Waves can be reflected at the boundary between different mediums. When a wave is reflected, it stays in the same medium instead of passing through.

Light reflection is when light waves bounce off a surface and change direction.

Diffuse and Specular Reflection

Specular reflection

Reflection from smooth, flat surfaces is known as specular reflection. In this case, light meeting the surface in one direction is reflected in only one direction. This can include surfaces like mirrors or calm water and in specular reflection, an image can form.

An example of specular reflection is in the picture below.

Specular reflection is taking place because the water is smooth and calm. So, there is a clear image of the trees and mountains in the water.

Diffuse reflection

Diffuse reflection occurs when a surface is rough. Each light ray is still reflected normally, but the reflected light is scattered in many different directions due to the rough surface. This is why an image does not form when diffuse reflection occurs.

For example, when you look at a rough wall, you do not see your reflection.

Reflection Ray Diagrams

We can use ray diagrams to show how light travels and what happens when it reaches a surface (boundary).

In the diagram above, the grey block is a mirror, which is called a plane mirror because it is a flat surface. The dashes at the bottom show the back side of the mirror.

  • On the left, there is a light ray going towards the mirror, which we call the incident ray
  • On the right, there is a light ray going away from the mirror, which we call the reflected ray

The incident ray hits the surface of the mirror and is reflected back as the reflected ray.

In ray diagrams, we draw a vertical line at a 90° angle to the mirror, which is called the normal. We use this line to calculate the angle of incidence and the angle of reflection.

  • The angle of incidence is the angle between the normal line and the incident ray
  • The angle of reflection is between the normal line and the reflection ray

The law of reflection states that:

The angle of incidence = The angle of reflection

So, if the angle of incidence is 40°, the law of reflection states that the angle of reflection is also 40°. They must always be equal.