Optical absorption

In physics, absorption of electromagnetic radiation is the process by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other form of energy, for example, to heat. The absorption of light during wave propagation is often called attenuation.

Physical process
When a material is illuminated, photons can make the valence electrons of an atom transition to a higher electronic energy levels. The photon is destroyed in the process and the absorbed radiant energy is transformed to electric potential energy. Several things can happen then to the absorbed energy, as it may be re-emitted by the electron as radiant energy (in this case the overall effect is in fact a scattering of light), dissipated to the rest of the material (i.e. transformed into heat), or even the electron can be freed from the atom (as in the photoelectric and Compton effects).

For most substances, the amount of absorption varies with the wavelength of the light, since the energy of the incident photon must be similar to an allowed electronic transition. This leads to the appearance of colour in pigments that absorb some wavelengths but not others. For example, an object that absorbs blue, green and yellow light will appear red when viewed under white light.

Quantitative measurements
The absorbance of an object quantifies how much of the incident light is absorbed by it (not all photons get absorbed, some are reflected or refracted instead). This may be related to other properties of the object through the Beer-Lambert law.

Precise measurements of the absorbance at many wavelengths allow the identification of a substance via absorption spectroscopy, where a sample is illuminated from one side, and the intensity of the light that exits from the sample in every direction is measured.

Earth surface
The specific phenomena involving absorption of electromagnetic radiation at the Earth's surface have several important aspects. These phenomena include regulating the temperature of the Earth's crust, surface waters and lower atmosphere. Changes in the Earth's crust such as glaciation, deforestation, polar ice melting, will necessarily alter the quantity and wavelength selectivity of electromagnetic absorption at the Earth's surface; correspondingly, changes in climate such as global warming may accompany changes in electromagnetic absorption or its inverse, the albedo. Regulation of the temperature of surface waters has been analyzed with respect to the influence of electromagnetic radiation absorption, indicating the effects of total solar insolation and the local albedo.