Reflection is the change in direction of a wave at a boundary between two different media, so that the wave moves back into the medium it came from.

The most common examples include the reflection of light, sound and water waves.

File:Reflection angles.svg
Diagram showing specular reflection

Specular reflection is a mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction. In the diagram to the right, a light ray is coming from the point P, and meets at the mirror at point O. The ray reflects at the point O outwards in the same medium towards point Q. Reflection is measured relative to a line through point O that is at 90˚ to the mirror. The angles made by the light ray (shown in the diagram as <math>\theta _i</math> and <math>\theta _r</math>) must be equal. Both angles have to be equal whenever reflection occurs. This is known as the "law of reflection".