1 - How light behaves

Summary

First gather the facts; then you can distort them at your leisure.

Mark Twain

First observations

How should we begin? Surely with Isaac Newton. He published his first scientific paper in 1672, and it was on light. In response to an objection raised against his paper, he wrote

• For the best and safest method of philosophizing seems to be, first diligently to investigate the properties of things and establish them by experiment, and then to seek hypotheses to explain them. For hypotheses ought to be fitted merely to explain the properties of things and not attempt to predetermine them, except in so far as they can be an aid to experiments.

So let us get a light source and begin investigating. I will suppose that you have access to some apparatus – at least in the sense of seeing lecture demonstrations – and so I will describe things as though we were doing the experiments together.

We take a strong flashlight or a 35-millimeter slide projector and shine the light horizontally. Clapping a pair of well-used erasers produces a cloud of chalk dust, and we see the light beam piercing the cloud as a bright shaft of light. This gives us our first observation:

Observation 1. Light goes in a straight line from a luminous source.

Reflection and refraction

Next we tip our source – the projector, say – so that the light enters the water in an aquarium tank. Figure 1.1 depicts what happens.