Opacity

Opacity is the measure of a material’s impenetrability to electromagnetic radiation, most commonly referring to visible light but also applicable to other forms of radiation such as ultraviolet light, X-rays, and gamma rays. In physics and applied sciences, opacity describes the extent to which radiation is absorbed, scattered, or reflected as it passes through a medium. An object or substance that is fully opaque transmits no radiation at the frequency considered.
Opacity is a fundamental concept in optics, radiative transfer, astronomy, atmospheric science, medicine, and materials science, where it is used both qualitatively to describe visual appearance and quantitatively to model radiation transport.
In everyday usage, opacity is contrasted with transparency, in which nearly all incident light is transmitted, and translucency, in which light is partially transmitted but scattered.

Physical basis of opacity

When electromagnetic radiation encounters the boundary between two substances, several interactions may occur simultaneously:

• A portion of the radiation may be reflected, either as diffuse reflection (for example, from a matte surface such as a white wall) or specular reflection (as from a mirror).
• Some radiation may be absorbed, converting its energy into internal energy of the material.
• Some may be scattered, changing direction due to interaction with particles or structural inhomogeneities.
• The remaining fraction may be transmitted through the material.

An opaque substance is defined by the absence of transmission at the relevant frequency; all incident radiation is therefore reflected, absorbed, scattered, or some combination of these processes. Both highly reflective materials, such as mirrors, and highly absorptive materials, such as carbon black, are considered opaque.

Frequency dependence

Opacity depends strongly on the frequency or wavelength of the radiation involved. A material that is transparent at one frequency may be opaque at another. For example, many types of glass transmit visible light efficiently but are largely opaque to ultraviolet radiation. This frequency dependence is even more pronounced in gases, where sharp absorption lines arise due to discrete atomic or molecular energy transitions.
As a result, opacity is never an intrinsic single-valued property of a material, but rather a function of radiation frequency, temperature, density, and chemical composition.

Visual appearance and perception

In visual perception, opacity is part of a broader framework describing how surfaces interact with light. Categories of visual appearance related to reflection and transmission have been organised under the concept of Cesia, an ordered system describing perceptual attributes such as opacity, transparency, and translucency. This framework is used in vision science and design to classify how materials are perceived under different lighting conditions.

Etymology

The term opacity derives from Late Middle English opake, from the Latin opacus, meaning darkened or shaded. The modern spelling, which became common in the nineteenth century, was influenced by the French form opaque.

Radiopacity in medicine

Radiopacity is a specialised usage of the term referring to the opacity of a substance to X-rays or similar high-energy radiation. In medical imaging, radiopaque or radiodense materials absorb or block X-rays and therefore appear light or white on radiographs.
Radiography and computed tomography have been transformed by the use of radiodense contrast media, which may be introduced into the bloodstream, gastrointestinal tract, or cerebrospinal fluid to enhance the visibility of internal structures. Radiopacity is also a critical design consideration for medical devices such as guidewires, catheters, and stents, as sufficient radiopacity allows these devices to be tracked accurately during interventional procedures.

Quantitative definition in radiative transfer

Beyond qualitative description, opacity has a precise quantitative meaning in fields such as astronomy, plasma physics, and atmospheric science. In this context, opacity is commonly used as a synonym for the mass attenuation coefficient or, in some cases, the mass absorption coefficient.
For electromagnetic radiation of frequency ν travelling through a medium with mass density ρ and opacity κ(ν), the intensity of radiation decreases exponentially with distance x according to the relation:
I(x) = I₀ e^(−κ(ν) ρ x)
where I₀ is the initial intensity at x = 0 and I(x) is the remaining intensity after travelling distance x. In this formulation, opacity has units of area per unit mass and can range from zero to very large values depending on the medium and frequency.
In air pollution studies, opacity is often defined differently, referring instead to the percentage of light blocked by particulate matter. In this context, opacity varies from 0 per cent (no light blocked) to 100 per cent (complete blockage).

Mean opacities: Planck and Rosseland averages

In many applications, radiation spans a wide range of frequencies, making it impractical to work with frequency-dependent opacity directly. Instead, average opacities are defined using specific weighting schemes.
The Planck mean opacity is calculated by weighting the frequency-dependent opacity by the Planck distribution for black-body radiation. This average is most relevant in situations where emission processes dominate.
The Rosseland mean opacity, named after Svein Rosseland, uses the temperature derivative of the Planck function as its weighting factor and averages the inverse of the opacity. This mean opacity is particularly useful in the diffusion approximation to the radiative transport equation and is valid when the radiation field is nearly isotropic, such as in conditions of local thermal equilibrium.
The Rosseland mean is closely related to the photon mean free path, which describes the average distance a photon travels before interacting with matter.

Opacity mechanisms in astrophysics

In astrophysical contexts, different physical processes contribute to opacity depending on temperature, density, and composition. Common mechanisms include:

• Thomson scattering by free electrons, which provides a relatively constant opacity in ionised gases.
• Free–free (Bremsstrahlung) absorption, arising from interactions between electrons and ions.
• Bound–free and bound–bound transitions, responsible for absorption edges and spectral lines.