Sensitivity of rods and cones

Medical Physics | AQA A-Level Physics

RodsPhotoreceptor cells in the retina that are highly sensitive to light. They work in low-light (scotopic) conditions but cannot distinguish colour. There are about 120 million rods per eye. are highly sensitive to light and work in low-light conditions (scotopic vision). They cannot distinguish colour. There are about 120 million rods in each eye, spread across the retina but absent from the fovea.
ConesPhotoreceptor cells in the retina that detect colour. They require high light intensity to function (photopic vision). There are three types: red, green, and blue, with about 6 million cones per eye concentrated at the fovea. require high light intensity and detect colour (photopic vision). There are three types of cone, each sensitive to a different range of wavelengths: red (long), green (medium), and blue (short). About 6 million cones per eye, concentrated at the fovea.
Fundamentally, rods are more sensitive because several rods connect to a single neurone (convergence). This means their individual signals add together, making the threshold lower, but at the cost of spatial detail.
Cones give better spatial resolution because each cone typically has its own dedicated neurone to the brain.
There are roughly 20 times more rods than cones. Night vision uses rods only, which is why you cannot see colour in very dim light.

Common Mistake MEDIUM

Students often: Saying rods detect colour and cones work in dim light.
Instead: Rods = dim light, no colour. Cones = bright light, colour. Think: Cones for Colour.