Q Why do some fabrics become transparent when wet?
A Cloth, like paper, is made up of discrete strands of fibre with air between them. Light hitting the dry material is somewhat scattered within it, but a good share is reflected to the eye. This renders the material visible.
Compared with the strands in the material, the air has a low refractive index - a measurement of how much light is bent. Water, on the other hand, has a higher refractive index, closer to that of the fibres.
When water or another liquid soaks fabric, the spaces between the fibres are filled with it. There is a smaller change in the angle of light that hits the material, resulting in more internal scattering. Less of that light comes back to the eye.
The result is less visibility for the fabric itself and more for whatever it is covering, whether a body clad in a T-shirt or a table covered by a wet tablecloth.