Photoreceptors are specialized nerve cells in the retina of the eye, which respond to light. A light sensitive compound (photopigment) within the cell undergoes changes in its chemical properties when exposed to light waves, resulting in changes in chemical release at the synapses of the cell.
The two main photoreceptor types are rods and cones, which differ both in shape and in characteristics.
Rods are highly sensitive to light, and are more responsible for vision in conditions of dim light. Rods do not enable colour distinction, as they do not differ in their sensitivity to specific wavelengths of light. Rods are most sensitive to a wavelength of around 500 nm.
Cones are less sensitive to light, and are responsible for colour vision. There are three distinguishable types in the human eye, which differ in which wavelengths of light they are most sensitive to, due to specific varieties of photopigment between each type. This enables an ability to distinguish between wavelengths, and is the basis for colour vision.