When light enters your eye it triggers a sequence of actions, ultimately causing a signal to be sent to your brain. Even a mere single photon can be detected in your vision system. It all starts with a photon interacting with a light-sensitive chromophore molecule. The interaction causes the chromophore to change configuration and this, in turn, influences the large, trans-membrane rhodopsin protein to which the chromophore is attached. This is just the beginning of the cellular signal transduction cascade. But before any of this begins, in some species the incoming light has already been filtered and focused. Read more