[Music] I'm going back to three size lessons cold UK by the end of this video you should be able to construct ray diagrams to show refraction of a wave and if you're a higher tier student you should be able to use wavefront diagrams to explain the fraction in terms of changes of speed so this video is all about refraction I'm showing you two examples here in both cases the light appears to change direction as it passes through the water and that's due to refraction here's a first key fact about refraction waves can change direction when they change speed moving from one medium to another so the best way to illustrate that is with the ray diagram this shows a ray of light entering a glass block now when light passes from air into glass the velocity of light decreases in other words the light waves slow down this causes the direction of the ways to change and we can show that by drawing the normal at right angles to the surface of the glass like this when waves slow down they bend towards the normal the live waves now passed through the glass block when the waves passed from the glass back to the air their velocity increases in other words the light wave speed or when waves speed up they Bend away from the normal and we can see that here now this causes the image of the object to appear to have shifted position remember that the fraction can happen when any wave changes speed as a passes form at one medium to another now there is one important exception to this if the waves enter or leave the medium are right angles to the surface in other words along the normal then they do not change direction and I'm showing you that here okay now if you're a higher tier student then you need to be able to use wavefront diagrams to explain the fraction so let's look at that now we're going to start by looking at what's meant by a wavefront this is actually quite straightforward idea imagine that a group of identical waves are travelling together like this I've shown transverse waves but this also applies to longitudinal waves the wavefront is an imaginary line that connects all the same points in a set of waves so here I'm showing the wavefront connecting the peaks of each wave however we could also draw a wavefront connecting the troughs and the idea would still apply wave fronts make it easier to visualize lots of waves moving together so now I'm going to remove the wave diagrams and just show the wave fronts we're going to use the idea of wave phones to explain why waves can change direction when they pass from one medium to another this shows light waves about to move from air into glass now when the first wave phones start to move into the glass those parts of the wafer and slow down this causes those part of the wave phones to get closer together as you can see the wavelength gets smaller this causes the waist to change direction towards than normal in other words the waves are refracting and remember that when waves speed up they change direction away from the normal and I'm shown that here this shows wave fronts approaching along the normal once again the waves slow down as they pass into the glass however in this case the whole wave front slows down at the same time and that means that the waves do not change direction remember your fun plenty of questions on refraction and my vision workbook and you can gather by clicking on the link above [Music]