There are two different questions here - how was the light actually detected, and how was this detection used in any kind of useful manner.
The former is easily answered by looking at literature on how eyes work. Many molecules and proteins change shape when light is incident on them. In human beings this shape change (in retinal) allows the molecule to bind to another molecule, fit into an enzyme, causing a cascade of chemical reactions resulting in the firing of a neuron.
The latter isn't so easily answered, at least not by wikipedia
However, you can get a relatively good insight into the question by looking at the article on Chemotaxis, which is how bacteria and so on move towards or away from certain chemicals.
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