Clothing... hair... paper... wood... Lots of things seem to be darker when they're wet. What's happening? The way we see an object's 'colour' is by the light which hits the object. For example, when white light (containing all colours) hits the pink shirt at the right, all the colours in the light are absorbed by the shirt ... except pink, which gets emitted. This pink light emitted by the shirt is why the shirt looks pink to our eye. Now suppose the pink shirt becomes wet. Water is now embedded in the fabric, including the outer layer of the material. Several things happen when light hits this water-fabric mixture. The simplest description would be to say that the fabric-water mixture will emit a different colour that the fabric would all by itself. Generally speaking, that is what happens. However, this is not strictly true, since the water and fabric don't form a new material, but remain as separate substances within the shirt. Light interacts with them separately. A better explanation of what actually happens is as follows: When white light from the room hits the surface of the material where it is wet, some of the light will scatter (reflect) from the water. This light doesn't make it back to our eye. Less light coming from the wet region means it will look darker. Some of the light that hits the water will pass through. When it does, it will be refracted (bent), so that its path changes. When pink light from the fabric is emitted, some of it will again be refracted as it passes through the water again. Not all of the light that is emitted by the pink fabric that would have reached our eye now does so, because some of it has changed direction. In other words, because there is less light coming from the wet area (some was refracted in a different direction), that area will look darker than the material around it.

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