Monochromatic Light

an electromagnetic wave of one specific and strictly constant frequency in the frequency range directly perceivable by the human eye. The term “monochromatic light” originated because a person perceives a difference in the frequency of light waves as a difference in color. However, the electromagnetic waves of the visible region do not differ in physical nature from those of other regions (such as the infrared, ultraviolet, and X-ray regions). The term “monochromatic” is also applied to the other regions, although such waves do not produce any perception of color.

The term “monochromatic light” (like “monochromatic radiation” in general) is an idealization. Theoretical analysis shows that the emission of a strictly monochromatic wave should continue indefinitely. However, real radiation processes are limited with respect to time, and therefore waves of all frequencies that belong to a certain frequency interval are emitted simultaneously. The narrower the interval, the more monochromatic the radiation. Thus, the radiation of the individual lines of the emission spectra of free atoms (such as the atoms of a gas) is very close to monochromatic light. Each line corresponds to a transition of an atom from a state m (with higher energy) to a state n (with lower energy). If the energies of the states had strictly fixed values E_m and E_n, the atom would radiate monochromatic light with a frequency ν_mn = 2πω_mn = (E_m − E_n)/h. Here h is Planck’s constant, equal to 6.624 × 10⁻²⁷ erg · sec. However, an atom can stay in states with a higher energy only for a short time Δt (usually 10⁻⁸ sec, called the lifetime at the energy level), and according to the uncertainty principle, for the energy and lifetime of a quantum state (ΔEΔt ≥ h) the energy of a state m, for example, can have any value between E_m + ΔE and E_m − ΔE. Because of this, the radiation of each spectral line acquires a frequency “spread” Δν_mn = 2ΔE/h = 2/Δt.

During emission of light (or of electromagnetic radiation in other bands) by real sources, a set of transitions between different energy states may take place. Therefore, waves of many frequencies are present in such radiation. Instruments used to isolate narrow spectral intervals (radiation that is close to monochromatic) are called monochromators. Extraordinarily high monochromaticity is characteristic of the radiation of certain types of lasers (its spectral interval may be much narrower than that of the lines of atomic spectra).