Hello, welcome to this lesson on refraction, and we're gonna be focusing on refraction through a transparent rectangular block in this lesson.

This lesson is from the Electromagnetic Waves Unit, and my name is Mr. Norris.

So refraction produces lots of interesting visual effects.

And of course a very well known one is just using a standard pencil into a glass of tap water.

And you get this interesting effect here, a discontinuity in a solid object, or an apparent discontinuity in a solid object.

So of course the pencil is still completely solid, and it just gets stranger the more you look at it.

So how does water produce effects like this, and glass and other transparent objects? How do they refract light to produce those effects? That's what we're gonna be looking at in this lesson.

So let's start.

The outcome of this lesson is, hopefully by the end of this lesson you should be able to describe what refraction is, draw accurate diagrams of refraction, and also use the idea of wavefronts to explain what causes refraction.

Here are some key words that will come up this lesson.

Refraction, normal, angle of incidence, angle of refraction, and wavefront.

Each word will be explained as it comes up in the lesson.

This lesson is divided into two sections.

In the first section, we will look at how refraction can be investigated using a transparent rectangular block.

And in the second section of the lesson, we'll look at the results from that investigation and what that tells us about how waves refract.

So let's get going with the first section.

So here's another interesting visual effect that's caused by refraction.

Now, there's only one brass cat in this tank of water.

Look, here's the proof.

Just one brass cat in that water.

But from that original photo, from that angle, it looks like there's two.

Why is that? Well, it's an illusion that happens because light can change direction when it's transmitted into a different medium.

Light that reflects off the cat, or the cat's underwater, so the reflected light from the cat first goes through water, then has to go through the plastic of the tank, and then has to go from plastic into the air of the room for the light rays to get to your eyes, for the light that's reflected from the cat.

But the light rays change direction when they change materials.

So light from the same part of the cat can appear to come from two different places, when in fact it's only just coming from one place, the same point on the cat.

So this change in direction caused when light rays change material that they're travelling through, change the medium that they're travelling through.

That change direction is called refraction.

And all waves can refract when they enter a new material or a new medium, not just light waves.

So don't forget that when light meets a boundary with a new medium, there might be a partial reflection with that surface and also some absorption by the surface, as well as transmission and refraction through the new medium.

So this animation shows that.

You can see the incident ray coming from the laser, and then there's a partial reflection from that surface between air and glass in the outlet.

But a good proportion of that ray is transmitted into the glass from the air, and there's a change in angle, which is the refraction, the change in direction of the light ray as it enters the new material and the angles of the light rays are measured to that line called the normal, which is an imaginary line at 90 degrees to the surface and is drawn at the point where the incident ray, so the ray that comes in, it's drawn at the point where the incident ray hits the surface.

So you can see that labelled in the diagram.

And then we've got that reflected ray there.

So there's a partial reflection from the surface and that angle is the angle of incident, the angle at which the light comes in, that angle at which the light is incident on the surface.

And of course the angle of incident is measured to the normal just like it was in reflection.

And that's the angle of reflection.

And of course the angle of incident should be equal to the angle of reflection for regular reflection.

However, this lesson we're focusing on transmission of light into a new material, and we're, in this case we're getting refraction, which is a change in direction as well as the transmission into the new material.

So that's the refracted ray and that angle of refraction.

So that angle of that refracted ray, the new direction it takes when in the new medium, the glass, is also measured to the normal, just like all the other angles that we've talked about in our work on light so far.

So that's the angle of refraction measured to the normal.

Okay, so let's do a quick check.

Which of these is the angle of refraction? A, B, C, D, E, or F.

Five seconds.

Make your decision now.

Well done if you said angle D.

Angle D is the angle of refraction in this case.

The light is coming out of the laser pen.

F is the angle of incidence and A is the angle of reflection measured to the normal for that partial reflection.

But a lot, most of that ray is transmitted into the new medium and is refracted.

There's a change of direction when it's transmitted into the new medium and the angle of reflection is D.

So well done if you've got that.

Time for a true or false question.

True or false, if the angle of incidence is zero degrees, like in the picture, so the angle of incidence is zero degrees to the normal, so along the normal, so 90 degrees to the surface, that's what normal means, 90 degrees to the surface.

