Reflection of waves

Waves - including sound and light - can be reflected at the boundary between two different materials. The reflection of sound causes echoes.

The law of reflection states that:

angle of incidence = angle of reflection

For example, if a light ray hits a surface at 32°, it will be reflected at 32°.

The angles of incidence and reflection are measured between the light ray and the normal - an imaginary line at 90° to the surface. The diagrams show a water wave being reflected at a barrier, and a light ray being reflected at a plane mirror.

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

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 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 are at different angles.

Diffuse reflection or scattering from a rough surface