- The spreading of light or other wave energy around obstacles. The longer the wave, the more the diffraction. Thus sound diffracts so easily that we are not surprised to hear noise from around a corner. Light, including starlight, also diffracts, especially around corners and edges of solid objects. The different rates of diffraction of light of different wavelengths is the principle behind the diffraction grating, which has replaced the glass prism for most spectrum-gathering. A diffraction grating consists of an array of fine lines ruled parallel to each other on a flat surface which can either reflect or transmit the light falling on it (to give a transmission or reflection grating). The diffraction of light around the edges of the lines on the grating produces the spectrum.
- The deviation of waves, and particles showing wavelike properties, around obstructions and edges. These include electromagnetic radiation, such as light waves or radio frequency signals, very small and rapidly moving particles, such as electrons or neutrons, and sound. Diffraction is a consequence of interference, and is most noticeable when the wavelength of the disturbance is comparable to the diameter of the obstacle. Diffraction provides, for instance, for sound to be heard around the corner of an object, or for light to be bent when incident upon the edge of an object.