So if the angle of incidence is zero degrees, like in the picture, light is transmitted but not refracted.

Is that true or is that false? Five seconds to decide now.

Okay, and before you settle on your final decision for true or false, I want you to justify your answer using either A or B.

A is refraction is only when transmitted waves change direction and B says refraction is the same thing as transmission.

So it's when waves pass into a new medium.

So you need to make a final decision now about whether the statement is true or false and then which out of A or B backs up your answer.

Five seconds to make sure you finalised your choices.

That's gonna give you the correct answers now.

The correct answer is true.

If the angle of incidence is zero degrees, light is transmitted, so it's going into the new material, but it's not refracted because the light ray doesn't change direction, and refraction is only when transmitted waves change direction.

So you can have transmission without refraction if the light rays travel along the normal, so the angle of incidence to the boundary is zero.

So well done if you went for true and A.

So just to be really clear, B was false.

Refraction is not the same thing as transmission because in this situation in the diagram, you can have transmission without refraction.

Transmission is when waves pass into a new medium and refraction is when waves change direction when they're transmitted, which doesn't always happen because waves can be transmitted without being refracted like in the diagram.

So now we've looked at what refraction is.

We're going to investigate the refraction of beams of light in a transparent rectangular block and it might be made of glass or it might be made of a plastic material like perspex or something like that.

So you're gonna use a ray box, so that's that piece of equipment with a single slit which might slot in the end of the ray box there.

You can just see the top of the slit in this diagram.

So you are gonna use a ray box with a single slit to create a narrow beam of light and just be careful because ray boxes can get hot when they're being used.

So light that's going to be incident on the transparent block will actually refract twice once on the way in when the light enters the block from air and there'll be an angle of incidence and an angle of refraction there.

But then the ray of light will also refract again when the ray of light leaves the block and reenters air and there'll be an angle of incidence and an angle of refraction there as well.

You might also notice that the first angle of refraction at this point is going to be or should be equal to the second angle of incidence, because there's a mathematical rule that links angles which form that shape.

It's the Z angles rule.

If you're not for sure about that, it doesn't hugely matter, it's just a nice link to make at this point.

What you're gonna do is you're gonna use a protractor to measure the angles of refraction for a range of different angles of incidence.

You're gonna change that angle of incidence every time and find out what the angles of refraction are in the block that you are testing.

So there are different ways of setting up and doing this practical.

Here's one way: You can mark out the angles of incidence on paper before you start.

So on paper using a ruler and a sharp pencil, draw out a baseline, that's what you're gonna line up the top of your transparent block with in a minute.

Mark a point on that line.

That's the point where you're going to aim.

All of your rays of light are gonna be aimed at the same point incident on the block.

You'll need a protractor and then using a pencil, mark different angles which are gonna be your angles of incidence.

And of course the angle which is at 90 degrees to the baseline is gonna be at 90 degrees to the surface of the block, which is gonna be your normal line.

Make sure you draw the normal all the way through the baseline above and below.

And then you can draw in lines which are going to be your incident rays at different angles of incidence.

I've done from 10 to 60 degrees, which is a good range of angles to test.

And then what you can do is you can place your block on the paper and draw around it.

So there's the block placed on the paper, and I placed it slightly off centre on our incident rays because we know that the rays of light are going to cross the normal.

So I've left lots of space on that side of the block for the rays to go into.

So draw around the block, so just in case the block moves, you can put it back in the same position.

Incident rays can then be aligned one at a time.

So use the ray box to produce a beam of light that goes along your first incident ray.

And then what you do is you mark the centre, try and get the exact centre of the rays that leave the block, but you've gotta mark them in two places so you can join them back to where they've come outta the block later.

There we go.

Two places marked using a sharp pencil to reduce the uncertainty in your marketing.

Try and mark the exact centre of the ray in two places.

And then you do that for each ray, two places, the next ray, two places, and so on.

And then when you, when you've done all of your angles of incidence, you remove the block to then draw in the path the rays took.

And after that, all the angles can be measured.

So I'm gonna show you removing the block now.

So the block's gone, but it's outlined where it was, it is still there.

