A measure of the amount of light absorbed or scattered by a substance, it quantifies how opaque a material is to electromagnetic radiation, specifically light. It is typically expressed as the logarithm of the ratio of incident light to transmitted light. For example, a solution with a high value indicates that less light passes through it, suggesting a greater concentration of the light-absorbing or scattering component.
This concept is crucial across various scientific disciplines, including physics, chemistry, and biology. It provides a non-destructive method for determining the concentration of solutions, assessing the quality of optical components, and analyzing the growth of microbial cultures. Historically, its development aided in the advancement of spectrophotometry, a technique essential for quantitative analysis and characterization of materials.