Index: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Home > Coherence (physics)
Coherence is a property of waves that measures the ability of the waves to interfere with each other. Two waves that are coherent can be combined to produce an unmoving distribution of constructive and destructive interference (a visible interference pattern) depending on the relative phase of the waves at their meeting point. Waves that are incoherent, when combined, produce rapidly moving areas of constructive and destructive interference and therefore do not produce a visible interference pattern.A wave can also be coherent with itself, a property known as temporal coherence. If a wave is combined with a delayed copy of itself (as in a Michelson interferometer), the duration of the delay over which it produces visible interference is known as the coherence time of the wave, Δtc. From this, a corresponding coherence length can be calculated:
-
where
c is the speed of the wave.
The temporal coherence of a wave is related to the spectral bandwidth of the source. A truly monochromatic (single frequency) wave would have an infinite coherence time and length. In practice, no wave is truly monochromatic (since this requires a wavetrain of infinite duration), but in general, the coherence time of the source is inversely proportional to its bandwidth.
Waves also have the related property of spatial coherence; this is the ability of any one spatial position of the wavefront to interfer with any other spatial position. Young's double-slit experiment relies on spatial coherence of the beam illuminating the two slits; if the beam was spatially incoherent, i.e. if the sunlight was not first passed through a single slit, then no interference pattern would be seen.
Spatial coherence is high for sphere waves and plane waves, and therefore is related to the size of the light source. A point source of zero diameter emits spatially coherent light, while the light from a collection of point-sources (or from a source of finite diameter) would have lower coherence. Spatial coherence can be increased with a spatial filter; a very small pinhole preceded by a condenser lens. The spatial coherence of light will increase as it travels away from the source and becomes more like a sphere or plane wave. Light from distant stars, though far from monochromatic, has extremely high spatial coherence. The science of stellar interferometry relies on the coherence of starlight.
Light waves produced by a laser often have high temporal and spatial coherence (though the degree of coherence depends strongly on the exact properties of the laser). For example, a stabilised helium-neon laser can produce light with coherence lengths in excess of 5 m. Light from common sources (such as light bulbs) is not monochromatic and has a very short coherence length (~1 μmA micrometre ( American spelling: micrometer , symbol m is an SI unit of length. It is defined as one millionth of a metre (1×10−6 m), equivalent to one thousandth of a millimetre. The symbol µ ( Unicode character U+00B5; HTML µ) is the " micr), and can be considered totally temporally incoherent for most purposes. Spatial coherence of laser beams also manifests itself as speckle patternIn optical systems, a speckle pattern is a field- intensity pattern produced by the mutual interference of partially coherent beams that are subject to minute temporal and spatial fluctuations. Note: In a multimode fiber, a speckle pattern results from as and diffractionIn physics, diffraction is a wave phenomenon: the apparent bending and spreading of waves when they meet an obstruction. Diffraction occurs with electromagnetic waves, such as light and radio waves, and also in sound waves and water waves. Diffraction als fringes seen at the edges of shadow.
HolographyTraditionally, a holograph is a document written entirely in the handwriting of the person whose signature it bears. That is not what this article is about; this is about the more modern concept, not introduced until the 20th century. This article is also requires temporally and spatially coherent light. Its inventor, Dennis GaborDennis Gabor Gabor Denes ( 5th June, 1900, Budapest 9th February, 1979, London) was a Hungarian physicist who is most notable for inventing holography. Gabor was educated at Budapest and Berlin. Having fled from Nazi Germany in 1933, Gabor was invited to, produced successful holograms more than ten years before lasers were invented. To produce coherent light he passed the highly monochromatic light from a gas-discharge lampA Sodium Vapor Lamp is a gas discharge lamp which uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure''. Low Pressure / LPS / SOX LPS Lamps (Low Pressure Sodium), also known as SOX Lamps through a pinhole.
Wave mechanics
Read more »