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# Law of Reflection

When light traveling in a medium encounters a boundary leading to a second medium, part of the incident light is returned to the first medium from which it came. This phenomenon is called reflection. Reflection of light from a smooth surface is called regular or specular reflection. To undergo the phenomenon of reflection following laws are there, these are called the laws of reflection. Let us study these laws in this section.

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## What is the Law of Reflection?

Let us first study what is law of reflection with respect to a single light ray. Look at the following diagram.

Let MM' be a plane mirror which is absolutely smooth. A light ray AO strikes the mirror at O (the incident ray), and it is reflected as ray OA', called as reflected ray. ON is the perpendicular to MM' at O. It is also called as the normal to the surface at the point of reflection.
The angle made by the incident ray with the normal ON is p and q be the angle made by the reflected ray with the same normal. These are called as 'Angle of Incidence' and the 'Angle of Reflection' respectively.
According to the Law of Reflection,Angle of incidence is always equal to angle of reflection.

## Law of Reflection Definition

Now, let us consider a set of rays assuming a plane mirror with a smooth and perfect surface. The situation is explained in the following diagram:

Let ABCD is a set of rays striking a mirror MM'. Obviously, the rays of parallel to each other. When each individual ray strikes the mirror it is reflected as per law of reflection, that is the angle of incidence and the angle of reflection are same for each ray. Thus the set of incident rays are reflected as a set of rays. In other words, a beam of light is reflected as a beam of light if the mirror is plane and its reflecting surface is smooth. This type of reflection of a set of rays is called Specular Reflection.
Thus, observing at this diagram we can Define Law of Reflection as:
When light falls on a surface which is plane, the angle with which the light falls on the surface is equal to the angle with which the light reflects back and the incident ray, reflected ray and the normal ray lies in the same plane.

## Does the Law of Reflection Hold for Diffuse Reflection?

What happens if the reflecting surface is rough or curved? Does the law of reflection hold for diffuse reflection? Does the law of reflection hold for curved mirrors? Diffused Reflection takes place when the incident light falls on the rough surface.
Let us study the situation again considering each ray in a set of rays through the following diagram.

Consider a reflecting surface MM' which is highly exaggerated rough for the purpose of study. A set of rays ABCD strike the reflecting surface. But when you consider the rays individually, the infinitesimally small reflecting surface on which each ray falls is plane. Hence as far as that ray is concerned the law of reflection holds good and the reflection obeys the law that angle of incidence is equal to the angle of reflection. For example, the ray A is reflected as A'. Overall, the set of rays ABCD is reflected as A'B'C'D' as shown in the diagram.
It may be noticed that while the incident rays are parallel, the reflected rays emerge in different directions. This is called scatter reflection or diffuse reflection. Thus the law of reflection still holds in case of diffuse reflection but the net is result different from specular reflection.
Thus, the Law of reflection does not apply to Diffuse Reflection because the diffuse angle spreads to various angular range is different from the incident angle as it has only one ray.