Eye and Vision
A light sensitive organ of vision that can distinguish minute variations of shape, colour, brightness and distance. The actual process of seeing is performed by the brain rather than the eye. The function of the eye is to translate the light into patterns of nerve impulses that are transmitted to the brain.
The retina has two basic types of receptors ? Rods and Cones ? for collecting this information. Cones can differentiate between the different wavelengths of light and therefore enable us to see in colour while the more sensitive Rods only give us black and white vision. The Cones operate during the day and normal daylight conditions and enable us to see in detailed colour. This is known as PHOTOPIC or daytime adaptation. As the light level drops, say to that of a well-lit street, the cones become less effective and are assisted by the more sensitive Rods. Therefore, the eye is using a mixture of Cones and Rods to see. However, as the Rods can only “see” a black and white image, the overall impression is much less brightly coloured. This is called MESOPIC vision. Finally, at even lower levels such as moonlight which is much lower than average streetlighting, the cones cease to function altogether. The eye looses all its facility to see in colour and the Rods take over giving completely black and white vision, called SCOTOPIC, of night-time adaptation.
This entry was posted on Friday, October 15th, 2010 at 5:35 pm
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