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Newton proved the human eye can only see wavelengths between 400
nanometers and 700 nanometers violet through to red. Why is that? White

light enters the pupil and passes through a transparent lens without

pigments, just like the prism it reflects individual wavelengths to the back of
the eye; the wavelengths are absorbed by the Retina which lines the back of

the eye. The Retina contains two different types of cells known as cones and

rods.

Cone cells accept the wavelengths red, blue and green also the wavelengths

nearest to them. It is the ability of the cone cells to accept red, blue and
green wavelengths and mix them that makes it possible for the human eye

to recognise over 300 different colours.

The rod cells do not recognise any wavelengths from the visible spectrum
so it is these cells that recognise brightly coloured objects in the dark as just

solid dark grey or black shapes. (Please see Fig. 6)

Fig. 6