Reflection of waves

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

Reflection at a plane boundary

A plane boundary, eg a plane mirror is a flat, smooth, reflective surface.

The law of reflection states that:

angle of incidence = angle of reflection

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

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

A ray diagram showing light rays coming from an object, reflecting at the surface normal of a plane mirror. the angles of incidence and the angles of reflection are also highlighted.

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 are 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