# Width of Gaussian Beam and Refractive Index

I know that in free space, the width of a Gaussian beam can be written as $W=W_0\sqrt{1+(\frac{z}{z_0})^{2}}$. However, I was wondering if it was possible to express this width as a function of refractive index instead (since I don't believe a Gaussian beam originating in say, glass, will diverge in the same manner as one in air). Anyone have any ideas?

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