And then for the first ray, which is the 10 degree line, which is coloured in blue, that's what you can draw in first, 'cause that's the path the ray took after leaving the block.

But then what path did it take through the block? Well, all you need to do is join those two lines together with another straight line there.

Okay, so that's not one line, that's two straight lines or actually a total of three straight lines, including the incident ray, which are all at slightly different directions.

It's a bit easier to see for the next angles of incidence.

So for the next ray, which is the green one on the screen, that is the ray as it left the block.

But what path did that ray take through the block? Well, it went from there to there.

So you need to use a ruler to make the, draw those rays in.

And the third one, which is the pink one on the screen, that was the direction it took outta the block.

But how did it get there? What route did it take through the block? So you use your ruler to draw a separate straight line to show its path through the block.

And you can do that for all of the angles of incidence that you've planned to test.

What you can then do to measure all the different angles, you might need to draw in some normal lines to measure the angles of incidence and angles of refraction as the rays leave the block there.

Let's do a little check on the angles you then need to measure.

So for that angle of incidence of 10 degrees, which is marked on the screen, and it's also in blue, which angle needs to be measured for each box in the results table? So we've got angle A, angle B, and angle C on the diagram, and each one of those goes in one column in the results table.

But which angle goes in which column of the results table? Five seconds to decide which letter goes in the first blank column, which letter goes in the second blank column and which letter angle goes in the third blank column? Off you go.

So I'm gonna tell you the answers now.

Pause the video if you're not ready yet.

So when the angle of incidence was 10 degrees, the angle of refraction is angle A, that's for entering the block, but for leaving the block, the angle of incidence is angle B and the angle of refraction is angle C.

So actually in the order of A, B, C in the table there.

So nice and simple when you do the experiment.

couple of notes on this experiment.

The slit in the ray box there should be as narrow as possible to reduce a very narrow beam of light, And a sharp pencil should also be used to try and mark the centre of each ray and to draw in the rays.

Both of those steps reduce the random error in marking the centre of the beam and then the uncertainties in the measured angles.

It's more difficult to measure the angle between two lines which are drawn with a thick line because you're not sure what part of the line to line up with the markings on the protractor.

So you need to make the rays as thin as possible using a sharp pencil and mark the centre of each ray as carefully as possible.

A sharp pencil can help you to do that to reduce the random errors.

We should also note that the method should not require the block to be moved between tests.

'cause accidentally knocking the block or moving it between tests could nudge it or you might not put it back in the right place and it might move it slightly off your marked angle of incidence, and drawing around the block allows it to be replaced back in the same position.

So that can help.

But hopefully you won't need that because you won't move the block, accidentally or otherwise.

Okay, time to do a little check on potential sources of error in this experiment.

So identify whether each of the following is a source of random error.

So it would cause results to be slightly different each time, but some might be too high, some might be too low.

Or would each of the below be a source of systematic error, which will make results be inaccurate by the same amount every time.

So pause the video now, read through each statement, one to four and decide would it be a source of random error or systematic error.

Pause the video now.

Off you go.

Okay, I'm gonna go through the answers now.

Pause the video if you need more time.

So statement one and two and four would be a source of random error because each of those would make the results potentially different every time, and sometimes too high and sometimes too low.

Whereas statement three would make the results be inaccurate by the same amount every time.

So it would be a systematic error.

And remember that errors in experiments are always either random or systematic.

All errors fall into those two categories.

So you should always have in your head random error or systematic error.

Right, so it's time for you to do this task now.

So here are the instructions.

Collect a rectangular transparent block and a ray box with a slit and a power supply if needed.

And then step two, with a sharp pencil, ruler, and protractor, mark out the angles of incidence on paper like I showed you before.

Step three, place the block so that a long side is on the base line and draw around it.

Then direct a ray into the block along each line in turn.

And then for each one, mark the centre of the ray that leaves the block in two places with a sharp pencil.

Step four, remove the block and use a ruler to draw in each ray including its path through the block.

And then use a protractor to measure the angles of each ray as it enters and leaves the block.

And on the screen we've got angle one, which is the angle refraction entering the block, and then angles two and three, which are the angles of incidence and angles of refraction leaving the block.

