With his theory of colour, Isaac Newton explained how the colours of a rainbow are formed. When Newton passed a white light through a glass prism, he found that it split into rainbow colours and showed that the colours were embedded in the white light rather than imprinted in the prism.
Colour Mix
Using the prism experiment, Newton demonstrated that light’s many colours could be mixed together to form white light. The colours were the base units rather than being made by the later mixing or by the prism glass itself. Newton was able to separate beams of red and blue light and show that these single colours were not split further if they were passed through more prisms.
During his experiments, Newton also realised that objects in a lighted room appear coloured because they scatter or reflect light of that colour, rather than colour being somehow a quality of the object.
Light Waves
For Newton, understanding colour was a means towards understanding the physics of light itself. After further experimenting, he found that light behaves in many ways like water waves. Light bends around obstacles in a similar way to how water flows around objects that are in its way.
Wave motions appear in many guises but the two main types are longitudinal and transverse waves. Longitudinal waves, such as those of sound, result when the pulses that produce the wave act along the same direction in which the wave travels. This variety of wave causes a series of high and low pressure crests. On the other hand, waves of both water and light are transverse waves where the original disturbance acts at a right angle to the direction of travel of the wave.
Colour Spectrum
The different colours of light reflect the different wavelengths of these electromagnetic waves. As white light passes through the prism, it separates into many colours because each colour is associated with a different wavelength and so is deflected to varying degrees by the glass. The prism bends the light waves by an angle that depends on the wavelength of light (red light is bent the least while blue is bent the most) to produce the rainbow colour sequence.
Of course visible light is just one part of the electromagnetic spectrum. The human eye has developed to be sensitive to this particular part of the spectrum. Since the wavelengths of visible light are on roughly the same scale as atoms and molecules, the interactions between light and atoms in a material are large. The eye has evolved to use visible light since it is very sensitive to atomic structure.
Sources:
Baker, Joanne (2007) 50 Physics Ideas You Really Need to Know (Quercus Publishing)
Holzner, Steve (2005) Physics for Dummies (John Wiley & Sons)
Kuhn, Karl (1996) Basic Physics: A Self-Teaching Guide (John Wiley & Sons)
