A ‘collimated’ laser beam has a slight divergence (the beam expands) as it propagates through a material or a vacuum. This effect is called ‘diffraction’ and results from light passing through the output aperture of the laser. For a laser beam having 1/e radius R0 and wavelength λ0 propagating though a material having a linear refractive index n0, the ‘diffraction length (Ldf)’ or ‘Rayleigh Length’ is defined as

Ldf = π (R0)2 n0 / λ0

If the sample thickness is less than Ldf, or optically thin, then the divergence of the beam as it passes through the sample is small and can probably be ignored. If the sample thickness is greater than Ldf, or optically thick, then the divergence of the beam will be noticeable.

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