Compute the spot size on the moon of an Earth based collimated Gaussian shaped beam whose wavelength is 0.8um and whose spot size equals 8mm.How long will the beam take to make the round trip?

FIber Optic
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Compute the spot size on the moon of an Earth based collimated Gaussian shaped beam whose wavelength is 0.8um and whose spot size equals 8mm.How long will the beam take to make the round trip?
1 mile = 1609m. D=238,900 miles