Opacity
Measure of impenetrability to radiation, especially visible light.
Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc. An opaque object is neither transparent nor translucent. Opacity depends on the frequency of the light being considered, and different processes can lead to opacity, including absorption, reflection, and scattering.
- field
- Physics, radiative transfer, astronomy, plasma physics
- known_for
- Quantitative definition as mass attenuation coefficient; Planck and Rosseland opacities; use in radiopacity for medical imaging
- related_concepts
- Transparency, translucency, cesia, radiodensity
Lore & Background
Opacity is a fundamental concept in radiative transfer, describing how a medium absorbs and scatters radiation. An opaque substance transmits no light, instead reflecting, scattering, or absorbing all of it. Both mirrors and carbon black are opaque, though opacity varies with frequency; for instance, some glass transparent to visible light is opaque to ultraviolet light. In air pollution work, opacity is defined as the percentage of light blocked, ranging from 0% to 100%.
Reader's Guide
Opacity has a specific quantitative definition in astronomy and plasma physics as the mass attenuation coefficient κν. For a beam of light traveling through a medium with constant opacity and density, intensity decreases exponentially with distance: I(x) = I₀ e^{-κν ρ x}. Average opacities are defined using weighting schemes: the Planck opacity averages κν weighted by the Planck black-body radiation energy density, while the Rosseland opacity averages the inverse of κν weighted by the temperature derivative of the Planck distribution. In medicine, radiopacity describes opacity to X-rays, and radiodense contrast media are used to highlight CT scan or X-ray images. Radiopacity is a key consideration in designing endovascular devices like guidewires or stents, allowing tracking during interventional procedures.
Did You Know?
- Opacity can range from 0 to infinity, with units of length²/mass.
- Both mirrors and carbon black are opaque, though they achieve opacity through different processes.
- Radiopacity is preferentially used to describe opacity of X-rays in modern medicine.
- Rosseland opacity uses a temperature derivative of the Planck distribution as the weighting function.
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