Hello, my name's Dr.

George, And this lesson is called Refracting light, which is different from reflecting light as you'll see.

And the lesson is part of the unit Making images.

The outcome for this lesson is I can describe and draw how a ray of light refracts when it moves from one medium to another, and explain how refraction can cause virtual images.

Don't worry if there are words in that that you don't understand yet.

I'll be introducing them during the lesson.

And if you want to come back and check the meanings of any keywords from this lesson, you can come to this slide anytime.

The lesson has three parts called Investigating refraction, Conclusions, and Refraction illusions.

So let's start on the first part.

When light hits the surface of a material, some of the light may be reflected, but some of it might be absorbed, so go into the material and not come back out.

And some may be transmitted through the material.

That means it makes it through.

So transparent materials like glass and water transmit most of the light that hits them.

They let most of it through.

So you can see through them.

And that's why we can see the stems of these flowers through the water in the glass.

Opaque materials don't let light through.

They reflect or absorb most of the light that hits them, such as this leaf for example.

It's reflecting green light and absorbing the other colours of light.

When light is transmitted through transparent materials, this can sometimes cause illusions.

So what happens is you see something that isn't really there.

For example, this transparent block, which could be made of glass or plastic, can make the middle of a person's finger look like it's slightly to one side of the rest of their finger as you can see here.

And if you put a pencil or a straw in water, the part of it that's underwater can look disconnected from the part of it that's in the air.

As you can see here, it almost looks as though the pencil is broken halfway along.

And the reasons these illusions happen is because light can change direction when it's transmitted into a new material.

And that effect is called refraction.

Now I'm going to be using the word medium in this lesson, and medium is the scientific word that we use to mean the material that light is travelling through.

So if it's travelling through glass, glass is the medium for that light.

If it's travelling through air, air is the medium.

And here's an example of refraction happening.

We have a beam of light changing direction when it goes into the block and changing direction again when it comes out.

And light only changes direction or refracts at a boundary between two media, two different materials.

That's the only time apart from reflection when light suddenly changes its direction.

If the medium doesn't change, then light travels in straight lines as you've learned.

So here's a question now.

Which of these diagrams correctly shows how a beam of light passes from air into and out of a glass block? And you already know enough about refraction to be able to work this one out.

Press pause if you need longer than five seconds to choose your answer.

And the correct answer is C.

The ray changes direction when it crosses a boundary from one material into the other and it doesn't change direction at any other time.

Now you can get transmission happening without refraction.

So transmission means the light gets through the material.

Refraction means it changes direction.

So these are two different things that could be happening at the same time.

But on the left here, that laser beam is going through that circular object.

So it's being transmitted, but it's not changing direction so there's no refraction.

In the picture on the right, transmission is happening, the laser beam is getting through.

But there is also refraction happening at the two boundaries.

Now which of these diagrams show refraction? And there's more than one correct answer here.

I'll give you five seconds, but if you need longer press pause and press play when you're ready.

And here are the correct answers.

In all of these diagrams, there's a change of direction when the light crosses a boundary that's refraction.

In A and D, there is no change of direction, there's transmission without refraction.

And what's happening in E is reflection.

The light isn't passing into a new medium, it's not refracting.

Now you are going to investigate refraction in a transparent rectangular block.

It might be made of plastic or glass.

And you're going to create a narrow beam of light using a ray box with a single slit.

But be careful because after a while the ray box can get quite hot.

Some of the light that hits the block from the ray box may be reflected, but you're going to ignore that because we're not investigating the reflection today.

We're going to focus on the refraction.

And you're going to trace the paths, record the paths taken by four different beams of light through the transparent block.

So first you'll draw a baseline in the middle of a sheet of paper with a ruler.

It's just a straight line that you draw with a sharp pencil.

And then you go to draw a line through it at 90 degrees and this will be the normal.

And you can use the corner of a block to make sure that it is at 90 degrees.

And draw the normal as a dash line because it's an imaginary line.

And then draw three lines at different angles to the normal that hit the baseline where the normal cuts it.

And you're going to use these to line up your beam of light from the ray box later.

So something like this, the exact angles don't matter, but they should be quite widely spread.

Then you're going to place the transparent block so that its top edge is against the baseline like this, and draw around it.

And now you're going to want to keep it in the same position.

So take care not to move or knock it from now on.

But if you do, put it back in the outline that you've drawn.

And then one at a time, you're going to line up the beam of light so that it passes along one of these three lines that you've drawn.

And you're also going to line it up with the normal as shown here.

And you'll trace the path that each beam takes through the block.

So you can line it up against the next line and the next one and the next one.

So you'll do these one at a time.

And you're going to mark the centre of each ray that leaves the block in two places.

In fact, it's a beam really rather than a ray, but you're going to draw a ray to represent it.

And it may be a little wide, a little fuzzy.

So you'll put a cross with a sharp pencil at the centre of the beam in two places as shown here.

