﻿Hello, my name's Mr. Benyohai, and welcome to this lesson about refraction. Make sure you've got somewhere quiet we can work and we are unlikely to be disturbed, and then let's get going. For this lesson, you're going to need a pen, pencil, paper, a protractor, and a ruler. Today, we're going to be looking at refraction, which is something that happens to all waves. We'll look at how you can measure refraction. We'll also look at how we can represent refraction using something called a wavefront diagram. And then lastly, we'll look at explaining why refraction occurs. Before we begin though, let's think about what can happen when waves arrive at a material interface. If you've done lesson five, this will be a recap. So you can think to yourself what will happen. If you haven't, that's fine. We're going to go through it. So what can happen when a wave arrives at a material interface? Well, one of three things can happen. If I draw my material in space there, so that's the boundary between two materials, a wave might be reflected, a wave might be absorbed, or it might be transmitted. Now sometimes, when a wave is transmitted, it will go straight through, as I've shown there. But other times, it can change direction. That changed direction as a wave goes from one medium to another is refraction. Here, I have my ray box, a mini whiteboard just to provide a white background. So if I turn on my ray box, here I have a ray of light that's travelling in a straight line, and it's travelling at the same speed as it goes through air. But I'm going to introduce another medium. So here I have some perspex. Now, it looks as though the light is just carrying on straight through that perspex, but actually, so it's being transmitted, but actually, something is happening. That light is slowing down as it goes through the perspex, and that's because the perspex is more optically dense. Now, if I'm travelling along the normal, and if we remember from last time, the normal is a line that's 90 degrees to the material interface, then it will just carry on a straight line. But if the light goes in at an angle, can we see that the light is bending as it crosses from air into the perspex? That effect is known as refraction. Now we can draw all this out to see what's happening. If I take my marker, I can draw on a line to show the change in medium. Here, I'll draw across and across to show the path that the light is taking. And then I'm just going to make a small mark here to show where the light has travelled out. So now, I can remove my ray box, and I can join up these lines. And what I get is this diagram. So my light was coming in on the right, so that was my incident ray, where this was the angle of incidence. My refracted ray coming out on the left with that is the angle of refraction. Now, because the light has slowed down as it's gone from air into perspex, it bends towards the normal. What that means is rather than carrying on in this direction, instead, it has followed a path closer to the normal than it would have done. We also see the converse. If lights goes from a more optically dense material to a less optically dense material, so from perspex to air, then it will bend away from the normal. Pause the video now to make a copy of this definition below and then press play when you're ready to continue. A chance for us to apply some of this knowledge now. Here, I've got a ray that's travelling from air into glass. And glass is more optically dense. That means it's more dense for light. So is it going to bend towards the normal, not change its path at all, or bend away from the normal? What do you think? In three, two, one. It would bend towards the normal, in that direction. This one now, from glass to air. It would bend towards normal, no change, or away from normal? In three, two, one. Well done if you said away from the normal. Because it's moving from something more optically dense to something that's less optically dense, it bends away. Now from air into glass, travelling along the normal. What do you think will happen? In three, two, one. There would be no change. There would be no change because there is no angle of incidence here. This only occurs if there is an angle of incidence and there's a change in optical density. What would happen in this example? In three, two, one. No change. There'd be no change this time because not only is the ray going along the normal but it's also going from air to air, so there's no change in medium. This example in three, two, one. No change. Although there is an angle of incidence, because the medium is the same, there's going to be no refraction. This example. Three, two, one. No change again. Along the normal. Even though there's a change in medium, we get no refraction. This example. Three, two, one. Well done if you said away from the normal. We're going from glass into air, so it's going to bend away. This example in three, two, one. Well done if you said towards the normal. The orientation doesn't matter. We're going from air into glass, so it's slowing down. So therefore, it bends towards the normal. What's