Tuesday, December 9, 2008

HOW DO ANIMALS SEE IN THE DARK?

Photoreception is activation of a biological process by light. Most organisms including man respond to light. Some animals react to light waves not perceived by man. 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. The cone cells, which are relatively small, mediate daylight vision and colour sensation. The retinas of animals active both day and night, as are those of humans, contain both rods and cones. In parts of human retina, the rods and cones are intermingled and the nervous system provides a switching mechanism that permits adjustment for light conditions. In nocturnal animals, the optical arrangement of the eyes suggests that resolution is scarified for high light-gathering power. For example, in animals such as dog, the lens is large, that is, it has a short focal length. 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 improved. The retinas of diurnal animals, have localized areas having a high density of cone cells. In nocturnal animals, the retina is mainly made of rod cells. Rhodopsin, a photosensitive pigment, present in rods is declourised by photons and slowly regenerated in the dark. This ensures better vision for them in dim light.

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