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Coherent and incoherent sources

Coherent light is light in which the photons are all in 'step' other words the change of phase within the beam occurs for all the photons at the same time. There are no abrupt phase changes within the beam. Light produced by lasers is both coherent and monochromatic (of one 'colour').

Incoherent sources emit light with frequent and random changes of phase between the photons. (Tungsten filament lamps and 'ordinary' fluorescent tubes emit incoherent light).

Conventional light sources are incoherent sources. The transitions between energy levels in an atom is a completely random process and so we have no control over when an atom is going to lose energy in the form of radiation.

The light that comes from a laser, however, is coherent, parallel, monochromatic and in unbroken wave chains.

We can make a normal light source more coherent by making it smaller, so reducing the number of atoms that may emit quanta, but if we do this the intensity is reduced.

Researchers then had the idea of using atoms and molecules as the resonant structures, but unfortunately the power available from just one electron transition is very small and it only occurs intermittently. Scientists therefore had to try and persuade all the atoms in a specimen to react simultaneously since this would produce a powerful coherent wave. This led to the development of the laser.


© Keith Gibbs