Why wet spots are dark?

Light is diffused from tthe dry cloth because it is constituted of thousands of micrometer sized cellulose-type fiber, which has an index of refraction of about 1.3 (ie, different from that of air, which is 1). In order for something to diffuse light, you have two preconditions:

 

- it should be rough on a micrometer length scale (about the wavelength of visible light)

- it should have an index of refraction different from that of the surrounding media.

 

What water does is :

 

- it flattens the surface - the wet clothe's surface essentially becomes flat

- it's refraction index matches that of the fiber (water : 1.33).

 

So the cloth becomes transparent : indeed, if you look at a piece of wet cloth in transmitted light (that is, with the piece of cloth betwwen you and the light source), it let's through a lot of light !

 

Coming back to the dark wet spots, the transmitted light then bounces off your skin, losing in intensity and goes through the shirt again (losing more intensity): this is why it appears darker.

 

The index of typical clothing fibers is best matched with oils: you've probably seen wax making paper transparent.

 

 

McCrunch