Absorption of light
It is the dissipation of light i.e. radiation within a surface or medium caused by the conversion of radiant (luminous) energy to a different form of energy, typically heat, by interaction with matter. Absorption is the missing portion of energy when comparing the total reflected and transmitted energy with the incident energy. In absorption, the frequency of the incoming light wave is at or close to the energy levels of the electrons in the matter. The electrons will absorb the energy of the light wave and change their energy state. There are following options as what can happen next:
- Either the electron returns to the ground state emitting the photon of light or
- The energy is retained by the matter and the light is absorbed
If the photon is immediately re-emitted the photon is effectively reflected or scattered. If the photon energy is absorbed the energy from the photon typically manifests itself as heating the matter up. The absorption of light makes an object dark or opaque to the wavelengths or colors of the incoming wave. Some materials are opaque to some wavelengths of light, but transparent to others like
- Wood is opaque to visible light.
- Glass and water are opaque to ultraviolet light, but transparent to visible light.
The material composition and properties can be understood by knowing which wavelengths of light are absorbed by a material. Another way to make absorption of light evident is by their color. If a material or matter absorbs light of certain wavelengths or colors of the spectrum, an observer will not see these colors in the reflected light. On the other hand if certain wavelengths of colors are reflected from the material, an observer will see them and see the material in those colors. For example, the leaves of green plants contain a pigment called chlorophyll, which absorbs the blue and red colors of the spectrum and reflects the green. Leaves therefore appear green.
Absorptance also referred to as absorption factor is the property of a body which determines the fraction of the incident radiation or sound flux absorbed or absorbable by the body. It is defined as the ratio of the luminous or radiant flux absorbed by a body to the flux falling on it. It is usually represented by the Greek letter α (alpha). It was previously known as absorptivity. In the case of a blackbody, absorptance i.e. α = 1. Standard unit of absorptance is percent (%) or a factor between 0 and 1.
It is the fraction of light absorbed per unit distance in a participating medium. The inverse of the absorption coefficient i.e. 1/α is the average distance traveled by a photon before it gets absorbed. In other words, it is the distance at which its intensity of light is reduced to a value 1/e i.e. 36% of its original intensity. In general, based on their absorption strength materials are classified as
For example, a glass is transparent, whereas a silicon wafer is opaque to visible-spectrum light. A transparent material has a smaller absorption coefficient than an opaque material.
The absorption coefficient determines how far into a material light of a particular wavelength can penetrate before it is absorbed. In a material with a low absorption coefficient, light is only inadequately absorbed, and if the material is thin enough, it will appear transparent to that wavelength. The absorption coefficient depends on the material and also on the wavelength of light which is being absorbed. Semiconductor materials have a sharp edge in their absorption coefficient, in view of the fact that light which has energy below the band gap does not have sufficient energy to raise an electron across the band gap. Accordingly this light is not absorbed. Standard unit of the absorption coefficient is fraction per meter (/m).