Absorption
Absorption is the taking in of wave energy by a material, converting light or sound into other forms, usually thermal energy.
Definition
Absorption is when a material soaks up light or sound instead of bouncing it back or letting it through, with the energy usually turning into heat, and the colors seen are the ones not absorbed: a green leaf absorbs red and blue light and bounces green back to the eye. It is the process by which a material converts wave energy into another form, most often thermal energy, with dark, rough surfaces absorbing more light, and an object that absorbs all visible light appearing black; thick curtains and foam absorb sound the same way. Absorption occurs when a photon's (or phonon's, for sound) energy transfers to the medium, exciting electrons, molecular vibrations, or lattice modes, with intensity decaying exponentially with depth by the Beer-Lambert law, $I = I_0 e^{-\alpha x}$, where the absorption coefficient $\alpha$ is wavelength-dependent because materials absorb strongly only where photon energy matches their internal transitions; Kirchhoff's law states that emissivity equals absorptivity at each wavelength, so a perfect absorber (a blackbody, $\epsilon = 1$) is also the ideal thermal emitter, and absorption spectra reveal composition from stellar atmospheres to blood oxygen levels.
Example
A black T-shirt gets hot in the sun because it absorbs almost all the light hitting it, while a white shirt stays cooler by reflecting most of it; on a clear day delivering about $1{,}000\ \text{W/m}^2$, a black car roof absorbing $90\%$ takes in $900\ \text{W}$ per square meter versus $200\ \text{W}$ for a white roof absorbing $20\%$, which is why the black roof can feel $20$ degrees hotter. Clear ocean water has $\alpha \approx 0.02\ \text{m}^{-1}$ for blue light but about $0.4\ \text{m}^{-1}$ for red, so at $10\ \text{m}$ depth blue retains $e^{-0.2} \approx 82\%$ of its surface intensity while red retains only $e^{-4} \approx 1.8\%$, which is why underwater scenes look blue and a red fish looks gray at depth.
Key Insight
Good absorbers of radiation are also good emitters, which is why a black pot cools faster than a shiny one once it leaves the stove. Greenhouse gases work by selective absorption: carbon dioxide is transparent to incoming visible sunlight but absorbs strongly at the $15\ \mu\text{m}$ infrared band Earth radiates, trapping outgoing heat while letting incoming light through.