 Now the tricky thing about light is that although it's well described as a wave, it turns out sometimes to behave like a particle. In fact this is one of the things that Einstein is famous for discovering and it's called wave particle duality. It turns out to apply to the electron and other tiny particles as well. So you can think of a beam of light from a torch as a wave of electromagnetic energy spreading out from the bulb. But you could also think about it as a stream of tiny particles, little packets of energy that have no mass. These light particles are known as photons. And it turns out that individual photons can be absorbed or emitted, which is the same as saying they're spat out, by atoms as though they were particles. However even when you have an individual photon and even though it is a particle it still has this property of wavelength and it can still spread out and travel across space like a wave. Now we've said wavelength determines color, but color is really just how animal eyes and brains experience some of the wavelengths of light. The crucial thing that the wavelength of a photon tells you is how much energy it's carrying. And the equation that gives you this information is E equals HC over lambda. C is the speed of light which is constant as we mentioned before. And H is called Planck's constant which has a value of 6.626 times 10 to the minus 34 joule seconds. And you do need to know that this constant, this Planck's constant is a phenomenally important constant in physics and chemistry. And if you go on to do more science at uni you will be meeting it again. Anyway you can see from the equation that if this part here is all constant then as wavelength goes up the right hand side of the equation gets smaller and smaller which means the energy is getting smaller. So large wavelengths mean small energy and small wavelengths mean large energy. This means that a photon that has a short wavelength carries a lot of energy which is why UV light and X-rays with their tiny little wavelengths are quite dangerous. So those photons carry enough energy in them to burn your skin or to get into your DNA and start messing with the molecular bonds. Long wavelength photons however like radio waves are pretty harmless which is why we don't mind broadcasting radio waves from hilltop to hilltop.