Opacity (optics)

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Opacity is the state of being impenetrable to light. An opaque object is neither transparent (allowing all light to pass through) nor translucent (allowing some light to pass through).

When light strikes an interface between two substances, some of the light is reflected, some is absorbed, and the rest is transmitted (see also refraction). An opaque substance transmits very little light, and therefore reflects or absorbs most of it. Both mirrors and jet are opaque. Opacity depends on the frequency of the light being considered. For instance, some kinds of glass, while completely transparent in the visual range, are largely opaque to ultraviolet light. More extreme frequency-dependence is visible in the absorption lines of cold gases. In general, a material tends to emit light in the same proportions as it absorbs it; this is the reason for the equivalence of absorption and emission lines.

[edit] Numerical definition

While many materials are so opaque (steel in visible light), and others so transparent (air in visible light), that opacity often seems to be a boolean property, others are "somewhat" opaque. Opacity is then in general a continuous variable: κ_ν, where the ν indicates the frequency in question. It has units of inverse length, and is also called the absorption coefficient (see also the similar extinction coefficient). It gives the proportional rate of absorption (or extinction) along a ray of light: . The mean free path is then 1 / κ_ν. The notation κ_λ is also used to discuss the physically equivalent description of opacity as a function of wavelength. The opacity of a material, and of a gas in particular, often depends on its temperature and density.

In astrophysics and plasma physics a term "opacity" often refers to an average of the above, calculated using a certain weighting scheme. Planck opacity uses normalized Planck black body radiation energy density distribution as the weighting function, and averages κ_ν directly. Rosseland opacity, on the other hand, uses a temperature derivative of Planck distribution (normalized) as the weighting function, and averages 1 / κ_ν, which is a photon mean free path; the average mean free path is then inverted to obtain opacity.

[edit] Applications

In astrophysics, the variations in opacity within a star are important to the understanding of radiation transfer in stellar atmospheres and the spectra we observe.

In several types of chemical analysis, the concentration of a sample in a transparent medium (typically air or water) is determined via measuring its opacity or absorbance. In spectrophotometry the device identifies the sample's constituent substances from their absorbances.

Opacity is also used as a measurement of particulate emissions.