optical absorption
/ OP-tih-kul ab-SORP-shun /
Leave a dark car in the summer sun and it turns into an oven; a white one stays far cooler. The dark paint is drinking in the sunlight while the white surface throws much of it back. That swallowing of light, with its energy turning into heat or into excited electrons, is optical absorption.
Optical absorption describes how a material soaks up light, and it is fussy about color. A particle of light, a photon, can only be absorbed if the material has somewhere to put its exact energy — an electron it can lift to a higher rung, or a vibration it can stir up. Photons whose energy matches an available jump get swallowed; those that don't pass through or bounce off. So each material has a characteristic absorption pattern across the rainbow, strong at some wavelengths and transparent at others, fixed by the spacing of its energy levels.
This matters because absorption is half the story of how everything looks and how light-based technology works: a solar cell must absorb sunlight to harvest it, a camera sensor must absorb light to record it, and a window must not absorb visible light to stay clear. The common confusion is to think a material is simply dark or clear; in truth almost everything is transparent to some wavelengths and opaque to others — window glass blocks ultraviolet, and many things that look black to our eyes are bright in the infrared.
Plants look green because chlorophyll greedily absorbs red and blue light to power photosynthesis but barely touches green, so the green is reflected back to your eye. The leaf's color is literally the light it refused to drink.
A leaf is green precisely because green is the color it does not absorb.
Absorption, reflection, and transmission always add up to the whole incoming light. A surface that absorbs little must reflect or transmit the rest — that is why a low-absorption material is either shiny like a mirror or clear like glass.