For the cloud in the title, could $\tau \gg 1$ in one spatial direction and $\tau \ll 1$ in another? Since its density is homogeneous, I think $\tau$ would be the same in any direction if we measure it under the same parameters.

However, could $\tau \gg 1$ for a given wavelength and $\tau \ll 1$ for another?

  • $\begingroup$ By “cloud” do you to restrict your question to vapor? For solids and liquids you could accomplish this by being clever with birefringence or optical activity. $\endgroup$
    – rob
    Jan 12 at 14:35


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