Why does a wet piece of paper let through more light than a dry one?
Most of the change in transparency of paper [and tee shirts] to allow more light thru when wet is probably because the water reduces the surface irregularities by holding them together, flatter, reducing reflection.....it makes the surface 'smoother'.
Yes. In fact, if you wet it with an organic solvent whose index matches that of the paper, it will become transparent and allow you to read the letter inside an envelope without opening it.
Because there is reduced scattering, due to the change in relative refractive indices of paper/air vs. paper water. Lower Δn, less scattering.
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