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.
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A measure of how easy it is for a
photon of
electromagnetic radiation to pass through anoptical medium. A dusty medium will have a high optical depth, since a photon will stand a high probability of interacting with a dust particle. This will prevent it from continuing along its path by either scattering it or absorbing it. This is known as
interstellar absorption when it occurs within the
interstellar medium . Low optical depths mean that photons can pass through the medium relatively unhindered. A medium of high optical depth may also be referred to as being optically thick and one of low optical depth may be referred to as optically thin.
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