ABSORPTION (Pr., Absorption; Ger., Absorp tion) This term is used both in a chemical and an optical sense. In the former sense it is used to designate the taking up of one substance by another, just as d. sponge absorbs or sucks up water. As a rule, this is not accompanied by any chemical, but merely a physical change.
Optically, absorption is applied to the sup pression of light, and to it are due all colour effects (see " Colour"). It is of great import ance from a photographic point of view, as on Draper's law, according to which only those rays which are absorbed by a substance act chemically on it, is based the whole of the photo chemical action of light. Light, when absorbed, is not lost but is converted into some other form of energy, either heat or chemical action. The absorption spectra of dyes are of great interest, as by their aid it is possible to prepare colour filters of any given tint. Many substances and dyes have simple absorption spectra—that is to say, more or less well defined continuous por tions of the spectrum are absorbed ; other sub stances, on the other hand, such as chlorophyll, have complicated absorption spectra, which change in character according to the concen tration of the solution, or the depth of the sole tion, which is practically the same thing.
The position and shape of the absorption bands of a substance are in many cases so characteristic that they serve as a means of identification. Obviously, the most opaque substances are the metals, but even these are translucent in thin films ; silver, for instance, appears blue, whilst gold in thin films is green. Even such trans parent and colourless substances as water, alcohol, glycerine, etc., possess characteristic absorption spectra, and therefore appear coloured when in sufficiently thick films. In studying the absorption spectra of coloured solutions, either the visual or the photographic method may be used, and the latter will be found not only more reliable, but considerably quicker. The visual method can obviously be applied only to the visible portion of the spectrum, whilst by the aid of photography the ultra-violet and infra red regions can also be mapped out.
Dr. Kenneth Mees and S. H. Wratten, who have made a special study of dye absorption spectra by photographic means, give the following outline of the methods which may be adopted : " (I) One may take a series of photographs with increasing dilution of the dye ; (2) one may take a series of photographs with a constant concentration of the dye, but an increasing thickness of the cell ; (3) one may take a series of photographs with a constant con centration and constant cell thickness, but with a varying exposure. These three methods
will all produce results differing slightly, though (I) and (2) are nearly equivalent to one another. (i ) is a very slow method, and it would be probably quicker to use a spectro-photometer. (2) and (3), though quicker, are still slow if carried out as described. But if in method (2) instead of varying thicknesses of cell there is used a cell of which the thickness varies through out the length—that is to say, a wedge-shaped cell placed in front of the slit so that the thick ness of the layer of dye solution in front of the slit varies from end to end of the slit—the method resolves itself into taking one or possibly two photographs of each dye. Method (3) is inferior to the two other methods, as it involves the inter pretation of the photograph of the plate curve. It is, however, a convenient way of examining the absorptions of coloured films and filters." This method is most conveniently carried out by placing directly in front of the slit a small wedge of black glass so that the intensity of the light varies from end to end of the slit. This black wedge consists of a narrow prism of neutral tinted glass cemented to a similar prism of white glass, which of course destroys the prismatic effect by forming a parallel plate. With this the intensity of the light varies from i to io,000.
For visual measurement of absorption spectra a spectro-photometer is used. This consists of a spectroscope and some means of comparing the brightness of two spectra of one light source. This can be effected in several ways, as, for instance, by two slits, which can be independently opened or closed, or by polarising prisms. The disadvantage of the variable slit system is that the two spectra are of unequal purity, and therefore accurate readings are impossible. In the polarising spectrophotometers the slit is usually divided across the middle by a small bar of metal, and the two light beams are polarised and dispersed, or dispersed and polarised, equality being obtained by rotation of a Nicol prism. The two spectra are brought into juxtaposition at the eyepiece, and equality of illumination obtained throughout its length. As one spectrum is con tinuous and the other darkened by the absorption band, the former is reduced in brightness till the two are equal and the necessary readings obtained from the varied width of the slit or the angle through which the Nicols are turned. The transmitted light, divided by the incident light, which is always taken as unity, equals the extinction coefficient.