your answer? In three, two, one. It would be going away from the normal. Well done if you said away from the normal. What do you think will happen here? In three, two, one. No change. Glass to glass, no change in medium, it would just keep going. Three, two, one. Towards the normal. Because diamond is more optically dense than glass, it's going to bend towards the normal. Here's some independent practise for you now. What I'd like you to do is for each of the scenarios below, determine what could be the lower medium. So is it air or glass or diamond for each of those different media, a, b, c, or d? Pause the video now to have a go at the task, then press play when you're ready to do the answers. And here are the answers for each of the different scenarios. When measuring refraction, what we want to do is find the relationship between the angle of instance and the angle of refraction. To do that, we'll need a ray box, some paper, pencil, protractor, a ruler, and a medium for our wave to go through. So I've just got a perspex block here. Ideally, you'd have a much wider one. I just have a very narrow one. What you need to do is draw a line where we're going to place our block. We need to mark where we're going to cause the ray to interact with it. And then with a protractor, we're going to mark on the normal. And I'm also going to mark on in advance where I'm going to direct my incident ray. So I, here, have got an angle of incidence of 20 degrees, 30 degrees, 40 degrees, 50 degrees, 60 degrees. Now, I can remove my protractor and I can join up this line to show where my normal is, and then I can do each of my incident rays. So I've got one incident ray, two, three, four, five. Now I want to set these up in advance so that I don't need to move my block or try and take any measurements while I'm working, because that will cause inaccuracies in the investigation. So now, I can place my block on the line there and I can direct my ray down the incident line. And what I need to do is mark where the ray is coming out of the block. And my block is mirrored on one side, so it's not actually coming out. If you were to do this, you might see that the ray comes out. What we're interested in though is where it comes out. So I'm just going to make a small mark there and I'm going to move my ray box. Let's move down the other incident ray and make a second mark, make a third mark like that, and a fourth, et cetera. Now, because my block is quite small, it's very difficult to get really accurate measurements, which is another reason you want this to be as big as possible. But then what we do, we can turn off our ray box, and then we join up where the ray went into the block and where it came out. So that is my refracted ray. I would measure the angle of refraction here between the normal and the refracted ray. So where the angle of incidence was 20, my angle of refraction, I can measure it to be 14 degrees. And I could record that on a table of angle of incidence in degrees, angle of refraction in degrees. It was 20 and 14. And I could do that for the next one. It comes out to be 20 degrees. So when the angle of incidence was 30, the angle of refraction was 20. And because we're going from air into glass, we expect the angle of refraction to be smaller than the angle of incidence in each case. To make this investigation as accurate as possible and to make sure we're not moving any of the blocks, we want to use a ray box so that the ray is as narrow as possible. And we also want to be using a sharp pencil because the largest source of inaccuracy here is judging the centre of the ray each time. A chance for some independent practise now. Have a go at these questions and pause the video. Press play when you're ready to go through the answers. Here are the answers to the questions. Use a different coloured pen just to go through them. This next section explaining why refraction occurs is for higher tier only. To explain refraction, we're going to introduce some more elements to our normal ray diagrams. So here, I've got a ray diagram of a wave when it's a light going from air into glass. Glass is more optically dense than air. That means it slows down. So we know from earlier in the lesson that it's going to bend towards the normal. Now a ray just shows the direction the wave is going in, but what it doesn't show is the width of the ray, because rays aren't really infinitely thin. They have some width to them. So I'm going to introduce an element onto this diagram called a wavefront. A wavefront represents all the peaks on the wave, and I can show it with some sort of width. So if I add them on, what I'm going to do is I'm going to consider my ray and I'm just going to draw a mark every two centimetres like this. And then I'm going to draw lines perpendicular to my ray every two centimetres along, like this. These lines are what are called wavefronts. You can imagine looking down on the wave and drawing a line across all the peaks. So there's a peak here, and there's a peak here, and there's a peak here, and a peak here. So we know the distance from peak to peak