Lesson video

Hello, my name's Dr.

George, and this lesson is called Water Waves.

It's part of the unit Waves.

The outcome for this lesson is, "I can explain how water waves consist of oscillations of water and describe how water waves can superpose and reflect." And don't worry if there are any words in there that you don't know yet.

I'll be explaining this as we go.

And here are the keywords that I'll also be explaining.

I'm not going to go through them now, I'll introduce them as we go along, but you can come back to this slide anytime to remind yourself of the meanings.

The lesson has three parts.

They're called oscillation of water as a wave passes, adding water waves, and reflecting water waves.

Let's start.

As you probably know, water waves can be very small or very large.

On the ocean, water waves are caused by the wind and ocean waves can break when they approach the shore or a beach.

As you can see here, we have breaking waves.

They sort of look as though they're folding over and they're turning white.

After a wave breaks, some of the water from it can travel up the beach before it drains back into the sea.

But before an ocean wave breaks, water is not actually being moved forwards.

So here are some waves that haven't broken yet.

And in these, you would actually find that each bit of water is moving up and down, not forwards.

Have a look at this animation.

You could focus on one of the red particles here and you can see that although the wave is moving forwards, each particle simply moves up and down.

What moves forwards is the wave shape or the pattern that these moving particles make, but the water itself isn't being transported along.

Now, between water particles, there are attractive forces and that's why when one part of the water moves up, it pulls the neighbouring parts up too and that's what keeps the wave moving.

The same thing actually happens if you make a wave on a string or a rope.

If you shake one end of the string, each part of it moves up and down in its place, but because it's connected to neighbouring parts, that makes this wave shape travel down the rope.

Have a look at this animation, and again, if you focus on one particle in the wave, you could look at one of the green ones, you'll see that they just move up and down.

They're not moving along with the wave.

So what moves forward with a water wave? Choose all of these that are correct.

And with short questions like this, I'll wait for you for five seconds.

But if you need longer, just press pause and press play when you've worked out your answer.

And the only thing that moves forward with a water wave is the wave shape or pattern that it makes.

The water itself isn't moving forward.

There isn't a force that's moving forward and the particles of the water aren't moving forwards.

So well done if you realised that.

If you've ever seen an object floating on the sea, you'll see that it's not pushed forwards by the waves.

It doesn't move along into the shore.

It bobs up and down.

And that's because the water beneath the object isn't moving forward.

It isn't flowing.

There is an exception to this, which is when waves are breaking on the shore.

You will have seen that people can surf on those waves and they do move forward, but that's a more complicated situation.

We're going to focus on waves that are not breaking.

So the water beneath a floating object moves up and down as the wave passes and that lifts the object up and down.

This wave is moving forward through the water.

It's moving towards the right.

What happens to this next bit of water that's shown in this small square? Is it pushed forward? Does it move up, then down, then up? Does nothing happen to it or does nothing happen until the dip of the wave reaches it? So take a look at the four options, press pause if you need to and press play when you've chosen your answer.

And the correct answer is B, it moves up, then down, then up again.

So in a water wave, the particles move up and down.

Now like all waves, including light waves and sound waves, water waves transfer energy without transferring material.

So the direction of energy transfer is the same as the direction the wave travels.

In this example, the wave is travelling from left to right and so it's transferring energy from left to right.

Quick recap.

What is transferred forwards by a water wave? Tick all that apply, and your choices are water, energy, material, anything floating on the water surface, and anything submerged in the water.

Press pause if you need to and press play when you're ready.

The only thing that's transferred forwards by a water wave out of this list is energy.

Now, any back and forth movement can be called an oscillation.

So an oscillation is a repeating movement back and forth, or you could say up and down, left, right, whichever way the repeating movement is happening.

In a water wave, the direction of oscillation is up and down and that's at 90 degrees to the direction of energy transfer.

Direction of energy transfer in this picture is left to right, and the direction of the oscillations is up down, and those are at right angles to each other.

