Absorption

ABSORPTION (from Latin absorptio — absorption). Absorption of radiant energy is the absorption of light and other electromagnetic waves by matter. If light falls on the surface of a certain material body, the energy of which we take as unity, then a part r of all this energy is reflected back, while a part 1-r enters the interior. In turn, a certain part a of the energy of the incident light may be absorbed inside the body. The remaining part s passes straight through. It is obvious that r+a+s=1. The greater a is, the greater the absorptive capacity of the body. It can be so great that the entire part 1-r is absorbed in the body. Such a body is opaque. Conversely, with complete transparency of the body, a=0; s=1-r. Each substance possesses a specific absorptive capacity, which moreover depends on the wavelength of the corresponding colored ray. If a substance possesses greater transparency for long waves (the red part of the spectrum), the body appears red in transmitted light; conversely, white light passing through a substance that transmits short waves best (the blue part of the spectrum) acquires a blue coloration. But besides the properties of the substance, absorption also depends on the thickness of the layer penetrated by the light. If the letter I₀ designates the energy of the incident light (minus the energy of the reflected light), and the letter I_x designates the energy of the light passing through a layer of thickness x, it turns out that I_x = I₀ · e^-kx. The magnitude k, depending on the substance and the wavelength of the corresponding colored ray, is called the absorption coefficient (e is the so-called base of the natural system of logarithms). If the dependence of the absorption coefficient on the wavelength of light is depicted graphically, curves are obtained with characteristic maxima indicating the position of so-called absorption bands. Often such bands are located at a great distance from one another. In such cases, the absorption curve usually resembles the so-called resonance curve, which expresses the relationship between the amplitude of forced vibrations of some system and their frequency: at a certain frequency, a sharply pronounced maximum is observed on the curve. This is the frequency to which the given system resonates. The similarity here is not accidental, since the absorption bands are caused by a kind of resonance of electrons oscillating inside atoms. Molecular absorption (more frequently adsorption) is the condensation of molecules (and sometimes ions) at the boundary between two substances: gas and liquid, gas and solid, liquid and solid. The body on whose surface absorption occurs is called the adsorbent. The molecules of the substance being adsorbed, falling into the sphere of action of attractive forces directed toward the molecules of the adsorbent, begin to gather near the interface and form a thin layer on it (according to some authors, one molecule thick, i.e., 10^-8 cm). Thermal motion tends to destroy this layer, and dynamic equilibrium is established only at a certain completely definite ratio between the concentration of adsorbed molecules in the surface layer and their concentration in the surrounding liquid or gaseous medium. The phenomenon of adsorption is used, among other things, in the construction of gas masks, as well as deodorizers.