Physics Theory : Plane Mirrors

Reflection of light is the phenomenon due to which a light ray traveling from one optical medium to another bounces off its surface with a change of angle.

Regular reflection takes place when a parallel beam of light strikes a smooth polished surface and it is reflected back as a parallel beam of light.
Reflections from mirrors and metallic surfaces are examples of regular reflection.

Diffused reflection or irregular reflection takes place when a parallel beam of light strikes a rough surface and it is reflected back in different random directions.
Reflections from objects such as trees and stones are examples of diffused reflection.

A plane mirror is a smooth polished surface that can turn the rays of light into the same medium.

The normal (denoted by line ON in the figure) is drawn at the point of incidence O perpendicular to the plane mirror surface.
It is an imaginary line.

The incident ray (denoted by line PO in the figure) corresponds to a ray of light that travels from an optical medium towards the mirror.

The reflected ray (denoted by line OQ in the figure) corresponds to a ray of light that bounces off the mirror surface and travels into the same optical medium in which the incident ray was traveling.

The angle of incidence (denoted by angle PON in the figure) is the angle that the incident ray PO makes with the normal ON.

The angle of reflection (denoted by angle NOQ in the figure) is the angle that the reflected ray OQ makes with the normal ON.

The First Law of Reflection states that the incident ray PO, the reflected ray OQ and the normal ON all lie in the same plane, at the point of incidence.
The point of incidence O is the point on the mirror surface where the incident ray strikes.

The Second Law of Reflection states that the angle of incidence PON is always equal to the angle of reflection NOQ.

When a divergent beam of light is incident on a plane mirror, it is reflected back as a divergent beam.
When the rays of light starting from a point travel in different directions, then the collection of such rays is termed a divergent beam of light.

When a convergent beam of light is incident on a plane mirror, it is reflected back as a convergent beam.
When the rays of light coming from different directions meet at a point, then the collection of such rays is termed a convergent beam of light.

When a parallel beam of light is incident on a plane mirror, it is reflected back as a parallel beam.
When a large number of rays of light travel parallel to one another, then the collection of such rays is termed a parallel beam of light.

The following characteristic properties of the image formed by a plane mirror are worth noting :
1. It is of the same size as the object.
2. It is formed as far behind the mirror as the object is in front of it.
3. It is always virtual (cannot be taken on the screen).
4. It is laterally inverted, but always erect.
For example, the letter p will appear as q after reflection in a plane mirror because of lateral inversion.

Lateral inversion occurs when the left side of an object appears as the right side of an image and vice-versa.