Why does wet sand look darker than dry sand?
Actually this is not just true for sand – it is also true for wet paper and many wet fabrics. Objects are darker if more of the incident light is absorbed, so we should ask what might enhance the absorption of light in wet sand. Imagine a ray of light entering a grain of sand from air (dry sand) and from water (wet sand) . In both cases the rays will be refracted, by Snell’s Law: The ray is refracted more if the refractive index difference, n1 – n2, is larger. Now air has refractive index that is 1, but water has a refractive index which is 1.33, whereas sand has approximately the same refractive index as glass, say 1.5. When an incident ray reaches the surface of dry sand it will be refracted through large angles as it passes from grain to grain. This results in a large probability that it will rapidly re-emerge from the sand surface. However, in wet sand it will only be deflected by a small angle at each refraction, so it will take many refractions before it can re-emerge. Thus an in
