Why do things look darker when wet?

Why do things appear darker when they are wet?

You must have come across this numerous times in your life when you spill coffee on shirt, paint the wall, throw water on sand and so on. The very first thought that comes into mind, well maybe my clothing brand is not so good. I should try some other manufacturer, whose clothes do not appear darker when wet. If you are thinking the same, then allow me to guide you through the science behind this. 

How do we perceive colors in the first place?

White light comprises of seven colors Violet, Indigo, Blue, Green, yellow, Orange, and Red- we must have read this acronym in school VIBGYOR. When light falls on an object, some of it gets absorbed while some of it gets reflected. The reflected light is the color we see. In the image below, the white light (comprising VIBGYOR) is striking the surface. All the components of white light are being absorbed by the surface but green. Green is reflected to us and that is nothing but the color of the surface in observation.

Fig.1. This is why the surface appears green (Source: udaix/Shutterstock)

This is how we see numerous colorful objects around us. When the light hits let’s say a piece of clothing, it is penetrating to the fabric, bouncing off the surface and reaching to our eyes.  

The question remains, why wet objects appear darker to us?

Fig.2. Wet clothes appear darker than dry ones (Source: iStockphoto)

                                                  

In the case of wet clothes, let’s say I have a blue shirt and I spilled water all over it. Now water will fill all the gaps of fiber which were initially filled with air. When light falls on this, some of the light will be absorbed by it while some of it will be reflected from water and fibers not air and the fibers. Light slows down when it goes through wet stuff. Due to the layer of water on my fabric, light will undergo multiple reflections inside it giving rise to a phenomenon known as Total Internal reflection. It can be very easily depicted from the diagram below, initially light strikes on the surface of water due to which light will bends and bounces in the medium. Then it penetrates the fabric, gets reflected from it and will again hit the water surface, leading to multiple reflections in the water. Because of the presence of water, less amount of light will reach to our eyes and therefore the material will appear darker in color. If the cloth is dry, meaning there is no water present, then there is no barrier to the light and most of it will be reflected by the surface and will reach back at us. 

Fig.3. The incident ray is hitting the surface of water and fabric 

This phenomenon is applicable to lot other things which absorb water for instance, sand, soil, paints and cement.

Fig.4. Wet sand versus dry sand (Source: youth.gov.hk)

Wet sand appears darker than the dry sand because of the aforementioned phenomenon. Next time you spray deodorant on your clothes, visit beach or paint walls, do not forget to acknowledge science behind these interesting phenomenas.