That's not true in all waves, but any wave where it is true is called a transverse wave.

Another question.

When water waves are moving forwards as shown, what is each bit of water doing? Is it moving forward, oscillating forward and backward, moving upward, or oscillating up and down? Press pause if you need a bit more time to think.

And the correct answer is oscillating, doing repeating motion up and down.

And now can you fill the gap here? The transverse wave is any wave where the direction of oscillation is what to the direction of energy transfer.

And what's missing is 90 degrees or you could say at right angles or you could use the word perpendicular.

Those all mean the same thing.

And here we see it happening again in a water wave on the left and a wave on a rope on the right.

So these are both transverse waves.

Look at the way their particles are moving up and down, but the wave is moving left to right.

Now, have a look at this animation, which also represents a wave.

True or false.

This is a transverse wave.

And I'd also like you to think about how you know.

So when you've answered both of those questions, I'll show you the answer.

And this is actually false.

And that's because in a transverse wave, the oscillation is at 90 degrees to the direction of energy transfer.

But in this wave, the oscillation is parallel to the direction of energy transfer.

It shows that the energy transfer is going left to right because that's the way the wave's moving.

But could you see that individual particles were moving left, right, right, left, which is parallel to the direction that the energy is being transferred? So that's not a transverse wave, although it is a wave.

And now a longer task for you to think about.

Some pupils are modelling a water wave.

They want to show how it's made.

They put their arms on each others' shoulders like this.

The first in the line bends their knees, each pupil bends their knees when they feel slightly pulled down by the pupil before them.

It's not the real thing, but it's a way of trying to represent or help us understand the real thing.

I'd like you to think about three ways that this is a good representation of a water wave.

So three ways in which it does match the way a real water wave is, and three ways in which this is not an accurate representation of a water wave.

So you're going to be evaluating this model.

So press pause when you write down your answers and press play when you're ready.

Now let's take a look at some of the things that you could have said.

Here are some examples of ways in which this is a good representation of a water wave.

The pupils only move up and down like each part of the water surface.

The pupils are connected.

So when one moves down, the next one is slightly pulled down too, like neighbouring parts of the water surface.

The pupils oscillating up and down creates a moving wave shape, just like the oscillations of the water surface do.

And the oscillations are at 90 degrees to the direction of wave travel, just like in a transverse water wave.

So well done if you managed to come up with two or three of those.

And here are some ways in which this is not an accurate representation of a water wave.

In a water wave, each part of the water surface is forced up and down by the neighbouring parts.

But in this model, the pupils are just bending their own knees at the right moment.

This means that energy from the first pupil isn't actually being transferred down the line.

Each pupil is just depleting their own energy store in turn, so not actually getting their energy to move up and down from the next person.

In this model, the pupils only move below their starting height.

In a water wave, each part of the water surface moves above its starting height as well as below.

And to make a water wave, the first bit of water has to be forced to move, but this is not shown in the model.

The first pupil just starts moving when they want to.

So well done if you've got some of these and you may have thought of some of your own as well.

Now let's move on to the second part of the lesson, adding water waves.

First, it's helpful to know the meanings of some words that we can use here.

The very top of water wave is called a peak or crest.

Two words meaning the same thing.

So these points here.

And the lowest point is called a trough as shown here.

When water is calm and flat, we can say that the water is in its rest position.

So on the left, we have some water with no wave.

On the right, there's a wave passing through and that dash line shows the rest position of the water where it is if there's no wave.

And when a wave travels through at any moment, some of the water is above that rest position and some of it is below as you can see.

So now three questions for you.

Have a look at the picture.

Which of these four labels shows a peak, which shows a trough, and which shows a part of the wave in its rest position? Press pause while you're thinking about that and press play when you're ready.

So let's look at the answers.

The only one showing a peak is labelled D, it's the very top of a wave.

The trough is the lowest point, so that's C, And a part of the wave in its rest position, well, that will be a part of the wave that's halfway up between the peaks and the troughs, and the only one there is A.

