Reflection

Reflection, in physics, phenomenon of wave motion, in which a wave is returned after impinging on a surface. When energy, such as light or sound, traveling from one medium encounters a different medium, part of the energy usually passes on while part is reflected. Regular reflection (in which the direction of the reflected wave front is sharply delineated) is governed by the law that both the incident, or striking, rays and the reflected rays travel in directions making equal angles with the normal, a line perpendicular to the reflecting surface at the point of incidence; and that the rays lie in the same plane as the normal. These angles are called the angle of incidence and the angle of reflection. Rough surfaces reflect in many directions, and such reflection is called diffuse.

To reflect a wave train, the reflecting surface must be wider than one-half the wavelength of the impinging waves. For example, a pile rising above the surface of the ocean may reflect ripples, but long waves pass around it. Shrill noises, which have very short wavelengths, are reflected by a thin windowpane, but sounds of longer wavelength pass through it. Small particles of dust in the atmosphere may reflect only the shorter blue wavelengths in sunlight.

For further discussion of the reflection of light, see Optics. For reflection of radio waves, see Ionosphere; Radio. See also Electromagnetic Radiation; Sound; Telescope.