And once you've done this for all four positions, you're going to remove the box, remove the ray box, and then use a ruler to join each pair of crosses taking the line back to the edge of the block like this.

So this is how you're going to show where the light went.

And then you can use a ruler to show where the light must have gone when it was passing through the block.

We know light travelled in straight lines except when it hits a boundary.

So it must have travelled in the straight line from where it went in to where it came out.

And now you've drawn, you've traced the whole path of that beam of light.

And you can do each of these in a different colour to make it clearer which was which.

Now before you do the investigation check that you've understood by trying to put the steps here in the correct order that you're going to do them in.

So press pause while you think about that and press play when you've decided on the order.

And the correct answer is B, then A, then D, then C.

And if you didn't choose that order, have a look again now and check that you understand the method for this investigation.

So now you're going to start.

I'll leave a summary here for you to check what you're doing.

But I'll just run through it very quickly again.

You're gonna need to collect a transparent rectangular block and a ray box with a slit.

And if the ray box needs a separate power supply, you'll have to collect that too.

Use a ruler to draw the baseline on paper and the normal and three other lines where you're going to put the incident beam.

Place the block on the paper, draw around it.

Shine a narrow beam of light from the ray box along the normal and put crosses to record where it went.

And then do that along each of the other three lines.

And then use a ruler to draw in the path of the beam each time.

And highlight each complete ray that you've drawn in a different colour.

So press pause, take as long as you need to do the investigation, collect your results.

And when you've done that, press play.

And here are some example results.

And here you can see different colours for each ray so that you can see which is which.

Now this next part of the lesson is called Conclusions.

And we're going to think about what we can learn from these results.

A reminder of what a conclusion is.

It states what you found out in an experiment and how you know.

This is going to help us think about that conclusion.

Two questions for you.

Which of these four beams is not being refracted and which of the beams is refracted the most? Press pause if you need more than five seconds to think, press play when you're ready.

And here are the answers.

D is not refracted.

It's not changing direction when it crosses the boundaries.

A is refracted the most, meaning that its direction is changed most strongly when it hits the boundaries.

So what we found is light that hits the surface of a transparent material along the normal, you could say straight on, is not refracted.

But what we can also say is that the greater the angle of a beam to the normal, you could call that the incident angle, the more that beam is refracted it's change in angle.

So that summarises what these results are showing us.

Now travelling into a transparent material from air, when that happens, we could say the ray is bent in.

It's bent towards the normal.

This dotted line shows where the ray would've gone if nothing had happened and it moves further into the normal than it would've been.

Travelling out of a transparent material, which you saw when your ray came out of the block.

So from the block back into air, again the dotted line shows where the ray would've gone if there had been no boundary, no change of medium.

And it's bent out compared with that.

We could say it's bent away from the normal.

And if you look carefully at your results, you should see that for each ray it bends as much out of the normal on the way out of the block as it did into the normal on the way into the block, the same angle.

So which option shows the correct path of the refracted ray in each diagram? So there are two questions here.

And in each one, pick A, B, C, or D for which one of these looks like it would be the refracted ray.

Press pause and press play when you've chosen your answers.

And the correct answer in question one is D.

D is the only ray that is shown bending towards the normal.

And that's what we expect when a ray goes from air into a solid material.

And in question two, we expect the ray to bend away from the normal when it goes from a solid material into air.

And B is the only ray that is doing that.

Well done if you got those right.

Now a ray that enters and leaves a block through parallel sides as we saw in the investigation, ends up travelling in its original direction when it comes out, but it's displaced to the side.

Have a look at the diagram to see what I mean.

The ray going in is parallel to the ray coming out, but they wouldn't join up in a straight line.

The ray coming out is displaced, it's moved across.

So the dash line here shows where the ray would come out if it hadn't been displaced.

And as we've already seen, the ray is refracted in towards the normal when it enters the block, and out away from the normal when it leaves the block by the same amount.

It's refracted by the same angle.

So which of these diagrams shows a ray refracting correctly through a transparent block? Only one of them is right.

Press pause and press play when you've chosen your answer.

And the correct answer is C.

If you look at that carefully, the ray coming out of the block is parallel to the ray coming in and it's displaced.

And also the ray is refracting in the right directions going into and coming out of the block.

Now I'd like you to draw the path of each ray of light in the three diagrams here.

So I'd like you to draw the path it takes going into and out of these rectangular glass blocks.

And each time you show the ray refracting, draw the normal.

That will help you think about which way the ray is going to go.

The amount of refraction doesn't have to be exact, you wouldn't know exactly how much the angle is going to change, but it's important that you just show it in the right direction.

So press pause, take as long as you need, and press play when you're finished.

I hope you realise that in the second diagram there is no refraction because the ray of light is travelling along the normal.

And in the other two diagrams there is refraction and the ray bends towards the normal when it enters the block, and away from the normal when it leaves the block.

So well done if you got most all of that right.

And now for the final part of this lesson, Refraction illusions.

