March 12, 2009
Moist dirt is darker than dry dirt (upper picture). Similarly, a wet spot on a tablecloth is darker than the dry area. Why? With an index of refraction between that of air and dirt, water acts like an anti-reflection coating. More light is transmitted into the wet surface so less light gets reflected back. The result is a dark wet spot. If more light is transmitted into the surface, then from the other side of the cloth the wet surface should be brighter, right? And indeed it is. The lower left picture shows reflected light on a piece of cloth with a dark wet spot. On the lower right is a picture taken from the back side of this cloth showing the same wet spot in transmitted light. Note that it's brighter, demonstrating that more light is transmitted by wet surfaces than by dry surfaces.