And just a reminder that hopefully you'll find that angle one and angle two are the same, or at least very, very similar when you measure them.

So that's the angle of refraction when entering the block, and that's angle one, and that angle of incidence when leaving the block should be the same angle hopefully.

So you should pause the video now and follow these instructions to complete this task and get a set of data for refraction.

Here are some sample results from the refraction experiment.

So that takes us to the second section of this lesson.

We've investigated refraction and now it's time to look at the results of this experiment and look at, put it all together and look at how waves refract.

Can we come up with some general rules for refraction? So a conclusion states what you found out in an experiment and how you know this.

So that's what we're gonna write now.

So here are the sample results for a ray of light going from air into the glass block, so entering the block.

And the sample results show that the greater the angle of incidence, the greater the angle of refraction.

So that's a very simple conclusion we can make.

We've also found that the angle of refraction is always smaller than the angle of incidence, in this case, when the light is going from air into the block, the angle of refraction is smaller, and it actually gets smaller by more when the angle of incident increases.

So the greater the angle of incidence, the greater the difference between the angles of incidence and refraction.

And you can see this most clearly on a graph of results.

Now the results would be plotted on a line graph because the independent variable, which is the angle of incidence, the thing you changed is continuous, not categoric.

When the variable's categorically, you do a bar chart.

But the variable's continuous the scale of numbers, so we do a line graph and the results don't quite form a straight line.

That dash line would be a straight line and the results curve away from that what would be the linear relationship.

So the results show a non-linear relationship.

So they're not directly proportional.

The angle of refraction is not always a set proportion of the angle of incidence.

The angle of refraction becomes a smaller proportion of the angle of incidence, the greater the angle of incidence gets.

So they were the results for light travelling from air into the block.

But I want you to do a conclusion for the other results that we collected, which was from light travelling outta the block and back into air.

So the sample results are there for you on the screen now, and all I want you to do in this check for understanding is to complete the conclusion for these results.

So the sample results show that the light travelling from a transparent block into air and then try and complete the gaps, please, for the three bullet points.

You should pause the video if you need to.

Right, I'll go through the answers now.

Pause the video now if you're not ready.

So when light is leaving the block and going back into air, we've got the greater the angle of incidence, the greater the angle of refraction again, because both columns increase as you go down, and the angle of refraction this time is always greater than the angle of incidence.

So that's the opposite to how it was when light went into the block.

When light comes outta the block, angular refraction is always greater than the angle of incidence.

And again, we've got the greater the angle of incident, the greater the difference between the angle of incidence and the angle of refraction.

So well done if you've got all of those.

So let's bring this all together now.

So the results showed the key rules for refraction.

When light travels into the block, the light is refracted.

So what shown at the moment is not where the light goes because that ray of light is gonna bend in from where it would've otherwise gone towards the normal.

So watch the diagram now as this now shows the correct refracted ray is bent in towards the normal like that.

So that's the correct refracted ray.

So that angle of incidence is bigger than the angle of refraction, when a ray of light travels into the block, the ray is bent in towards the normal.

And what about travelling out of the block? So the ray of light is travelling from left to right, there's a partial reflection from the surface again, so ignore the partial reflection.

And what we've got this time is the angle of incidence is always smaller than the angle of refraction.

So the angle of refraction is bigger.

So this diagram doesn't yet show the correct refractive ray because the ray has to be refracted or bent out away from the normal.

So watch that diagram on the right now.

There that is the correct refractive ray with the ray bent out away from the normal.

So the angle of incidence is smaller than the angle of refraction or the angle of refraction is bigger when the ray is travelling out of the block.

So to summarise, when light travels into a glass or plastic block, the ray is bent in towards the normal.

So think going in it's bent in towards the normal, and then coming outta the block the light is bent out away from the normal.

Okay, so think in, is bent in, coming outta the block, is bent out away from the normal.

So let's check that we've got the right idea on this.

Which is the correct refracted ray for each of the following diagrams. We've got diagram and diagram two.

Pause the video and choose the correct refracted ray for each of the following diagrams. Off you go.

Right, I'm gonna go through the answers now.