Well done if you pick those.

So the water oscillating at the surface moves just as far above the rest position as it does below the rest position.

Take a look.

And that's why the peaks are the same distance above the rest position as the troughs are below it and that distance is called the wave amplitude.

It's the greatest distance that the water moves from the rest position.

So both of these arrows show the amplitude.

They're both the same length.

The amplitude of a real water wave can decrease as the wave travels.

In science lessons, we often ignore this effect and we stick to the simple situation where the amplitude is staying the same.

The reason why the altitude does decrease in reality is because water particles dissipate some energy as they collide with surrounding air and water particles.

So some energy spreads out from the wave.

As the wave travels, the oscillations of the wave decrease sightly and the random motions of the surrounding water and air particles increase slightly.

So there's actually a tiny heating effect of the water and the nearby air, and that's where the energy is going as the altitude decreases.

Now, which of these arrows show the amplitude of this wave? There might be more than one.

So press pause while you're thinking and press play when you're ready.

There are actually four correct labels here.

A, D, E, and G are all showing the same distance.

It's the amplitude, it's the distance from the rest position up to a peak or from the rest position down to a trough.

Now that we've learned those keywords, let's look at adding water waves.

If two waves arrive at the same point, at the same moment, the water is displaced, it's moved by both waves at the same time and that makes a larger wave at the moment when two peaks meet.

So have a look at these two peaks.

When they meet, they're joining to make a larger peak.

And when two waves are in the same position, it's called superposition.

Super here actually means on top of, so superposition means the two waves are on top of each other.

The moment when the peaks overlapped, we had this larger peak.

And after this, the individual waves continued.

So waves can pass through each other in this way.

At the moment when a peak meets a trough, the superposition causes the two waves to cancel each other out.

So here's that moment in the picture on the right, the two waves completely cancelled out because they had exactly the same amplitude.

If the waves have different amplitudes, then the trough of one only partly cancels out the peak of the other.

I'll show you what that might look like.

So here the trough is smaller, has lower amplitude than the peak, so they never exactly cancel out.

When they're on top of each other, we see what's in that picture there, a small peak.

And the wave you end up with when two or more waves superpose is called a resultant wave.

What happens when two water waves collide? Tick all that apply.

Do they hit each other and bounce off, pass through each other, superposition, adding up or cancelling out, or do they superimpose? Press pause if you need more time to think.

There are two right answers here.

They do pass through each other and they do superpose.

They add up or cancel out.

They don't superimpose, it's a similar word, but it's not the right word to use here.

Now these two waves on the left are shown at the moment when they pass through each other in opposite directions.

So what is that going to look like? Which of these options shows the correct resultant wave? The right answer is A.

Can you see that in these two waves on the left, the peaks are in the same place and the troughs are in the same place and they're going to add together.

We have a peak that is two squares high, combining, superposing with a peak that is one square high, we're going to get peaks that are three squares high.

And similarly, troughs that go down three squares.

And now a longer question for you.

Each of these three examples shows a moment where two water waves pass through each other.

On the blank grids, draw the resultant wave for each moment.

And you may find the squares helpful here.

So press pause while you're doing this and press play when you're finished.

Let's look at the answer to question one.

Here's what we get.

We had two waves superposing.

Their peaks are in the same place, their troughs are in the same place, and we have two peaks that are one square high.

We have an amplitude of one square for both.

We're going to get a resultant wave with an aptitude of two squares.

Now in the second picture, we have peak meeting trough and the amplitudes are the same.

So we have a one square high peak meeting a one square low trough.

We're going to get nothing at all.

If these waves haven't stopped existing, they will actually pass through each other and come out the other side.

And in three, we have peak meeting trough, but they're not the same size.

Notice that the amplitude for the first wave is twice the aptitude of the second wave.

So on the left of the picture where a first wave peak meets the second wave trough, the peak is two squares high, the trough is one square low, so we're going to end up with a peak that's one square.

We end up with a wave with amplitude one square.

