Optical depth is a measure of transparency, and is defined as the fraction of radiation (or light) that is scattered or absorbed on a path. One way of visualizing optical depth is to think of a fog. The fog between you and an object that is immediately in front of you has an optical depth of zero. As the object moves away, the optical depth increases until it reaches a large value and the object is no longer visible. Transparent glass ball In optics, transparency is the property of allowing light to pass. ... Radiation as used in physics, is energy in the form of waves or moving subatomic particles. ... This article does not cite any references or sources. ...

The optical depth expresses the quantity of light removed from a beam by scattering or absorption during its path through a medium. If I₀ is the intensity of radiation at the source and I is the observed intensity after a given path, then optical depth τ is defined by the following equation: Scattering is a general physical process whereby some forms of radiation, such as light, sound or moving particles, for example, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which it passes. ... In physics, absorption is the process by which the energy of a photon is taken up by another entity, for example, by an atom whose valence electrons make a transition between two electronic energy levels. ... A transmission medium is any material substance, such as fiber-optic cable, twisted-wire pair, coaxial cable, dielectric-slab waveguide, water, or air, that can be used for the propagation of signals, usually in the form of modulated radio, light, or acoustic waves, from one point to another. ... In physics, intensity is a measure of the time-averaged energy flux. ...

In atmospheric sciences, one often refers to the optical depth of the atmosphere as corresponding to the vertical path from the Earth's surface to outer space; at other times the optical path is from the observer's altitude to outer space. Since τ refers to a vertical path, the optical depth for a slant path is τ' = mτ, where m is called the airmass factor, and for a plane-parallel atmosphere it is determined as m = 1 / cosθ, where θ is the zenith angle corresponding to the given path. Therefore Atmospheric sciences is an umbrella term for the study of the atmosphere, its processes, the effects other systems have on the atmosphere, and the effects of the atmosphere on these other systems. ... Plot of the airmass calculations using the formulas discussed, The airmass in astronomy quantifies the path length that the light from a celestial source must travel through the Earths atmosphere to get to the observatory, relative to that for a source at the zenith. ... In astronomy, a celestial coordinate system is a coordinate system for mapping positions in the sky. ...

The optical depth of the atmosphere can be divided in several components, ascribed to Rayleigh scattering, aerosols, and gaseous absorption. The optical depth of the atmosphere can be measured with a sun photometer. See also: Beer's law. Rayleigh scattering causing the blue hue of the sky and the reddening at sunset Rayleigh scattering (named after Lord Rayleigh) is the scattering of light, or other electromagnetic radiation, by particles much smaller than the wavelength of the light. ... Particulates, alternatively referred to as particulate matter (PM), aerosols or fine particles, are tiny particles of solid or liquid suspended in a gas. ... In physics, absorption is the process by which the energy of a photon is taken up by another entity, for example, by an atom whose valence electrons make a transition between two electronic energy levels. ... A sun photometer is a type of photometer conceived in such a way that it points at the Sun. ... In optics, the Beer-Lambert law, also known as Beers law or the Beer-Lambert-Bouguer law is an empirical relationship in relating the absorption of light to the properties of the material the light is travelling through. ...

Another example occurs in astronomy where the photosphere of a star is defined as the surface where its optical depth is 2/3. This means that each photon emitted at the photosphere suffers an average of less than one scattering before it reaches you. At the temperature at optical depth 2/3, the energy emitted by the star (the original derivation is for the sun) matches the observed total energy emitted. A giant Hubble mosaic of the Crab Nebula, a supernova remnant Astronomy (also frequently referred to as astrophysics) is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earths atmosphere (such as the cosmic background radiation). ... The photosphere of an astronomical object is the region at which the optical depth becomes one for a photon of wavelength equal to 5000 angstroms. ...

Note that the optical depth of a given medium will be different for different colors (wavelengths) of lights. The wavelength is the distance between repeating units of a wave pattern. ...

For planetary rings, the optical depth is the proportion of light blocked by the ring when it lies between the source and the observer. This is usually obtained by observation of stellar occultations. A planetary ring is a ring of dust and other small particles orbiting around a planet in a flat disc-shaped region. ...

External links

• optical depth equations