Quantum mechanics is able to resolve this dilemma by assigning probabilistic characteristics to the motions of atoms and photons. The energy associated with an individual photon is quite minute. The term monochromatic is applied to the idealized situation in which the light in a beam is all of one wavelength. The visual response to intensity is brightness. There is some speculation that certain migrating birds may also respond to this quality of light.
Coherence is therefore an interferometric property of light.
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. On the other hand, light from a continuous-wave gas laser has produced fringes at a distance of over 100 meters.