Why does wet sand look darker than dry sand?
Water is colourless and transparent, but why is wet sand darker than dry sand?
Is it because water absorbs more light than air? Not entirely correct.
All else being equal, wet sand looks darker because not much light is coming out.
For sand grains in water, the change in the direction of light is smaller than in air.
On average, it takes a much longer path for light in wet sand to come out (left) than in dry sand (right). The longer the path, the greater the chance that light gets absorbed.
Hence, wet sand looks darker than dry sand.
(For more details, please click here to read the article written by Hong Kong Observatory)
(Information provided by Hong Kong Observatory)