88. How do we see?


In everyday speech, “I see” means “I understand”. Eyes are the direct extensions of the brain. Eyes are among the most sensitive and vulnerable organs in a human body. Eyes act like camera but they are also similar to a radio receiver.

Visible light is a member of the family of “Electromagnetic waves”—ranging from the long but low frequency radio waves, to the small but high frequency X Rays and Gamma Rays. Eyes can detect wavelengths shorter than one-thousandth of a millimeter.

The wavelength of red light is 0.00072 mm. and that of the violet light is 0.00038 mm. All the other colours have wavelengths within this range. There are more than 125 million light sensitive cells in each eye. These nerve cells are of two types—the Rods and the Cones.

Cones work in bright light and can detect the colours. Rods can work in dim light and near darkness but they cannot detect colours. All the objects appear monochromatic. The momentary blindness we feel when we move from a bright light to darkness or vice versa, is due to the time needed by the eyes to switchover one system to the other ( from cones to rods or vice versa).

The Rods are scattered around the edge of the retina, away from the centre. So it is much easier to see the objects in the dark, when we are not directly looking at them! The Rods are amazingly sensitive and detect the light from a candle flame kept 5 miles away, in complete darkness.

To process the information they receive, the nerve cells in the eye perform 10 billion calculations every second. During that one second the eye ball moves in it socket 100 times. This eye movement does not blur the vision. It actually helps to view the finer details and allows the brain to process the light falling on the eyes at different angles.

Rays of light pass through the Cornea, at the front of the eye. The lens focuses them onto the retina—a region packed with light sensitive cells, The Rods and The Cones. A chemical reaction triggers the cells to send signals through the optic nerve to the brain. The brain integrates the information received and understands it.

To be able to see, the eyes which are a rare combination of a radio receiver and a camera must be connected to a computer called the brain.

Visalakshi Ramani

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