Rods are sensitive to dim light and colourless vision(since they are unable to detect colours).Rods contain a sensitive pigment called visual purple(rhodopsin). The visual purple contains vitamin A and protein. When light falls on the visual purple,chemical reaction(bleaching)takes place in the visual purple that releases yellow pigment called retinene and energy that stimulates the optic nerve which then carries light rays to the brain. The brain interpretes the light rays as light. When the eyes are closed,the purple colour is regenerated with the help of vitamin a. That is why the deficiency of vitamin a causes night blindness.
Cones are sensitive to bright light and are able to detect colours. They contain a photochemical substance called iodopsin which is not easily bleached by high light intensity. The yellow spot (fovea) contains only cones hence the yellow spot gives the best vision.
The difference between rods and cones form the basis of nocturnal(active by night)and diurnal(active by day)habits of animals. Man has both rods and cones,and therefore has both day and night visions. Many animals have only one vision. Chickens have only cones hence they are active in the day. That is the reason why they go to roost very early. Owls have only rods hence they are active at night during which they see better. Cat and deer see very well at night because they have far more rods than cones.
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