Light absorption

A physical interaction in which light gives off its energy to matter is referred to as light absorption . Light absorption is a special case of the more general physical phenomenon of absorption . When electromagnetic radiation is absorbed, an electron of an atom changes from an energetically more favorable state to a state with higher energy, which happens through the "electron jump". One speaks of electronic excitation. The reverse process to light absorption is spontaneous light emission . Light is emitted by matter, whereby the internal energy of the matter decreases by the corresponding amount of energy.

Historical meaning

The special importance of light absorption as a particularly important source of energy in the earth's ecosystem is reflected in the history of the development of physics. In 1864 almost all the phenomena about magnetism, electricity and light observed up to that point were standardized by the theory of classical electrodynamics, and in 1888 Heinrich Hertz succeeded in experimentally confirming the predictions of this theory in key points.

Max Planck's improvement to Wilhelm Wien's energy distribution law of black body radiation, which became famous after its publication in 1900 as Planck's law of radiation , however, showed a discontinuity. While the leading physicists of that time endeavored to explain the auxiliary quantity of Planck's quantum of action introduced by Planck in the classical context, Albert Einstein proposed the photon hypothesis in 1905 , according to which this quantum of action ( ) represents the physical reality of a light particle of energy ( = frequency of light in the wave image). ${\ displaystyle h}$ ${\ displaystyle E = h \ nu}$ ${\ displaystyle \ nu}$

Einstein not only succeeded in explaining the classically indescribable photo effect with a closed quantum approach. In addition, a few years later he applied the thermodynamic energy balance for the emission and absorption of light by matter based on the findings of Ludwig Boltzmann . In it Einstein showed that the classical representation of the interaction of light and matter is incomplete. The previously known interactions absorption and spontaneous emission lead in this energy balance to an occupation of the energy levels of the matter interacting with light, which, according to Boltzmann, does not occur in nature. Einstein recognized the need to adopt a previously unknown process of stimulated emission , which creates a thermodynamic equilibrium for the interaction of light with matter in the Boltzmann sense. He specified the previously known emission of light as spontaneous emission. From this view of Einstein followed decades later the development of the burl and laser .

However, Einstein's contribution in the context of basic research is more important than this technological aspect. The complete and consistent explanation of light absorption as a quantum effect by Einstein represented the actual beginning of quantum physics .

Examples

Physics :

The emission spectra of the stars show characteristic flaws. Joseph von Fraunhofer found, measured and cataloged many such Fraunhofer's lines for our sunlight. Physicists who worked later were able to explain this with the characteristic light absorption of certain atoms in the photosphere .
Dark bodies absorb more light than light ones, so they warm up faster in the sun.

Technique :

Most color pigments get their appearance by mainly absorbing the light of the complementary colors.
In the photoelectric effect of solar cells , the individual absorption processes of light quanta in a semiconductor provide electrical charge separation and represent a power source .
The same effect is used electronically in photodiodes to measure the brightness or to switch a switch when light falls.
Solar collectors absorb light and thus heat a reservoir from which useful heat can be drawn at a later point in time .
Yellow glasses absorb the blue components of light that are more strongly scattered by haze.

Biology :

During photosynthesis, the energy of the absorbed photon excites a chlorophyll molecule and thus triggers a reaction, at the end of which the chemically largely stable energy carrier sugar is created.
Rhodopsin , the visual pigment, absorbs the light that is visible to it and uses the energy for chemical reactions that lead to an action potential in the adjacent nerve cell.
Reptiles sun themselves and use the energy of the light absorbed in this way to reach the most favorable temperature for them.

Chemistry :

Some dyes show different acidity in aqueous solution by changing color or color .
Some berries and plums appear red when unripe (and sour), lose acid as they ripen, and turn blue or black (and sweet).
By chemical splitting off of water, suitable organic molecules form (additional) double bonds, whereby absorption bands increasingly extend from the UV into the blue and further into the visible range ( bathochromic effect !).
Maillard reaction: starch, sugar and protein become increasingly yellowish to brown and black when heated in the kitchen when they are cooked hot.
Synthetic dyes contain chromophores , the absorption of which is in the visible region of the spectrum.

Individual evidence

↑ Reinhard Baltes, Werner Matissek: Lebensmittelchemie , 8th edition: ISBN 3662471116 .

literature

Peter W. Atkins: Physical Chemistry . VCH, 1987/88, ISBN 3-527-25913-9 , p. 305.
Georg Wittke: dye chemistry . 3rd edition, Diesterweg, Frankfurt am Main 1992, ISBN 3-425-05368-X .
HGO Becker: Introduction to Photochemistry . Deutscher Verlag der Wissenschaften, 1991, ISBN 3-326-00604-7 .
Paul A. Tipler, Gene Mosca: Physics for Scientists and Engineers . 2nd edition, 2004, ISBN 3-8274-1164-5 , pp. 994-996.

[1] Reinhard Baltes, Werner Matissek: Lebensmittelchemie , 8th edition: ISBN 3662471116 .