How do animals see in the dark?

Subject :Environment

Section: Species in news

Photoreception:

Photoreception is the activation of a biological process by light.
Most organisms, including humans, respond to light. Some animals react to light waves not perceived by humans.
Vertebrates have two types of photosensitive cells, rods and cones, so called because of their shape.
- The rods, which are long and fat, contain large amounts of visual pigment and they mediate vision under dim illumination (scotopic vision).
- The cone cells, which are relatively small, mediate daylight vision (photopic vision) and colour sensation.

Vision of Human Eyes:

The retinas of animals active both day and night, as are those of humans, contain both rods and cones.
In parts of the human retina, the rods and cones are intermingled and the nervous system provides a switching mechanism that permits adjustment for light conditions.

Vision in nocturnal and diurnal animals:

In diurnal animals, the lens is smaller and the front surface is flatter. Thus the focal length is longer and so the image on the retina is larger and dimmer than in the nocturnal eye. As a result, the resolution is higher.
The retinas of diurnal animals have localised areas with a high density of cone cells. (The eyes of most birds have two such areas.)
In nocturnal animals, the optical arrangement of the eyes suggests that resolution is sacrificed for high light-gathering power.
In nocturnal animals, the retina is mainly made up of rod cells.Rhodopsin, a photosensitive pigment present in rods, is decolourised by photons (light particles) and slowly regenerated in the dark. This ensures better vision for them in dim light.