Waves transfer energy from one place to another. Right now you're listening to my voice. My voice, the energy is coming from the speaker in your headphones or in your computer. That energy is transferring from the computer through sound waves to your ear. And all sound waves or all waves in general, they all involve some kind of oscillation. Something is vibrating, whether it's a water wave, the water would be vibrating or sound air is vibrating or even light itself. There's electromagnetic waves that are vibrating. Now there's lots of different types of wave experiment. For example, water waves, ripples on a pond. If you've ever played with a slinky as a child, you can make those oscillate. Light itself is a wave, part of the electromagnetic spectrum. And even if you've got maybe a rope, you tie it to a tree and you vibrate it up and down, you'll see waves occurring all the way along the rope. Now we need to learn a few terms to do with waves for the course. And our first one is the wave front. Now the wave front is a line where all the waves are in phrase. If you look at this photograph of a water wave coming through a harbor wall, through the gap in the harbor wall, you can see there's a line where the wave is all doing the same thing. It's all at a peak. That line there and also behind it there, that line there would represent the wave front. So it's a line where all the waves are doing the same thing that all in face. This speed is the measurement of how fast a wave moves forward. It's measured in meters per second. And here you can see a jet and it's actually going faster than the speed of sound. And around it, the sound waves are trying to catch up with the jet and it forms this pressure wave called a sonic boom. Frequency is the number of wave cycles per second and it's measured in hertz. Only human beings can hear from a range of 20 to 20,000 hertz. Wave length is the distance it takes for the wave to repeat itself. Basically the length of one wave. Wave length is measured in meters and here in our diagram you can see two waves. We've got this blue wave here. Wave length, you can say from one peak to another. That distance there is one wave length. Or from where the wave is starting, this distance here would be one wave length. This unusual symbol here, lambda, lambda is the symbol for wave length. The red light, the red wave here. Well, it's got a much smaller wave length, if you can see from there to there, from peak to peak, or from trough to trough, or from where the wave starts to work. Here you can see it's much smaller than the wave that's in blue. Amplitude. The distance from the peak of the wave to the rest position is what we call the waves amplitude. And the amplitude is measured in meters. So here, if there was no wave, the medium would be nice and still. And the medium, maybe if it was water, it would rest at this position here. So when the wave is moving through the medium, the amplitude is from this rest position to the peak, or from the rest position to the trough. If you look, the distance from here to here is the same as the distance from here to here. So that's the waves amplitude. Now one thing you have to learn is the wave equation. And the wave equation states that the wave speed is equal to the frequency times the wave length. So the wave speed is in meters per second. Frequency is in hertz, or per second. And then the wave length is in meters. The triangle here that I've just drawn in the left hand corner. Well, if you want to work out the frequency, so if you want to work out wave speed, it's frequency times the wave length. If you want to work out the frequency, it's wave speed divided by wave length. And finally, if you want to work out the wave length, it's the wave speed divided by the frequency. Now reflection is a property of waves. And reflection occurs when a wave meets a boundary. So here's our chap stuck on this nice little desert island here. When he shouts for help, the sound wave moves forwards. It hits the boundary, which in this case is a cliff, and it'll be reflected back. So here he's showing the sound wave moves forwards and it echoes back. And this is called, you know, an echo. This effect is called an echo. Refraction. When waves travel from one material to another, the wave speed can change. It can either go faster or slower. And this causes an effect called reflection. What happens is, for example, here when light travels into a glass or plastic block, the wave will actually slow down. And what happens is that as it slows down, the wave will bend towards the normal. When it enters, when it leaves the glass, it then continues in the original direction. So it's going through here, slowing down, and the slowing down causes the wave to bend. Here, if you look, I've got an example of maybe water waves going from an area that's deeper to an area that's shallower. As they come through, you can see that what happens is the wave slows down. So it's coming in here, the wave slows down. And because the wave slows down, this causes the wave to bend towards the normal line. Our final property of waves is something called diffraction. Now when a wave encounters a slit, the wave will spread out, and this effect is called diffraction. Here you can see a wave traveling through, and it encounters a boundary, while the boundary is here and here. And there's a gap in the boundary, and as it passes through, it will spread out around the wall here. Now what's interesting about diffraction is, the smaller the gap, the bigger the spreading occurs. You might think it could be the other way around, but it's quite unusual, but the smaller the gap, the more spreading you get, it doesn't mean there's more energy going through. In fact, the energy from the wave will actually be less, but there's more spreading that occurs with this smaller gap. If you also will make the wave spread more, is the lower the frequency of the longer the wavelength of light, the more spreading that will occur. So the longer the wavelength, the more spreading, the shorter the wavelength, the less spreading. Now the diffraction explains why you can hear someone, maybe if you imagine that you've got a person stood here, and then this is maybe a doorway here. You can actually hear someone because as the wave enters through, that wave will spread out and diffract around.