Light and general radiation laws. Coherence and incoherence. Emission, absorption and amplification of radiation. Units and physical constants

Coherence and incoherence

In the previous section, we discussed only those properties of light that can be described in terms of intensity. Being electromagnetic radiation, light is characterized not only by intensity, but also by polarization and phase. Leaving the polarization aside for the time being, we will follow the problems concerning the phase.

Evidence of the phase existence is proved by classical interference experiments in which it is demonstrated that the amplitudes of two rays from one point source after passing through the rays of different distances should be added according to the known rules of vector addition, the direction of the added vectors depending on the length of the traversed paths.

A common characteristic of all classical sources of light is the lack of coherence between light waves emitted from different points of the radiator. By the term "coherence", or "spatial coherence", we mean the correlation of phases of monochromatic radiation emitted from two different points. We accept, as the established experimental fact, that light emitted from two points of an ordinary source, the distance between which is greater than the wavelength of the radiation, can not give interference, even if extremely narrow "monochromatic" components are isolated. From this fact, we conclude that there is no correlation between the phases of spatially separated radiators.

The other side of the problem is coherence over time. Under normal conditions, interference can be observed when a light emitted from one point of a source is separated and then the separated rays are combined in one region after they have passed through different paths, if only the lengths of these paths differ by not more than a few centimeters. Interference is not observed if the path length difference exceeds, say, 30 sm, since the phase of radiation is not conserved by the source during the time of passage of light by this distance. In an ideal monochromatic wave, the amplitude of the oscillations at any fixed point is constant, while the phase varies linearly with time. This case does not take place in a wave produced by a real source; the amplitude and phase undergo chaotic fluctuations whose velocity depends on the width of the spectrum . The time of coherence is the interval of time . During a time interval much shorter than , the radiation is almost a monochromatic wave, which is unfair for a longer time interval.

So, summarizing the above, we can add that the electromagnetic wave is called monochromatic, if the components of the electromagnetic field E and H vectors of the electromagnetic wave make harmonic oscillations of the same frequency, called the frequency of the wave. In the nature of strictly monochromatic radiation does not exist.

Two waves are called coherent if the difference in their phases is independent of time. This condition is satisfied by monochromatic waves whose frequencies are the same.

Two waves are called incoherent if the phase difference varies with time. Monochromatic waves of different frequencies, as well as waves consisting of a number of groups of wave trains, starting and terminating independently of each other with random phase values ​​at the time of the onset and termination of each group, are incoherent.

An almost coherent beam can be focused into a spot with dimensions on the order of the wavelength. If a coherent beam is obtained, then its energy can be concentrated, and the degree of concentration depends on the degree of coherence of the beam.

Now we can evaluate some of the advantages of a coherent or almost coherent source that produces radiation in the form of a spherical or plane wave of a limited cross section. This radiation can be concentrated with the help of lenses and mirrors into an image whose brightness is greater than the brightness of the original source. Moreover, the radiation emitted by the source in the form of an almost plane wave can be directed to a remote object with very small diffraction losses, while only a small part of the radiation from an incoherent source can be transformed into an almost plane wave.