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

Units and physical constants

In principle, it would be desirable to take any one system of units and always use it. However, it is difficult to follow such a method in describing the subject chosen by us, since we must use both atomic units and technical ones. Thus, the wavelength of the visible radiation is usually expressed in angstroms (10^-8 sm), whereas in the infrared region, the wavelength in microns (10^-4 sm) is usually expressed. Nobody measures the energy of atomic levels in ergs or joules. They are expressed either in electron-volts (1.602 • 10^-12 ergs), or in reverse centi-meters. The origin of the unit of the inverse centimeter follows from the relation , which can be written as

(1.8)

The quantity E/hc has the dimension of reciprocal length. Usually the inverse centimeters denote energy, although more accurately they should designate the wave number. Tables of energy levels in inverse centimeters allow us to directly subtract two tabulated values ​​to obtain an inverse wavelength (in vacuum) corresponding to the transition between these levels. One electron-volt is equivalent to 8066 reverse centimeters. If the energy of the level E is expressed in inverse centimeters and it is necessary to find the energy required to transfer the gram-atom of the substance from the ground state to this state, then the value of E must be multiplied by Nhc, where N = 6.02 • 10²³ is the number of Avogadro and hc = 1.986 • 10^-16 erg • sm. Numerical values ​​of wavelengths, frequencies, wave numbers and quantum energies are given in Table 1.1.