Why does sand look darker when it is wet? Water is clear, so the color does not come from the water. Dry sand has air between the grains.

Light hits the grains and quickly bounces back to our eyes. When water fills the spaces, light goes deeper into the sand. It bounces many more times, and more of it is absorbed. So less light comes back.

When sand, jeans or a road gets wet, it usually looks darker, even though water itself is clear. The reason is how light travels through the material. In dry sand, the gaps between grains are filled with air. Because sand and air bend light very differently, light changes direction quickly and returns to our eyes from near the surface.

When water fills the gaps, the difference becomes smaller, so light travels deeper before coming back. On the way, it is scattered many more times, and each time some of it is absorbed. In 1986, American scientists tested this idea by wetting sand with various liquids. The same physics explains why a wet white shirt becomes see-through.

Sand, denim and many other materials darken when wet, even though water is nearly transparent. Physicists have studied this puzzle for about a century and proposed two complementary mechanisms. The first was suggested in 1925 by the Swedish physicist Ångström. A wet surface is covered by a thin film of water; some light scattered by the surface strikes the boundary between the film and the air at certain angles and undergoes total internal reflection, returning to the surface and gaining another chance to be absorbed.

In 1988, researchers in New Zealand refined this model and found good agreement with experiments. The second mechanism, described by American scientists in 1986, concerns light inside the material. In dry sand, the large difference in refractive index between grains and air makes light change direction sharply, so it re-emerges after a short path.

Water reduces this difference, so light moves more strongly forward, is scattered many more times and is more likely to be absorbed before escaping. Measurements with sand soaked in different liquids broadly supported this explanation. The same principle makes wet white fabric translucent, though how much each mechanism contributes likely varies with the material.