would be the wavelength of the wave. But let's think about this wave as it's progressing along. And I'm just going to use a pencil to indicate the width of the ray. So the wave is coming along, and every second, it's moving two centimetres. But notice the bottom of the wave strikes the glass first. So this side of the wave is going to slow down. So in the next second, it's not going to move as far, but this side of the wave will continue to move quite quickly. And in the next second, the bottom part is going to move only a small amount and the top part is going to move a large amount. Hopefully, you can see that my pencil is changing direction because one side of the wave is slowing down before the other side of the wave. So let's look at the effect that has on the rest of the diagram. I can continue to draw my wavefronts on my refracted ray by lining up my ruler with this point here where they interact and drawing a line that's at 90 degrees to my ray because a wavefront is always 90 degrees to the ray. That same effect will happen each time these wavefronts interact with the new medium. So I can add on some other ones, and each time, I'm making sure is that each wavefront is parallel to the last. That gives me my wavelength for inside the glass. And I can continue to draw some more, making sure they remain the same distance apart. So this diagram now holds quite a lot of information. Not only does the ray bend towards the normal as it goes from air into glass, we can also see that the wavelength changes. If the wave is slowing down, the wavelength will decrease. If the ray was speeding up, then the wavelength would increase. What must they stay the same on both sides is the frequency, and that's 'cause if we can imagine, at the boundary, if the wave is moving up and down, there's no way on just either side of the boundary, they could be out of sync with one another, so the frequency is fixed. Only the wavelength and the speed can change. This diagram is quite tricky to draw, so what I would do is pause your video now to make a sketch copy of this diagram, making note of the key points, that the wavelength decreases as it's going from air into glass. Some quick fire questions now just to practise some of that information. So I'm going to make a statement and I want you to tell me whether it's true or false. When a wave travels from one medium to another, its speed can change. Is that true or false? In three, two, one. Well done if you said true. The speed can change. When a wave travels from one meeting to another, its wavelength can change. Is that true or false? In three, two, one. Well done if you said true. the wavelength can change. If it's going from fast to slower, then its wavelength will decrease. When a wave travels from one medium to another, its frequency can change. Is that true or false? In three, two, one. Well done if you said false. The frequency is fixed. When a wave slows down, its wavelength will increase. Is that true or false? In three, two, one. That is false. When its speed decreases, its wavelength will decrease. When a wave slows down, its frequency will decrease. Is that true or false? In three, two, one. False. Its frequency is fixed. It cannot change. Some independent practise for you now. Pause the video to have a go at these questions and then press play when you're ready to view the answers. Here are the answers. Just use a different coloured pen to check your answers against them. Some application practise now. Here's a diagram where the question relates to the diagram. Have a go at answering this question. Pause the video and then press play when you're ready to view the answer. Just use a different coloured pen to mark your answers. Press play when you're ready to continue. Well done, everyone. You've done really well in this lesson. We've been through refraction. We've looked at how you can measure refraction. We've looked at how you can draw wavefront diagrams. And we've looked at how we can use those to explain why refraction takes place in terms of which side of the wave is hitting the medium at which point. Make sure to click next at the bottom of the lesson to complete the quiz to show how much you've learned in this lesson.

In this lesson we will be looking at what may happen when a wave moves from one medium to another. We will look at how we can measure the effect of refraction and also how to explain it.

Combined Science

Y11 Combined Science (Higher)

Key Stage 4

Year 11

Refraction

Waves (HT)

UNIT

Wave properties

Calculations with waves

Measuring the speed of waves in water

Measuring the speed of waves in solids

Electromagnetic spectrum (Part 1)

Electromagnetic spectrum (Part 2)

Infrared

Combined science review

Y11 Combined Science (Foundation)

Waves (FT)

Physics

Y11 Physics

Waves

Reflection

Sound

Ultrasound and seismic waves

Lenses

Colour

Black body radiation

Physics content review

Lesson overview: Refraction

Video

Presentation

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Transcript

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16 lessons in Waves: