Why is a wet T-shirt darker than a dry one?

Take a bright white or brightly coloured T-shirt. Dip it in the water. What happens? As if by magic, it seems to become darker. But why does this happen? Hold on to your T-shirts, we're going to explore this enigma (using words that we understand as we write them, even though! (A keen sense of science that everyone at Quat'rues understands!)

First of all, let's talk about light. When a T-shirt is dry, it reflects light in a certain way. The fibres of the fabric reflect the rays of light, giving our T-shirt its original colour. On the other hand, when we get it wet, small droplets of water are deposited on the fabric. These droplets act like mini-prisms (not prims!), changing the way light is reflected.
Instead of bouncing off in all directions, the light penetrates a little further into the damp fabric, giving it a darker appearance.

In addition to this, the water itself has an effect on the colour. By moistening the fabric, the water fills the spaces between the fibres, which can make the T-shirt more translucent. The colour of the fabric then appears more intense as we see a combination of the dye in the T-shirt and the colours of the background behind it.
If you look at a T-shirt that is completely soaked, the fabric may appear even darker due to the saturation of the colours.

Finally, don't forget that if your t-shirt is made from organic, ethically-traded cotton, it won't look any darker when it's wet, but it will be even more pleasant to wear (physically and emotionally!) when it's dry!

In conclusion, a wet t-shirt looks darker because of the way the light interacts with the water droplets and the fabric fibres.
And a dry t-shirt is much nicer to wear when it's a Quat'rues!