 Our understanding of the structure of the atom has developed by the use of many different tools. Thompson used electricity to separate electrons out from atoms, showing for the first time that an atom was not a homogenous, indivisible ball. Rutherford and many other scientists used the phenomenon of radioactivity, still not well understood at the time, as a tool to probe further, leading to the discovery first of the nucleus, and then of the separate nucleons, protons and neutrons. Since then, most of the progress in our understanding of atomic structure has been all about the electrons. How are they arranged in the atom? What shifts and changes and forces are involved as they whiz around the nucleus? And the tool that we use to probe the electrons, to stimulate them, to get responses from them, to make them share their secrets with us, is light. So to understand how the 20th century got to grips with the electron in its atomic habitat, you first need to have an appreciation of what light is. I should say that the title of this video, The Nature of Light, is kind of misleading. There's no way I can do justice to the beauty and the complexity and the sheer usefulness of light in a short presentation like this. For that, you need to go and do more physics and chemistry at university and become a laser physicist or a solar engineer or a spectroscopist. But hopefully, I can at least give you a feel for how light accidentally became the most useful tool that we have for learning about the atom. Firstly, you need to know that the light you experience, you as a human, is only one tiny family inside a much larger ecosystem called the electromagnetic spectrum. A spectrum is something that can have any one of a continuous series of values. People talk about a spectrum of political opinion, meaning an opinion that lies somewhere in the range from progressive to conservative thinking. Or there's the autism spectrum, describing a range of different levels of autism. The electromagnetic spectrum describes electromagnetic waves and the range of different wavelengths that they can have. Electromagnetic waves are a kind of wave that can carry energy through empty space. This is in contrast to waves such as sound or earthquakes which can only travel through matter. You're familiar with many forms of electromagnetic energy even if you didn't realize that that's what they are. X-rays, UV light, visible light, infrared light, which you experience as heat, microwaves and radio waves. In fact, all of these things are electromagnetic energy or in fact forms of light. Being electromagnetic waves, they are waves of energy that can travel through empty space, as I said, and that can be absorbed or emitted or reflected by particles of matter.