Glossary · Sinh học
Photoreceptor
Cells in the retina that turn light into electrical signals. There are two kinds: rods, strongly photosensitive, handling dim-light and peripheral vision but seeing only shades of grey; and cones, less sensitive, needing bright light, and giving us colour.
Rods sit toward the outer edges of the retina and are strongly photosensitive, so they carry dim-light and peripheral vision. In exchange, they distinguish only shades of grey.
Cones cluster near the centre of the retina and are less sensitive, so they need bright light. Their primary role is daytime and colour vision. The fovea — the region in the centre back of the eye — is the region responsible for acute vision, and in humans it has a high density of cones, which far outnumber rods there.
Whichever kind, what leaves the retina is electrical signals, not images. They travel along the optic nerve to the brain, and the brain is what builds what we call seeing.