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Why are rod cells more useful than cone cells in dim light?

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details of the mammalian eye — Photoreception The mammalian eye has a cornea and a lens and functions as a dioptric system, in which light rays are refracted to focus on the retina.

Rod cells are more useful than cone cells in dim light primarily because they possess a much higher sensitivity to light. While both cell types can respond to single photons, rod cells generate large electrical signals in response to these stimuli, whereas the signals produced by cones are significantly smaller.12

This difference in sensitivity is central to the duplicity theory of vision, which explains that rods are the primary photoreceptors utilized under scotopic (low-light) conditions. Because rods have a much lower threshold for activation, they can detect light intensities far too weak to trigger a response in cone cells.3

Additionally, the biochemical composition of rods supports this function. They contain the photoreceptor molecule rhodopsin, which remains in a high concentration in dim environments because there is little breakdown of the molecule. In contrast, bright light causes rhodopsin to break down, which reduces the sensitivity of rods and prevents overstimulation. Consequently, rods are specialized for vision in dark-adapted states, while cones are better suited for higher light intensities and daylight conditions.123