When light or sound waves hit a surface and bounce back, this is called reflection. Light reflection is a change in the direction of light waves that hit a reflective surface.

Keep in mind that reflective surfaces are smooth and shiny, like glass or a mirror. In the diagram above, the shiny silver blocks represents the mirror.

Diffuse and Specular Reflection

Specular reflection

Reflection from smooth surfaces is known as specular reflection. In this case, light meeting the surface in one direction is reflected in only one direction.

Specular reflection is taking place in the diagram below. Three light rays hit a smooth surface and bounce off in one direction.

This can include surfaces like mirrors or calm water, and in specular reflection, an image can form. An example of this is in the diagram below.

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

Diffuse reflection

However, reflection from a rough surface is known as diffuse reflection.

With diffuse reflection, the ray is scattered in many different directions. This is why an image does not form when light reflects from a rough surface.

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

Ray Diagrams

To see how light travels in reflection, we use ray diagrams. A ray diagram contains an arrow which shows the direction that the light travels.

In the diagram above, the grey block is a mirror, which is called a plane mirror as it’s a flat surface. The dashes at the bottom show the back side of the mirror, but diagonal lines can also be used.

  • On the left, you’ll see a light ray moving towards the mirror; this is known as the incident ray
  • On the right, you’ll see a light ray moving away from the mirror; this is known as the reflected ray

The incident ray hits the surface on 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; this line 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 the angle between the normal line and the reflected ray.

The law of reflection states:

The angle of incidence = The angle of reflection

This means that if the angle of incidence is 40°, according to the law of reflection, the angle of reflection must also be 40°. The two angles are always equal.