurrences when it strikes a surface: reflection, refraction, or absorption. Most of the light is absorbed when it hits a regular surface. Mirrors are thus polished surfaces that have been covered in mercury such that the majority of light that strikes them is reflected. Now, we can classify mirrors as concave, convex, or flat mirrors depending on the type of reflecting surface. Here, we'll limit our discussion to the plane mirror. Therefore, at least two rays from the object must meet or appear to contact at a place in order to create an image.

Characteristics of a plane mirror image:
1. The obtained image is a digital one.
2. The image has been inverted laterally.
3. The picture is upright.
4. The object's size and the image's size are the same.
5. The object's distance from the mirror and the distance between the image that was obtained are equal.

Note: Because mirrors are smooth on a tiny scale, only mirrors are reflective and not other flat white surfaces. Diffuse reflection occurs when light energy strikes a rough surface and bounces back in all directions. But over a smooth surface, light only reflects in one direction, a phenomenon known as specular reflection.