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

Keep in mind, the reflective surface is smooth and shiny, such as a glass or mirror.

In the diagram above, the shiny silver block is the mirror.

There are four light rays hitting the surface that bounce off, which is called reflection. However, reflection from a rough surface is diffuse reflection. This is where the ray is scattered in many different directions, which 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.

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. 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.

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, 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 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. 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:

The angle of incidence = The angle of reflection

This means that 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.