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Sci Tech
Night vision
Question: Why are we unable to see in the dark?
C. Valliammal, Kanyakumari, T.N.
Answer: Cells in the eye which aid in vision are the rods and cones. There are about 250 million rod cells and 6 million cone cells. These cells are found in the inner layer of the eye called the retina.
Cones are the photoreceptors that respond differentially to light across the visible spectrum, providing colour vision and visual acuity in bright light because of higher light threshold simulation. They contain a pigment iodopsin which is a conjugated protein of photopsin and retinal.
The rods help us in scotopic vision or night vision. Each rod cell has a pigment called rhodopsin (visual purple) which is a conjugated protein of opsin and retinal.
The rods and cones are present around a small yellow pigmented spot called fovea centralis which is the area of greatest visual acuity of the eye.
At the centre of the fovea, the sensory layer is composed entirely of cones. The cones become fewer towards the periphery. At the outer edges only rods are present. The cones help to distinguish very fine details.
The rods do not have the ability to separate small details of the visual image. So in the visual field of the eye a small central area of great sharpness is surrounded by an area of lesser sharpness.
Just in front of the fovea centralis is the pupil, which is a contractile opening in the iris (pigmented diaphragm). The amount of light falling on the eye is regulated by the pupil. The pupil generally dilates in the dark and constricts in brightness.
Every object the eye perceives acts as a source of light. Generally in the dark the intensity of the source of light is lesser than the light minimum (minimum amount of light energy which can induce a visual sensation).
Therefore we cannot see till we get used to the darkness. Moreover since only the rod cells are stimulated in dim light we are unable to discriminate colours and also far less quanta of light falls on the retina, out of which a fraction falling on just one or two rods is sufficient to initiate a visual response.
Dim objects can be seen at night on the peripheral part of the retina when they are invisible to the central part. The seeing mechanism in the dark involves a resynthesis of rhodopsin. Visual purple is bleached by the action of light and is reformed by the rod cells under conditions of darkness.
So it takes time for the pigment to begin to form. When the pigment is formed the eyes are sensitive to low levels of illumination and the eyes are said to be dark-adapted.
Under normal circumstances, there is a routine & rapid process of rhodopsin synthesis in the dark, because an equilibrium is maintained in the retina such that the rate of breakdown of rhodopsin is equal to the rate of its synthesis.
Vitamin A plays a major role for dark-adaptation. But if there is a deficiency of vitamin A, the rate of resynthesis is delayed or there is a delay in the dark adaptation. This is the defect in night blindness (Nyctalopia). — The Hindu S & T Desk
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