So for diagram one, the incident ray is going into glass, it's refracted in towards the normal.

So the answer is D.

Well done if you've got that.

And for diagram two, the incident ray is coming out of the block, so it's refracted out away from the normal.

So it's ray B.

Well done if you've got both of those right? So we're now gonna look at why refraction happens.

And the answer is refraction happens because waves travel at different speeds in different materials.

So the table shows the speed of light in different materials.

The speed of light is always very, very fast, but it's fastest in air or a vacuum and a bit slower in water, glass and diamond.

And if we look at a representation of refraction using wavefronts like in this animation, so there's rays, but that's wavefronts now, that can help us understand how a change in speed of waves, or represented as wavefronts, can cause a change in direction.

So let's look at that now.

So you can see in the first diagram that one side of the wavefront arrives at the block first, so slows down first, whereas the other side of the wavefront continues at the higher speed for longer 'cause it's not entered the block yet.

So it's gonna travel further in the same time.

So compare the length of those two arrows.

The top arrow, that side of the wavefront travels further than the bottom arrow.

And again, you can see that in the third diagram, the top side of that wavefront has still not entered the glass.

So it has travelled further in the same time as the other side of the wavefront, the bottom side of the wavefront, which hasn't travelled as far, 'cause it's slowed down 'cause it entered the glass earlier.

And you can see that in the fourth diagram as well.

By the time both sides of the wavefront have entered the block, the wavefront has turned.

So that's what causes the change in direction towards the normal when waves slow down.

One side of a wavefront basically enters the block and slows down before the other side causing the turning effect.

And this could be modelled by using two students holding a metre rule both facing forwards.

The metre rule represents a wavefront.

And if the students walk forwards at the same speed towards a boundary drawn with an angle on the floor in front of them, then one of the pupils, Jacob in this case, is gonna reach the boundary and slow down before the other pupil, Sam.

So Jacob had slowed down and Sam continues at her faster speed for longer.

So she's gonna travel further in the next few moments than Jacob does, and the metre rule or wavefront ends up turning.

And the turning effects happens because of the different speeds.

One side of the wavefront hits the boundary before the other, so slows down before the other causing the wavefront to turn.

And the opposite happens when waves leave the block and go back into the air.

They're gonna speed up and and turn away from the normal.

That's because one side of the wavefronts, it's the left side in this diagram on that yellow line, leaves the block and speeds up before the other side.

And that explains the change in direction away from the normal when the waves speed up.

And of course if the angle of incidence is zero, then all parts of the wavefront enter or leave the block at the same time.

So there'd be no change in direction.

Let's do a check of what we just explained.

So complete the two part explanation of what causes refraction.

Here's the first part, what word goes in each gap.

Five seconds, pause the video if you need to.

Off you go.

Okay, here's the answer.

Refraction is when waves change direction as they enter a new medium, you could have said material, because the speed of the waves is different.

Well done if you've got that.

Now here's the second part.

What words fill in these gaps? Five seconds, off you go.

Okay, here's the answer.

Refraction happens if one side of each wavefront enters the new medium and changes speed before the other.

Now it's not direction in that last gap because we're looking for what causes the change in direction, which is the change in speed.

Well done if you've got that.

We're going to look at some final ideas about refraction now.

So at an air/glass boundary, the wave speed decreases and that causes the wavelength, which remember, is represented by the Greek letter, lambda, to decrease too, but by the same proportion.

So why is that? Well, let's think of an example.

If the wave speed halves, then the waves will travel half as far as they did in air in the same time period, which is the time before the next wave arrives at the boundary.

So that wavelength would become half the size.

So the wavefronts would be half as far apart in the new medium because they've gone half as far in the same time period before the next wave arrived.

So when the wave speed halves, that means the wavelength also halves where one changes other, the other changes by the same proportion.

However, the frequency of a wave does not change after transmission or refraction.

So let's think about why that is.

That's because if the wave speed halves, for example in the new medium, there'd be twice as many wavelengths in the same distance.

We've just seen that.

So look at those equal distances, and there's twice as many wavelengths in the same distances.

So the same number of waves will pass a point per second.

They're half speed, but there's twice as many in the same distance.

