Reflection, the bending back of waves, such as light waves, radio waves, or sound waves, from a surface. Reflection plays an essential role in radar, sonar, numerous optical devices, and some types of heat insulation.

The angle at which the waves are reflected from a smooth surface is equal to the angle at which they strike the surface. For example, if a beam of light strikes a mirror at an angle of 40°, the beam will be reflected at an angle of 40° in the opposite direction. Reflection from a smooth, polished surface, such as that of a mirror, is called regular, or specular, reflection. When the reflecting surface is rough, the reflected waves are scattered in all directions; this type of reflection is called diffuse reflection.

The reflection of light makes it possible to see objects. An object's color results from the way the materials at its surface reflect some wavelengths of light and absorb others.

Under certain conditions, waves are completely reflected from a boundary between two transparent materials. This phenomenon, called total internal reflection, is used in fiber optics.