diffraction

When waves pass around a barrier or through a gap, they spread out. The effect, known as diffraction, will be more pronounced at a narrow gap than at a wider gap.

The spreading out of waves when they pass through a small gap or around a small object, resulting in some change in the direction of the waves. In order for this effect to be observed, the size of the object or gap must be comparable to or smaller than the wavelength of the waves. Diffraction occurs with all forms of progressive waves – electromagnetic, sound, and water waves – and explains such phenomena as the ability of long-wave radio waves to bend around hills more easily than short-wave radio waves.

The wavelength of visible light ranges from about 400 to 700 nanometres, several orders of magnitude smaller than radio waves. The gap through which light travels must be extremely small for diffraction to be observed. If light passes through two narrow closely spaced parallel slits, the slight spreading of the light beam from each slit causes the different wavelengths of light to interfere with each other, producing a pattern of light and dark bands. A diffraction grating is a plate of glass or metal ruled with close (some diffraction gratings have from 2,000 to 7,000 lines per inch), equidistant, parallel lines used for separating a wave train such as a beam of incident light into its component frequencies. White light passing through a grating will be separated into its constituent colours. Red light is diffracted more as it has a longer wavelength; blue light is diffracted less as it has a shorter wavelength.

The wavelength of sound is between 0.5 m/1.6 ft and 2.0 m/6.6 ft. When sound waves travel through doorways or between buildings they are diffracted significantly, so that the sound is heard round corners.

The regularly-spaced atoms in crystals diffract X-rays, and by exploiting this fact the structure of many substances has been elucidated, including that of proteins (see X-ray diffraction).