REFLECTION. When waves of any kind travelling in one medium arrive at another in which their velocity is different, part of their energy is, in general, turned back into the first medium. This is termed reflection. If the surface of separation is smooth, i.e., if the irregularities in it are small compared with the wave length of the incident disturbance, the reflection is regular; if the surface is rough each facet reflects the rays incident upon it in accordance with the laws of regular reflection (see LIGHT) and the reflection, as a whole is irregular or diffuse. Consider, for example, the reflection of light at smooth and ruffled water surfaces. From the ruffled surface rays enter the eye after reflection from suitably inclined facets distributed over a wide area on the surface and a diffuse band of light is seen, while from the smooth surface only those rays reflected at the requisite angle from a small area on the surface can reach the eye.
In the first case the optical system of the eye focusses the water surface on the retina just as it does all other rough surfaces. In the latter the rays appear to come from a laterally inverted replica of the luminous source situated as far below the surface as the source itself is above, and it is this image which is then focussed on the retina. The diffusion of light by matt surfaces in general is an example of the same phenomenon differing only in the number and size of the facets which scatter the light.
Echoes and reverberations are caused by the reflection of sound (q.v.) and the "fading" of wireless signals (in part) by variations in the conditions under which the electromagnetic waves are reflected by the Heaviside layer in the upper atmosphere. (See WIRELESS TELEGRAPHY.)