opacity

Opacity is a measure of how much light is blocked as it passes through a material. An object with zero opacity is transparent, allowing all light to pass; complete opacity blocks most or all light. In physical terms, opacity relates to transmittance (the fraction of incident light that emerges on the far side) and to how the material absorbs or scatters light. The two main processes are absorption, where photons are removed by the material, and scattering, where photons are redirected out of the forward path. The combined effect is often described by an attenuation coefficient α, with light intensity I decaying roughly as I = I0 e^{-αx} along a path of length x. The optical response is also influenced by the complex refractive index n = n' − i k, where the imaginary part k reflects absorption.

In different contexts, opacity is used slightly differently. In computer graphics, opacity (or alpha) is a value between 0 and 1 that governs pixel visibility and blending with other layers; 0 means fully transparent, 1 means fully opaque. In materials testing and radiology, opacity helps characterize how tissues, minerals, or coatings attenuate diagnostic radiation, forming contrasts in images.

Opacity can be affected by wavelength, temperature, and surface roughness, leading to color changes and selective transmission. Understanding opacity is important for design of lenses, coatings, packaging, and visualization in science and art.