Ray diagrams

Specular reflection

Reflection from a smooth, flat surface is called specular reflection. This is the type of reflection that happens with a flat mirror. The image in a mirror is:

  • upright
  • virtual
  • the same distance behind the mirror as the object is in front

In a virtual image, the rays appear to diverge from behind the mirror, so the image appears to come from behind the mirror.

A ray diagram showing light rays coming from an object, reflecting at the surface normal of a plane mirror and producing an image in the eye.A ray diagram showing how an image forms in a plane mirror

Diffuse reflection

If a surface is rough, diffuse reflection happens. Instead of forming an image, the reflected light is scattered in all directions. This may cause a distorted image of the object, as occurs with rippling water, or no image at all. Each individual reflection still obeys the law of reflection, but the different parts of the rough surface reflect at different angles.

Light rays are directed onto the cross section of a rough surface and are reflected off in different directions.Diffuse reflection or scattering from a rough surface

Refraction

Refraction is the change in direction of a wave at a boundary.

It is important to be able to draw ray diagrams to show the refraction of light at a boundary.

A light ray travels from air to glass. The angle of incidence is 55 degrees. The angle of refraction is 33 degrees.A ray diagram showing refraction at the boundary between air and glass

Dispersion

When light is refracted as it enters a prism, each colour is refracted by a different amount. Because each colour has a different wavelength, the speed of each colour is different. This means that the light leaving a prism is separated into its spectrum colours. This a process called dispersion.

Graphic showing a prism. White light disperses within the glass prism to produce a visible spectrum of colours.