7.1. Discrete Energy and Radioactivity

Here is my very oversimplified history of physics up until the 20th century:

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So up until about the end of the 19th century (...also until the end of GCSE physics), most people thought light was a wave. In a nutshell, there was plenty of evidence (see this video for some of this evidence) to suggest light behaved as waves, and it mostly obeyed the mathematical rules of what we now call 'classical physics' (Newton, Galileo, Huygens and the like). However, when exploring mathematical models of something called 'black bodies' (stuff that radiates energy, explored in Section 8.2) , the rules of classical physics started to fall apart. This was termed the 'UV Catastrophe' - a rather melodramatic term for when the maths goes wrong - like when you end up dividing by zero. A fudge was found (... well, not quite, it was a bit more involved) by a guy called Max Planck to make the maths work - treating the emitted light as 'discrete' particles.

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These discrete particles of light were termed 'photons'  - 'photo' meaning relating to light, an -ons being a standard suffix that Physicists add when talking about particle'y things (neutrons, electrons and so on).