Light is characterized not only by wavelength, essentially a temporal quality, but also by state and degree of polarization , a geometric or directional quality, and by intensity, essentially a physical quality. The visual response to intensity is brightness. There is some speculation that certain migrating birds may also respond to this quality of light. Light is further characterized by its degree of coherence . Coherence, closely related to the degree of polarization and to the degree of monochromaticity, refers to the ability of a beam of light to interferewith itself.
Coherence is therefore an interferometric property of light.
By the use of a Michelson Interferometer, most light sources can be made to produce interference fringes. These are clearest when the length of the two arms of the interferometer are equal. As one arm is lengthened, however, the contrast of the fringes is seen to decrease until they are no longer visible. Unfiltered light from an incandescent source will barely produce fringes under any circumstances. Light from a mercury arc lamp will produce fringes over a range of one or two centimeters.