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Definition: the geometric thickness times the refractive index
The term “optical thickness” is ambiguous, as two totally different definitions occur in the literature:
- The optical thickness of a light-absorbing medium is the inverse intensity absorption coefficient, i.e., the propagation length over which the optical power is reduced to 1/e (∼ 37%) of its original value. This definition is often used e.g. in atmospheric optics.
- The optical thickness of a transparent medium is sometimes understood to be the same as the optical path length, which is its geometric thickness multiplied by the refractive index. This is the quantity which determines the phase delay for light passing through the medium and is therefore relevant e.g. in interferometers.
See also: refractive index, interferometers
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