When light reaches the interface between two different media, some of it is reflected back into the medium from which it came. This phenomenon is known as reflection. The law of reflection states that:

• The incident ray, the reflected ray, and the normal line all lie in the same plane.
• The incident ray and the reflected ray lie on opposite sides of the normal line.
• The angle of reflection is equal to the angle of incidence (\(\theta_r=\theta_i\)).

Here, the normal is an imaginary line perpendicular to the interface. Both the angle of incidence and the angle of reflection are measured between the corresponding ray and the normal.

Image Formation by a Plane Mirror
When we look into a mirror, the image we see is a direct result of the law of reflection.

Every point on an object emits light rays in all directions. When these rays strike a smooth mirror surface, they are reflected according to the law of reflection. The backward extensions of the reflected rays form corresponding virtual image points, which together produce the complete virtual image of the object.