The tapetum lucidum is a layer of tissue in the eye of many vertebrate animals. It lies immediately behind the retina. It reflects visible light back through the retina, increasing the light available to the photoreceptors, though blurring the initial image of the light on focus. The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores that hunt their prey at night, while others are deep sea animals. Eyeshine is a visible effect of the tapetum lucidum. When light shines into the eye of an animal having a tapetum lucidum, the pupil appears to glow

Why do different species of cats have different pupils? All big cats and some small cats (cougars and lynx) have round pupils. But all house cats and the species they descend from (caracals, African sand cats, etc.) have vertical, almond-shaped pupils.

By day, the circular pupil is inefficient at blocking light. Pupil shapes have evolved that limit incoming light, the most advanced being the vertical slit. The slit pupil can shut out all light except a narrow band. Its vertical orientation is of significance too, as it works well with eyelids. As an animal squints, partially closing its lids at right angles to the vertical slit pupil, it further reduces the amount of light entering its eye.

Lions hunt by day and have circular pupils, while smaller cats hunt by night and have slit-like pupils. Their vertical pupils help them to endure the bright daylight by restricting the amount of light that penetrates the lens. Lions are less concerned with this and spend much of their time asleep.

The pupil of the clouded leopard never gets fully round like those of big cats, but never shrink to vertical slits .

The Tiger and cheetah has round pupils although the tiger hunts at night.