Opacity (optics)

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Opacity is the measure of impenetrability to electromagnetic or other kinds of
radiation In physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department ...
, especially visible
light Light or visible light is electromagnetic radiation In physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of ...
. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma,
dielectric In electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge Electric charge is the physical property of matter that causes charged matter to experience a force In ...
, shielding material, glass, etc. 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, in general some may be reflected, some absorbed, some scattered, and the rest transmitted (also see
refraction In physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department o ...
). Reflection can be diffuse, for example light reflecting off a white wall, or specular, for example light reflecting off a mirror. An opaque substance transmits no light, and therefore reflects, scatters, or absorbs all of it. Both mirrors and carbon black are opaque. Opacity depends on the
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz The hert ...
of the light being considered. For instance, some kinds of
glass Glass is a non- crystalline, often transparent, amorphous solid In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid Solid is one of the four fundamental ...
, while transparent in the visual range, are largely opaque to
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength In physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the re ...
light. More extreme frequency-dependence is visible in the absorption lines of cold gases. Opacity can be quantified in many ways; for example, see the article mathematical descriptions of opacity. Different processes can lead to opacity including absorption, reflection, and scattering.

# Etymology

Late Middle English opake, from Latin opacus ‘darkened’. The current spelling (rare before the 19th century) has been influenced by the French form.

''Radiopacity'' is preferentially used to describe opacity of
X-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation In physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior t ...
. In modern medicine, radiodense substances are those that will not allow X-rays or similar radiation to pass. Radiographic imaging has been revolutionized by radiodense contrast media, which can be passed through the bloodstream, the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans ...
, or into the cerebral spinal fluid and utilized to highlight CT scan or X-ray images. Radiopacity is one of the key considerations in the design of various devices such as guidewires or stents that are used during radiological intervention. The radiopacity of a given endovascular device is important since it allows the device to be tracked during the interventional procedure.

# Quantitative definition

The words "opacity" and "opaque" are often used as colloquial terms for objects or media with the properties described above. However, there is also a specific, quantitative definition of "opacity", used in astronomy, plasma physics, and other fields, given here. In this use, "opacity" is another term for the mass attenuation coefficient (or, depending on context, mass absorption coefficient, the difference is described here) $\kappa_\nu$ at a particular frequency $\nu$ of electromagnetic radiation. More specifically, if a beam of light with frequency $\nu$ travels through a medium with opacity $\kappa_\nu$ and mass density $\rho$, both constant, then the intensity will be reduced with distance ''x'' according to the formula $I(x) = I_0 e^$ where * ''x'' is the distance the light has traveled through the medium * $I\left(x\right)$ is the intensity of light remaining at distance ''x'' * $I_0$ is the initial intensity of light, at $x = 0$ For a given medium at a given frequency, the opacity has a numerical value that may range between 0 and infinity, with units of length2/mass. Opacity in air pollution work refers to the percentage of light blocked instead of the attenuation coefficient (aka extinction coefficient) and varies from 0% light blocked to 100% light blocked: $\text = 100\% \left(1-\frac \right)$

## Planck and Rosseland opacities

It is customary to define the average opacity, calculated using a certain weighting scheme. Planck opacity (also known as Planck-Mean-Absorption-Coefficient) uses the normalized Planck black-body radiation energy density distribution, $B_\left(T\right)$, as the weighting function, and averages $\kappa_\nu$ directly: $\kappa_\left( \right) \int_0^\infty \kappa_\nu B_\nu(T) d\nu ,$ where $\sigma$ is the Stefan–Boltzmann constant. Rosseland opacity (after Svein Rosseland), on the other hand, uses a temperature derivative of the Planck distribution, $u\left(\nu, T\right)=\partial B_\nu\left(T\right)/\partial T$, as the weighting function, and averages $\kappa_\nu^$, $\frac = \frac.$ The photon mean free path is $\lambda_\nu = \left(\kappa_\nu \rho\right)^$. The Rosseland opacity is derived in the diffusion approximation to the radiative transport equation. It is valid whenever the radiation field is isotropic over distances comparable to or less than a radiation mean free path, such as in local thermal equilibrium. In practice, the mean opacity for Thomson electron scattering is: $\kappa_ = 0.20(1+X) \,\mathrm$ where $X$ is the hydrogen mass fraction. For nonrelativistic thermal bremsstrahlung, or free-free transitions, assuming solar metallicity, it is: The Rosseland mean attenuation coefficient is:George B. Rybicki and Alan Lightman, Alan P. Lightman,
$\frac = \frac.$