Opacity refers to the extent to which an object or surface hinders or blocks light from passing through and so obstructs light from reaching objects or space beyond.

  • Opacity can be caused by various factors, such as absorption, reflection, and scattering.
  • An entirely opaque substance reflects and absorbs all incident light, with no transmission or scattering.
  • When light strikes an interface between two media, some light is reflected, some is absorbed, and some is scattered:
    • The remaining light undergoes refraction and is transmitted through the second medium.
    • Opacity therefore measures the ability of the second medium to obstruct light.
  • An opaque object is neither transparent (allowing all light to pass through) nor translucent (allowing some light to pass through).
  • Opacity of some media varies with the wavelengths of light. For example, certain types of glass are transparent in the visual range but mostly opaque to ultraviolet light.