Coherence is attained when there is a constant phase difference in two or more waves over time.

Two waves are said to be in phase if their crests and troughs meet at the same place at the same time, and the waves are out of phase if the crests of one meet the troughs of another.

The waves are incoherent if the crests and troughs meet randomly.

To be coherent waves must be:

• of the same frequency - monochromatic - temporally in phase
• have a constant phase difference - have a constant spatial phase relationship

Laser light is coherent.

A 'red light' can be monochromatic but still be incoherent because the waves are in random phase... it can also be a range of frequencies in the 'red' region of the spectrum!

White light is incoherent both because the phase of the waves are random and because white light is made of many different frequencies simultaneously.

If a laser is not available for an experiment it is possible to use a sodium lamp (virtually monochromatic) and a fine slit to create the spatial coherence.

There are two types of coherence - spatial and temporal coherence.These are studied at degree level NOT at 'A' Level.

Spatial coherence describes the correlation between signals at different points in space. Spatial coherence means a strong correlation (fixed phase relationship) between the electric fields at different locations. For example, within a cross section of a beam from a laser with diffraction-limited beam quality, the electric fields at different positions oscillate in a totally correlated way, even if the temporal structure is complicated by a superposition of different frequency components. Spatial coherence is the essential prerequisite of the strong directionality of laser beams.

Temporal coherence describes the correlation or predictable relationship between signals observed at different moments in time. Temporal coherence means a strong correlation between the electric fields at one location but different times. For example, the output of a single-frequency laser can exhibit a very high temporal coherence, as the electric field temporally evolves in a highly predictable fashion.