Light is a form of electromagnetic energy. Our chief source of light is the sun. Every second the sun converts four thousand million kilograms of matter into energy by chemical and nuclear reactions. We can see the light energy and feel the heat energy.

The energy is radiated in waves or pulses and can be measured in Nano meters (nm).

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The variation of the intensity of solar radiation (irradiance, H₂) with wavelength. The values for H₂ are obtained by measuring the total energy in a wavelength interval of one nanometre. The red line is the curve for solar radiation outside the atmosphere, and the dashed line is the curve for a black-body radiator at 5 900 K. The continuous black line is the curve for solar radiation at sea level. The shaded areas show which wavelengths are partially absorbed by chemical species in the atmosphere. Note particularly the absorption by ozone at short wavelengths (source: Handbook of Geophysics and Space Environments, b Air Force Cambridge Research Laboratories, edited by Shea L. Valley, McGraw-Hill Book Company. 1965)

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The sun emits energy at all these frequencies, a lot of which do not reach us because of absorption by gases in our atmosphere.

Our eyes detect light in the wavelength HOD-700 nm with two detection mechanisms. On the retina, the material at the back of the eye, are sensers called rods and cones. Rods are responsive to very low concentrations of light and give us what is known as dark vision, the ability to see (when our eyes have adjusted to it) under very poorly lit conditions. Rods do not have colour vision and only pass on information to the brain in black and white. Cones only work well when there is sufficient light energy available.

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There are three different types of cone detecting at maximum wavelength 450 nm, 550 nm and 620 nm that is one responding to blue, green and red. 80 our eyes only see the three primary colours blue, green and red and the brain computes the combination of these primary colours to indicate the colour we see.

The eye cannot be likened to a radio where the whole range of frequencies can be scanned and each transmitting station recorded for intensity and frequency. The eye only receives three stations and computes intensity and frequency from those three detectors.

The colour television complements our eye detectors, using three colour emitters of blue, green and red, but with various combinations we are able to see all colours.

Light below 400 nm is removed by the lens and above 650 nm is of insufficient energy to generate the detectors into action.

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