Why does water make a shirt darker?
Category: Physics Published: November 6, 2013
Getting a spot wet on your shirt does not make that spot permanently darker or intrinsically different. Water just makes fabric appear darker by making the fabric more transparent, allowing you to see darker objects behind the fabric.
Let's first consider a white shirt. The individual fibers that make up a white shirt are really not that white. They are actually mostly transparent. However, transparent materials do reflect a small amount of light. For instance, a glass window is transparent (allowing us to see the outside world) but still reflects some of the light that hits it. This is why you can see your reflection in a window at night. Shirt fibers are the same. They are mostly transparent and slightly reflective at the same time. A single fiber therefore looks mostly clear, not white. However, each fiber is surrounded by more fibers which give the light more chances to be reflected. It's important to realize that the reflection of light happens at the interface between the fiber material and air. Therefore, a material that is made out of many small fibers has a large surface area, giving the light many opportunities to be reflected. Furthermore, the weave of the fibers creates many layers of reflecting surfaces. Therefore, the portion of the light that successfully makes it through the first fiber without being reflected can still be reflected by the lower layers. As a result, most of the white light falling on an undyed shirt eventually gets reflected back away from the shirt in a random fashion. The shirt is therefore seen as a diffuse white color. This also means that very little light makes it all the way through the shirt. We therefore see the material as opaque. The main factor that turns a mostly transparent material (the individual fibers) into a mostly opaque material (the shirt) is the array of multiple reflecting surfaces created by the weaving pattern.
This effect is quite common. Take any solid chunk of mostly transparent material, introduce small structures that create little pockets of air, and you end up with a white material. For example, a solid block of ice is clear, but ice formed into microscopic structures (snow) is white. A solid block of salt (halite) is transparent, but salt ground down to a pile of grains (table salt) is white. A solid block of quartz is clear, but quartz filled with air bubbles (milky quartz) is white, and quartz ground down to a pile of grains (sand) is white. A cup full of water is clear, but a collection of tiny liquid water droplets (a cloud) is white. In each case, it is the existence of a microscopic material structure filled with tiny air pockets that leads to multiple reflecting surfaces, and therefore to strong reflection.
Spill some water on your white shirt and the situation changes. In the spot that is wet, the water fills all the little pockets in the fabric that used to be filled with air. In terms of its interaction with light, water behaves very similar to fiber material. Adding water to fabric effectively removes the multiple reflecting surfaces and effectively turns the shirt into a solid chunk of material which is mostly transparent. Water does not really make the fabric chemically darker. Instead, water just temporarily makes the fabric more transparent. Underneath the shirt is usually something that is darker than white, such as skin or another shirt. Therefore, you perceive a wet spot of fabric that has become more transparent as a wet spot of fabric that has become darker. If you wear a white shirt over a bright orange shirt, spilling water on your belly will make your belly look more orange and not more black. If you hold a white shirt with a wet spot right in front of a bright light, the wet spot will appear brighter than the rest of the shirt because it is more transparent and lets more light through. For the same reason, getting any white, micro-structured material wet will make it look darker, more colorful, or lighter; depending on what's behind it. Pour water on a pile of snow, granulated sugar, or table salt that is sitting on a dark table and the pile will become more transparent and appear darker.
There is another effect at work which contributes to the change in visual appearance of a wet spot. When light reflects off of a rough surface, the light reflects in all directions. As a result, the surface looks bright when viewed from any direction. This is called diffuse scattering. In contrast, when light reflects off of a very smooth surface, the light reflects only in the mirror direction. As a result, the surface only looks bright when viewed from a single specific direction, i.e. when viewed at the mirror angle. At all other angles, the surface looks dark. This is called specular scattering. Since there is a low probability that you will just happen to be looking at a smooth surface at exactly the right angle, this means that smooth surfaces generally look darker than rough surfaces. When water spills on a shirt, it fills in all the little pockets of air and turns the shirt effectively from a rough surface to a smooth surface, leading the wet spot to generally look darker.
If you add dye to a shirt the situation gets more complicated. Dyes strongly absorb certain colors and strongly reflect other colors. However, the surface of each individual dyed fiber still reflects a little bit of all of the colors. This means that a dry blue shirt is actually whitish blue (which we call "lighter blue"), but the same shirt when wet is just blue (which we call "darker blue"). Adding water to a dyed fabric inhibits the multiple reflections, so more of the light has a chance to be absorbed by the dyes. Therefore, adding water to a heavily dyed shirt does not make it more transparent. It makes the shirt have a darker, more saturated color.