The major pigment in these cells is rhodopsin, which is composed of a protein molecule called opsin, and a molecule made from vitamin A called retinene. When light strikes rhodopsin, the retinene portion twists and turns, splitting away from the opsin portion. This process requires only a few picoseconds it may be one of the fastest biological processes known, but, it still takes us that much more away from now. The splitting of rhodopsin acts as a switch triggering a nerve impulse to the brain. Each rod and cone is a light-sensitive switch with its own wire into the brain.
Visual resolution is limited to the number or rods and cones.
When light strikes each cell, the chemical switch in it is turned on, sending a signal along the optic nerve to the visual cortex in the brain. If the signal to the brain traveled at the speed of light, which is the fastest that any information is supposed to be able to travel, it would still take the signal about one nanosecond to reach the brain. The transmission along the optic nerve is governed by a mixture of electrical transmission and chemical transmission. Consequently, it takes a bit longer than that. The velocity of propagation of the signal along the optic nerve is roughly 10,000 centimeters per second.