A measure of how far light will travel through a partially transparent medium, such as the atmosphere of a star or a planet, before it is absorbed or scattered. A completely transparent medium has an optical depth of zero. A medium with a low optical depth is described as optically thin, whereas one with a high optical depth is optically thick. Absorption and scattering vary with wavelength (λ), so optical depth is usually specified for a particular wavelength and written as τλ.