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

Light and general laws of radiation

The concept of the light flux in ordinary optics corresponds to the radiation flux. This flow is characterized by the speed with which the radiant energy passes through the surface. It is measured in terms of power, i.e. in watts (joules per second). The radiation flux density characterizes the intensity of radiation emitted by the surface and is measured in watts per 1 m². To characterize the direction of radiation emitted by the surface, it is necessary to determine the concept of radiation in a given direction. This is the radiation flux in a given direction at a unit solid angle from the unit area of ​​the projection of the radiator to a plane perpendicular to the propagation of the beam.

Fig.1.2. Reduction of the duration of ultrashort laser pulses obtained by different methods.
Usually, radiation in a given direction is denoted by the symbol N and its meaning can be explained as follows. Let there be a radiating surface of area A and the direction is given by an angle to the normal drawn to this surface. Then the radiation flux in a small cone of steradians around the given direction is NAcos . If N does not depend on the direction, then it is customary to say that the surface radiates or scatters according to Lambert's law. In this case, the total radiation from the surface . The value of N is associated with the radiation energy density u, which is simply the radiation energy per unit volume. We recall the existence of filters and monochromators, with the help of which one can characterize the radiation from its frequency or wavelength. All quantitative characteristics of radiation can be represented as a function of frequency or wavelength .

As in physical experiments, and in the case of technical applications, it is customary to characterize radiation by its wavelength. However, in theoretical calculations, especially when dealing with energy, the concept of frequency is more often used. If the electromagnetic radiation inside the resonator is in thermodynamic equilibrium at an absolute temperature T K, then the frequency distribution of the radiation density follows Planck's law:

(1.1)
where is the Planck constant, k is the Boltzmann constant, and c is the speed of light.

The total radiation of a black body is determined by the Stefan-Boltzmann law:

(1.2)
· Vt/sm²·deq.⁴ .

It follows from what has been said that a heated solid is the source of radiation, and the radiated energy is not concentrated in any narrow frequency interval. Naturally, for each temperature there exists a wavelength for which the emitted radiation reaches its maximum value. This wavelength is determined by the Wien displacement law:

·10⁷ А⁰ К (1.3)
At a temperature of 5200 K, the maximum black body radiation is at a wavelength of 5575 A⁰, which corresponds to the center of the visible spectrum to which the human eye is most sensitive. Nevertheless, only 40% of this blackbody radiation falls on the visible part of the spectrum, about 6% - on the ultraviolet region, and the rest of the radiation falls on the infrared region.

Let us formulate the main limitations when using classical light sources (gas light sources, discharge arcs, ...).

The energy radiated by an intense source is distributed in a relatively wide spectral interval. There are no powerful monochromatic sources.

The radiated energy, as a rule, does not have an advantageous direction, and its collimation can not be carried out without loss in intensity.

The brightness of the image can not be greater than the brightness on the surface of an extended source.

Next, we will see how these restrictions will be removed in the case of coherent sources.