One of the most well-known forms of diffraction is single-slit diffraction, which occurs when waves pass through a narrow gap. The waves bend and spread from the gap, and if coherent?, may cause a pattern of interference beyond the gap (known as a diffraction pattern). Diffraction depends on the size of the gap compared to the wavelength of the waves involved; the greatest effects are observed when the gap equals the wavelength.
From monochromatic waves of wavelength λ incident on a slit of width d, the intensity I of the diffracted waves at an angle θ is given by:
where sinc function is given by sinc(x) = sin(x)/x. If there are two or more slits through which the waves pass, a more complex diffraction pattern occurs which is the result of the coherent superposition of the diffraction patterns from each slit. A two-slit experiment is described in Young's? Double-slit experiment.
Light does not have to pass through an aperture to diffract; a beam of light of a finite size also undergoes diffraction and spreads in diameter. This affect limits the minimum size d of spot of light formed at the focus of a lens:
where λ is the wavelength of the light, f is the focal length of the lens, and D is the diameter of the lens. (See [Rayleigh criterion]?).
Diffraction also occurs when waves are scattered from a periodic structure, such as atoms in a crystal or rulings on a [diffraction grating]?. Each scattering center (e.g., each atom) acts as a point source of spherical wavefronts; these wavefronts undergo constructive interference to form a number of diffracted beams. The direction of these beams is described by Bragg's law:
where λ is the wavelength, d is the distance between scattering centers, θ is the angle of diffraction and n is an integer known as the order of the diffracted beam. Bragg diffraction is used in X-ray crystallography to deduce the structure of a crystal from the angles at which X-rays are diffracted from it.
The most common demonstration of diffraction is the spectrum? of colors seen reflected from a compact disc: the closely-spaced tracks on the surface of the disc form a diffraction grating, and the individual wavelengths of white light are diffracted at different angles from it, in accordance with Bragg's law.