Therefore there'll be just as many passing a point per second.

So the frequency is unchanged in the new medium.

So we can see all of this in the wave speed equation which links wave speed, frequency, and wavelength.

Wave speed is equal to the frequency times the wavelength.

V, the wave speed, F, the frequency, and the lambda symbol for the wavelength.

So in a new medium, the wave speed changes, but frequency is constant and wavelength changes by the same proportion as the speed changes by.

So which diagram correctly shows the wavefronts when the waves move into a medium with slower wave speed, think about what happens to wavelength.

Five seconds to decide.

Off you go.

Okay, I'll tell you the answer now.

The answer is B.

It has to be B because the waves should be refracted because they are incident on that boundary at an angle.

So it can't be A, which doesn't show any change in direction.

Slower wave speed means the wave speed is a smaller number, so the wavelength must also be a smaller number, as in B, not D, which shows an increase in wavelength.

And in C, the refraction is occurring the wrong way for when waves slow down.

So well done if you put B.

We should just mention that all kinds of waves refract when they change speed across a boundary.

But look carefully 'cause light waves are faster in air and slow it in liquids and solids.

But it's the opposite for sound waves.

Sound waves are faster in solids and slower in air.

What that means is the rules of refraction are opposite for sound waves as they are for light waves with the same materials.

Water waves are faster in deeper water and slower in shallower water.

So identify whether each of the following statements link to diagram A or diagram B.

Pause the video now and have a go at all five.

Okay, here are the answers.

Statement one, wave speed changes from faster to slower, or slower wave speed means shorter wavelength, that's diagram A, and turning towards the normal, whereas the waves turn away from the normal In diagram B.

Light travelling from air to glass, or light is faster in air and slows down in glass.

So that's diagram A.

Whereas sounds travelling from air to glass, sound is slower in air and faster in glass.

So when waves speed up, that's diagram B, turning away from the normal.

And water waves travel faster in deep water and then slow down in shallower water.

So that's diagram.

Well, very, very well done if you've got all five.

Let's do a task now.

There's two parts to this task.

In the first part I'd like you to draw the path that each ray of light takes into and out of each rectangular glass block.

And you should include a normal in your diagram each time the ray changes direction.

And the angles of refraction don't have to be exact, just show the light ray bending in the correct direction towards the normal or away.

And then part two of this task explain why this ray of light is refracted as it enters glass from air.

Pause video now, have a go at this task with your best effort.

Okay, I'll give you some feedback now.

So for the first task at the first ray, when it goes into the glass block, it's gonna be refracted in towards the normal.

When it comes outta the block, it's gonna be refracted out away from the normal.

And one little detail actually is that when rays of light are refracted twice, going into and out of a rectangular glass block with parallel sides, when the ray exits the glass block, it will be parallel to, so the same direction as the original ray that went into the glass block.

So it's refracted in and then out by the same amount.

So it ends up parallel to the original ray.

So if you need to make that little improvement to yours, then you could do that.

Now in the second diagram, the ray isn't refracted 'cause it's incident along the normal, so there's no change of direction.

And in the third diagram, the ray is refracted in when it goes into the block towards the normal and when it comes outta the block, it's refracted out away from the normal and it ends up parallel to the original ray that went into the block, just like we said for the first diagram.

And then the second part of the task explain why this ray of light is refracted as it enters the glass from air.

We're looking for an explanation along these lines.

The speed of light in glass is lower than in air, and in this diagram, the lower side of each wavefront enters the glass and slows down before the upper side, which continues in air at the higher speed until it too enters the glass.

So the wavefront turns because different parts of it are travelling at different speeds.

Here's a summary of the lesson.

When waves are transmitted into a new medium, they can change direction.

This is called refraction.

When light enters glass, it turns towards the normal and the angle of incidence is bigger than the angle of refraction.

When light reenters air from the glass, it turns away from the normal and the angle of incidence is smaller than the angle of refraction.

And this is all to do with the different wave speeds in the different media.

The refraction of light can be investigated using a transparent rectangular block, a ray box, and a single slit in paper, a sharp pencil, and a protractor.

And refraction occurs when one side of a wavefront enters the new medium, so it changes speed before the other side.