Now we saw earlier that your finger can look strange if you see part of it through a block.

And this happens because a ray that enters and leaves the block through parallel sides, although it does end up travelling in the original direction, it's displaced to the side.

And that effect leads to the appearance of part of your finger being displaced to the side.

Now have a look at the second diagram.

Your eye sees the light coming from the finger and traces back to where the rays seems to have come from.

And that's where you see an image of that path of the finger which now looks displaced.

So here's the virtual ray showing a path that the light seems to have travelled along but didn't really.

The actual path the light took is shown by the rays that are drawn in the normal way as solid lines with arrows.

So now can you pick out which of these sentences explains why this illusion happens? Press pause and press play when you've decided.

The correct answer is that it happens because light changes direction as it enters and leaves the glass.

A only gives part of the reason.

So that part of the finger that we see in the wrong place is a virtual image.

It appears where rays of light seem to have come from but not where they've actually come from.

And that's what makes a virtual image.

The pencil-in-water illusion works in the same way.

So we see the part of the pencil that's in the water where the rays seem to have come from.

So our eyes just don't know that along their way to us, these rays had their direction changed.

They're receiving rays coming in a particular direction and they trace back to where those rays seem to come from.

And there's a virtual image.

Another place where you can see virtual images is when you look in a mirror.

The image in a mirror looks as though the light is coming from behind the mirror, but it isn't really.

So water often looks shallower than it really is.

And objects underwater don't look as far down as they really are.

And that's for a similar reason.

In this diagram, the object at the bottom of the pool is scattering light, and light from the object refracts when it passes from water into air.

So by the time it reaches our eyes, it's not travelling in the direction that it originally was.

And so we see a virtual image of the object where the light seems to have come from.

And that is less deep than the object itself.

So images underwater always appear to shallower depth than where the object really is.

And now a question for you.

Andeep is standing by a pond and there is a fish underwater near Andeep.

Where does the fish appear to be? And I'll give you four choices.

So think about that.

Pause while you're thinking and press play when you're ready.

And did you choose B? And here's the reason why it's correct.

That is where the light appears to be coming from.

And so that's where Andeep sees the fish.

And now a couple of sentences for you to complete.

So read those.

Choose suitable words to fill the gaps.

Press pause while you're thinking about that and press play when you're ready.

In the first sentence, here's the missing words.

When rays of light leave the water and enter air, they refract away from the normal.

That's what happens when light passes from a solid or a liquid into air.

Andeep sees a virtual image of the fish where the rays of light appear to have come from.

So well done if you pick those words.

And now you're going to try out a refraction illusion for yourself.

You'll need a mug that's not transparent and two coins, any type of coins will do.

And you place the coins at the bottom of the mug.

Then position yourself so that you can only see one of the coins.

And the top diagram here shows how that can work.

Light rays from one of those coins can reach the person's eye and rays from the other can't because the side of the mug is in the way.

You're going to stay exactly in that position while someone else carefully fills the mug with water.

And they need to do it slowly so that the coins don't move about.

I'd like you to predict what you think you'll see when the mug is filled with water.

And obviously this has something to do with refraction.

Explain why you think you'll see this and then try it out and see what happens.

And afterwards, see if your prediction was correct.

And do you think your explanation was correct? And if you like, you could improve your explanation.

So press pause for as long as you need when you do all that.

And when you're finished, press play and I'll show you some example answers and explain how this works.

I hope you saw the illusion.

Now let's have a look at what happens and why.

One possible way you could write the prediction is.

As the water was poured in, the edge of the second coin slowly became visible.

The first coin remained visible.

It appeared to move slightly in my field of view.

So did you see or did you predict that you would see that the coin you couldn't see became visible when water was added? And why might that happen? Here's an explanation.

With a mug full of water, rays of light reflecting from the coins refracted away from the normal when they moved out of the water into air.

And the diagram at the bottom is showing that.

Rays of light from the second coin could now enter my eyes so I could see it.

What I was actually seeing was a virtual image of the second coin located where the rays of light from that coin appeared to be coming from.

So your answer doesn't have to be exactly the same words as that or as long as that.

But did you include the idea that light refracts away from the normal when it enters air and that your eyes trace back to where those rays seem to come from and you see a virtual image there and it's higher up than the position of the real coin? So well done for doing that.

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

So here's a summary.

Light changes direction when it is transmitted into a new medium, material, at an angle to the normal.

This is called refraction.

When light enters a transparent material from air, it turns towards the normal.

When light leaves the transparent material and reenters air, it turns away from the normal.

A ray that enters and leaves a transparent block through parallel sides ends up travelling in the original direction but displaced to the side.

You can see that in this diagram.

Refraction can cause virtual images of objects to appear where light seems to be coming from, e.

g.

water appearing shallower than it really is.

So I hope you enjoyed the lesson.

And I hope you now understand why things sometimes look strange when they're partly underwater.

I hope I'll see you again in a future lesson, so bye for now.