Why do rod cells help people see in dim light?

Rod cells are highly specialized photoreceptive cells that enable vision in dim light primarily due to their extreme sensitivity and the properties of the pigment rhodopsin.2

Unlike cone cells, which require higher light intensities to function, rods are capable of responding to single photons of light. This sensitivity is facilitated by the high concentration of rhodopsin within the rod's outer segment. Rhodopsin is composed of the protein opsin and 11-cis-retinaldehyde, a photosensitive chemical. When light strikes the eye, it triggers a photochemical reaction known as isomerization, where 11-cis-retinaldehyde converts to all-trans-retinaldehyde, activating the opsin and initiating a biochemical cascade that sends signals to the brain.12

In bright light, rhodopsin is rapidly broken down, which reduces the cell's sensitivity and prevents overstimulation. Conversely, in dim light, there is minimal breakdown of rhodopsin. Its persisting high concentration ensures that the rods remain primed to capture even the faintest available light, allowing for effective dark-adapted vision.2