KnowHOW team explains: Slit pupils are often found among nocturnal animals, such as the cat. These creatures need to be able to open their pupils very wide to let in as much light as possible when they are operating in darkness. However, because of their very sensitive retinas, they need to be able to make the pupil very small indeed when they are out during the day.

Such a wide range of adjustment is hard to achieve with a circular pupil, because of the amount of tissue stretching involved.

For example, a ring of tissue small enough to form a pinhole pupil of 0.5 millimetre across would have to stretch by a factor of 10 to form a pupil 5 millimetres across. A neat solution to this engineering problem is to use a slit pupil. Simple geometry shows that a completely shut slit pupil 5 millimetres long can turn into a circular aperture 5 millimetres across simply by stretching its edges by a factor of only 1.6.

The total length of the two sides of a closed slit pupil is 10 millimetres. When the pupil opens to form a circle of 5-millimetre diameter, its circumference will reach 15.7 millimetres, so the pupil muscles have stretched by a factor of only 1.6.

Not enough is known about what different animals see to provide a final answer, but it can be concluded that slit pupils do provide advantages in the range of light adaptation and the depth of field they provide.

The question was sent by Subhash Mukherjee from Jamshedpur