A three-month-old black Labrador puppy with eyes set aglow by a camera flash.
| Photo Credit: Jazzjohnn (CC BY-SA)

A: Nocturnal animals such as cats are adapted to dim light. Their retina has more rod cells, which aid scotopic vision or vision under dark conditions, than cone cells, which aid photopic vision or vision under bright light.

Behind their retina is a layer called the choroid coat, which has a reflective surface called the tapetum lucidum (Latin for ‘shiny carpet’). The cells of the tapetum lucidum are made of reflective materials such as guanine crystals (in insects), yellow crystals of riboflavin (in some mammals) or white collagenous fibres. As a result, the tapetum lucidum layer acts as a mirror.

When light falls on the eye, it is first incident on the retina. The light not absorbed by the retinal cells then proceeds to the tapetum lucidum cells, which reflect it back through the rod cells to the outer layers, where it becomes available once again for the retina to process.

The net effect or benefit for the animal is that the retina has two opportunities to receive the light, once when it is incident and next when the tapetum lucidum sends it back. As a result, the animal can see clearly even in dark conditions.

When you are looking at, say, the cat, the glow in its eyes is the reflected light.

— M. Ramalingam, Coimbatore