 Previously we looked at the absorption of light to create chemical changes and then the emission of light as black body radiation, which we can extend across the electromagnetic spectrum. The electromagnetic spectrum extends far out beyond the visible light that we can see. Physically there's no difference between photons of higher or lower frequency, a radio wave and a gamma ray are mostly the same thing, the difference is in how they interact with matter. We can't see radio waves in X-rays not because they're special, but because they don't have the right energy to trigger a chemical reaction. That only happens for the visible and UV region. So in this part we're going to go all the way down to the lowest energy that we're going to deal with, the microwave region. But before kind of getting into what microwaves do to molecules, we're going to get an appreciation for their scale. So you can see hot spots inside a microwave oven with some regularity and this is due to the wave-like nature of microwaves. These hot spots are spaced a few centimetres apart and as each represents a peak and trough of a wave, the complete peak-to-peak wavelength is therefore twice this measured distance, giving us a wavelength of about 12 centimetres. The cold spots correspond to nodes in the standing wave. Here there's no oscillation of the field, so therefore no force to interact with the molecules. Now we'll get to the nature of that interaction shortly, but first some maths. If you look on the back of a microwave oven, you'll find some indication of its operating frequency. Now with cold frequency before, it's the number of times per unit time that the field oscillates backwards and forwards. But this experiment here gets us wavelength. That's a distance. And if you take a distance and divide it by time, or multiply it by reciprocal time, you get speed. Specifically if we do this calculation of 12 centimetres, that's 0.12 metres, multiply it by 2,500 megahertz, that's 2.5 times 10 to the 9 reciprocal seconds. So we get 3 times 10 to the 8 metres per second. If that number sounds familiar, it's because it's the speed of light. The frequency and wavelength of electromagnetic radiation is related by the speed of light. I would strongly recommend against ever memorising equations for this because there are actually many different unit conversions and conversions that we can do. Instead, look to the units for guidance. If you memorise equations, you'll invariably do something really stupid like, I travelled 10 kilometres in 20 minutes, 10 divided by 20 is 0.5, therefore I travelled at 0.5 miles per hour. Clearly, that's nonsense. So if you have frequency in per second, you can convert to wavelength by taking the speed of light in metres per second and dividing it by that frequency. The per seconds cancel, leaving you with metres, which is distance. So to keep it at proof, work in metres wherever you can. Now, bear in mind that we do use different units and standard units depending on what's convenient for different regions of the electromagnetic spectrum. So to keep it at proof, work in metres if you can. So now we've established a link between frequency, wavelength and the speed of light and seeing how big microwaves are. What's the nature of that microwave-molecule interaction? We know it can't be a chemical change because there isn't enough energy to move electrons around. So what form of useful spectroscopy can we get out of microwaves?