Well done if you're starting to get these right.

Now, let's take a look at the third part of this lesson, reflecting water waves.

First, I'm going to show you another way of representing a wave that's easier than drawing this.

Any wave can be represented by drawing lines called wavefronts, and each wavefront marks the position of a peak like this.

So they're just parallel lines representing the peaks and the distances between the wavefronts is a fixed distance for that particular wave.

Notice there's the same distance here every time, and that distance, by the way, is called the wavelength.

The wavefront representation doesn't show the amplitude of the waves.

We can't tell by looking at those parallel lines how high or how low those waves are going.

Water waves reflect when they hit a hard surface.

In this film, the wave starts at the left and can you see that when it hits the right-hand end of this tray, it bounces back, so travels to the right, hits the end and travels back towards the left.

When water waves reflect, they obey the laws of reflection.

You might have heard about these when studying light.

So the laws of reflection say that the angle of reflection r is always equal to the angle of incident i and the two angles are on opposite sides of the normal line and the two angles are in the same plane.

What that means is we're going to end up with reflection looking rather like this.

So here we have blue lines representing the direction of travel of the wave before and after it's reflected.

And these are very much like light rays that we draw to represent the direction of travel of a light wave.

And here we can see that the angle between the incoming wave direction and this imaginary normal line is i and the angle between the reflected wave direction and the normal line is r.

And what the law of reflection is saying is that these two angles are an opposite sides of the normal, they're the same size and they're in the same plane, and let's see how to draw a wavefront diagram for reflection.

So this is the sort of thing you don't normally do for light reflection, but we quite often do for water reflection.

So look at picture one.

Here, we have some wavefronts moving towards this barrier that the wave is going to reflect from.

It's useful to draw the wave direction and that's shown in picture two with the blue lines.

One has been drawn at each end of the wavefront.

This is gonna help us draw more wavefronts.

And in diagram three, the rest of the wavefronts are drawn in up to the barrier.

Then we need to draw the reflected wavefronts and we can continue the blue lines to help us do that.

So here when we draw the reflected blue lines showing the reflected wave direction, we follow the laws of reflection.

And so that's been done in picture four.

And in picture five, we can start filling in the reflected wavefronts, drawing them in between the two blue arrows.

And in six, that's been finished.

You can see that the reflected wavefronts are overlapping with the incoming wavefronts in one area and it creates this crisscross pattern.

And you do actually see that with water wave reflection.

Make sure when you're drawing these that the rays reflect at the correct angle, angle of incidence equals angle of reflection, and that the wavefronts stay parallel with each other.

So which of these four pictures shows water waves reflecting correctly? Press pause if you need more time to think.

The only correct answer here is B.

In A, the angle of reflection is correct, but the wavefront distance has changed.

I mean, the wavelength has changed, and there's no reason why that would happen to the wave.

In C, again, the wavelength has changed after reflection and the angle's wrong and somebody hasn't used a ruler.

And in D, the angle of reflection is not right.

Now I'd like you to have a go.

Complete the diagram to show water waves reflecting from a hard barrier.

Press pause while you do this and press play when you're finished.

Let's have a look at what the answer should look like.

Yours could look like either of these.

If you've drawn arrows for the wave direction, that's fine.

If you've done it without them, that's also fine.

So well done if your diagram looks like one of these.

You may find that you need a little practise to get these right.

And now we've come to the end of the lesson.

So I'll give you a summary.

As a water wave moves forward, each bit of water is moving up and down, oscillating, not forward.

Water waves are transverse waves.

The direction of oscillation is 90 degrees to the direction of energy transfer.

The amplitude of a water wave is the greatest distance that water moves above and below the rest position.

Waves that pass through each other can add up or cancel out.

This is called wave superposition.

And water waves reflect from hard barriers, obeying the laws of reflection.

So well done for working through this lesson.

There was a lot to learn.

I hope you've enjoyed it and I hope to see you again in a future lesson.

Bye for now.