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Part II - Introduction to light
1. The nature of lightThe modern theory of light begain sometime in the late 16th century. It was the Englishman Thomas Harriot who first explained diffraction, although it was much later that Snell apparently rediscovered the rule that bears his name, and eventually published it (which Harriot did not). About 1635 Descartes first explained in a reasonable way how rainbows are created (although he may have obtained some ideas from medieval experimenters). This, even though his theory of colour was completely wrong. Isaac Newton began the subject more systematically, and among other things first separated white light under laboratory conditions. He believed very strongly that light was propagated by particles, although an alternative theory was proposed at about the same time by Huygens, who pointed out that diffraction could be explained elegantly on the assumption that light was propagated in waves.
The current theory is that it is neither wave nor particle but a weird combination of both. This is a part of the theory of qauntum mechanics. We shall not see it again. Instead, most of the time we shall assume that light travels in rays. This is a valid approximation for most everyday phenomena, and ceases to be so only when distances involved are roughly the same size as the wavelength, which is really very small.
If light is made of waves, what are the waves? The answer is, transverse vibrations of electric and magnetic force fields in space. This is unlike sound waves, for example, for which the vibrations take place along the direction of propagation. The difference between the two accounts for the polarization of light. Most of the time, we shall ignore this effect.