rayAbsorption

Ray absorption is the process by which radiant energy is removed from a beam as it travels through a medium, typically converted into heat by the medium’s atoms or molecules. It is one mechanism that contributes to the attenuation of light; the others include scattering and reflection. When photons are absorbed, their energy excites electronic, vibrational, or rotational transitions within the material. The rate of absorption depends on wavelength and the material’s composition and structure.

Quantitative description is given by the absorption coefficient mu_a(lambda) and the Beer-Lambert law. In a homogeneous medium, the intensity I of radiation of wavelength lambda decays with path length x as I(x, lambda) = I0(lambda) exp[-mu_a(lambda) x]. The optical depth tau(lambda) is defined as tau = ∫ mu_a ds, and transmittance T = exp(-tau). In more general cases that include scattering, the radiative transfer equation is used, with absorption and scattering terms.

Absorption is strongly wavelength-dependent. Materials with discrete energy levels show absorption lines or bands; broad absorption arises from vibrational or electronic transitions. The color of an object arises from the wavelengths it transmits or reflects; for example, a substance that absorbs strongly in the blue may appear yellow or red.

Applications and contexts include atmospheric science (absorption bands by gases such as ozone and water vapor), spectroscopy (characterizing materials via absorption spectra), medical imaging (X-ray and infrared absorption contrasts), astronomy (intrinsic and interstellar absorption), and solar energy devices (absorption of photons to generate heat or electricity).

Measurement units: mu_a has units of inverse length. The concept of optical depth tau provides a dimensionless measure of absorption along a path. Limitations: simple exponential models assume a uniform medium and neglect scattering; real media may require full radiative transfer treatments.