Opacity

Opacity is the property of a substance or an object to block light, i.e., to prevent light from getting through it. In contrast, an object transmitting light can be called transparent or at least translucent.

There can be perfect opacity, i.e., complete blocking of light, or some degree of opacity, meaning that light is more or less attenuated. Partial opacity is in most cases substantially dependent on the considered optical frequency. An object or a material may be completely opaque in certain spectral regions, while well transmitting light in other spectral regions. Light outside the visible spectral range may be considered. For example, semiconductors are opaque in the visible spectral regions while substantially transmitting light in certain regions of the infrared.

Opacity is often understood as a qualitative term, but in some cases it is taken as a quantity. However, there is not a generally agreed definition of what opacity means in a quantitative sense:

• In some cases, the opacity of an object can vary between 0 and 1, the latter meaning complete blocking of light, and 0.8 would mean that 80% of the light is prevented from passing through.
• In other cases, e.g. in astronomy or plasma physics, opacity is considered as a property of the substance, and defined such that the exponential attenuation coefficient is the opacity times the density of the substance.

Opacity can have different physical origins. A typical origin is absorption, but light may also be reflected back or strongly scattered. This shows that blocking of light does not necessarily mean making it disappear; it may only be sent to other directions.