Hello, my name's Dr.

George.

Welcome to this lesson.

It's part of the unit, "Sound, light and vision" And the lesson is called, "Reflecting light from mirrors." Now, you've known about mirrors almost all your life, but do you really know how they work? Let's take a look.

Here's the outcome for the lesson.

"I can investigate the reflection of light from mirrors by tracing beams of light and measuring angles and compare results to the known laws of reflection." Don't worry if you dunno what that all means yet, I'll help you understand it and be able to do it by the end of the lesson.

And here are the key words for the lesson.

I'm not going to go through them now because I'll be introducing them through the lesson, but this slide is here in case you want to come back at any time to check the meanings.

The lesson has three parts, which are called, "Using a protractor", "Investigating reflection by mirrors" And, "Laws of reflection." This first section is about angles because it turns out that measuring angles is going to be useful when we investigate reflection.

So a reminder about angles.

Small angle, the marked angle between the lines on the left.

Larger angle on the right.

We measure angles in degrees and the angle of a full turn is 360 degrees.

A quarter turn is 90 degrees, and we also call that a, "Right angle." It's in the diagram at the bottom.

And the two lines in that diagram are at right angles to each other, but another way to say that is they are normal to each other.

So normal is a key word here.

In everyday life it means ordinary.

But when we use it like this, it means at right angles.

We'll be using that later.

And of course we use protractors to measure angles.

Here's a picture of one.

We can name the parts.

It has a baseline along the bottom, a centre, scale markings, an outer scale and an inner scale.

So if you want to measure an angle with a protractor, there's a particular way to do it.

You put the baseline of the protractor on one of the lines that makes the angle, and then you move the protractor along the line.

You position the protractor so that its centre is on the point of the angle, the place where the two lines meet to make the angle.

And then you use the scale that starts at zero to measure the angle.

That might be the inner scale or it might be the outer scale.

It depends on the situation and how you place the protractor.

Here's an angle, the angle between the two lines in the top picture.

Which of these protractors have been placed correctly to measure this angle? I'll give you five seconds to think, but if you need longer, pause the video and press play when you're ready.

Let's check the answers.

Protractor B has been placed correctly.

The point where the two lines meet is at the centre of the protractor, the point we call the centre, and one of the two lines is along the baseline of the protractor.

We would be able to measure the angle that way.

In D, the protractor's been placed quite differently, but this would still work.

Again, the point where the two lines meet is at the protractor's centre and one of the lines, the other line this time, is along the baseline of the protractor.

Can you see why A and C wouldn't work? In C, the point where the two lines meet isn't at the protractor's centre.

We could get a reading off that, but it would be the wrong angle.

And in A, neither of the lines is along the protractor's baseline.

It would be possible to take readings and calculate the angle, but I don't recommend doing it that way.

Now, what is the angle? You could read it from B or D.

I'll give you five seconds.

And the answer is 52 or 53 degrees.

If you're off by one degree when you measure with a protractor, that's okay.

It's not easy to get it exactly.

But you wouldn't want to be off by more than that.

So notice that in A and C, those are not the readings that we get.

Also, notice that in B, we have to read the outer scale.

And in D, we have to read the inner scale because of which of those scale starts at zero on one of the lines.

Well done if you got those.

Now some more angle measuring for you.

If you're able to get a protractor up against the screen, I'd like you to measure these three angles.

Pause the video and take as long as you need and press play when you're ready.

And here are the angles.

19 degrees, 70 degrees, 42 degrees.

But if you're one degree either way, that's an acceptably close measurement.

So for example, if you got 18 or 20 for the first one, that's okay, but if you were further off than that, perhaps try again and try to make more accurate measurement.

And now for the second part of this lesson, investigating reflection by mirrors.

So if you have a glowing object, a luminous object, it emits light and that light comes out in all directions, and light travels in straight lines at an extremely high speed.

So those are useful things to know.

Now, if light hits a mirror, it reflects off it.

We can represent this using an arrow showing the direction the light is going before and after the reflection.

So it sort of bounces off the mirror.

This ray shows the direction of some of the light leaving the torch, and this shows the direction after the reflection from the mirror.

Now in reality, there are very many rays of light you could draw coming off the torch, hitting every point on the surface of the mirror all at once, all reflecting off.

But we don't need to try to draw all of these.

We can understand how any light reflects from a mirror by looking at just one single light ray and seeing how that reflects.

So we can think about these rays one at a time.

Remember, light rays aren't actually real objects.

They just represent the way light is travelling.

Now there are three laws that describe how a single light ray reflects from a mirror, and these apply to every reflection.

So these aren't laws that make something legal or illegal.

These are laws of the universe.

These are things that we have discovered that are just ways that the universe works.

So you're going to be investigating these and the kind of equipment that we can use here.

We can use a ray box which connects to a power supply and it has a narrow slit.

Inside it, there's a bulb.

And so light from the bulb comes through the narrow slit and you get a thin beam of light.

Be careful when using a ray box, they can get quite hot.

All the light in the beam is travelling in the same direction.

So in diagrams, you can represent this beam by a single light ray.

If you want to draw a mirror in a diagram, you can represent it the way shown here.

So a straight line which shows the flat side, the shiny side of the mirror.

And then on the other side we have these little markings and that represents that that's the back side of the mirror.

And now a question.

Here's a picture of a ray box making a beam of light.

That's what a ray box is for.

It's just to give us an narrow beam.

And that light is reflecting off a mirror.

Which of these four is the correct scientific diagram of the light beam reflecting? How should we draw this? I'll give you five seconds, but if you need longer, press pause and press play when you're ready.

The answer is C.

Can you see why? An A and B, the mirror is not drawn the way we draw mirrors in scientific diagrams. And D shows the ray reflecting off the back of the mirror instead of the shiny side.

Now let's look at how angles are going to come into our investigation of reflection.

Look at the diagram which shows light hitting a mirror represented by a ray called the, "incident ray." That's one of your key words for this lesson.

An incidence just means coming in.

So this is a ray that's coming in to hit the mirror.

And an angle that we're going to measure is the angle between that ray and the dashed line, which is an imaginary line drawn at 90 degrees to the surface of the mirror.

It's dashed because it's imaginary, and it hits the mirror where the ray hits the mirror.

And the line is called the normal.

Remember that word? It means at 90 degrees to.

And the angle we can measure here has its own name.

It's called the, "Angle of incidence" The angle between the ray and the normal.

And we often use a simple i for that angle.

And you might think, "Why don't we measure the angle between the ray and the mirror? Then we don't have to draw an imaginary line at all." But have a look at the diagram with the curved mirror.

With a curved mirror, you can still draw a normal line, a line that's at 90 degrees to the mirror at the point where the ray hits.

And so you have two straight lines and you can measure the angle between them.

If you wanted to measure the angle between the ray and the mirror surface, you have to try and measure an angle between a straight line and a curve.

And how do you even do that? So we always measure the angle between the ray and the normal, not the ray and the mirror.

By the way, a flat mirror is also known as a, "Plane mirror." And now a question for you.

In this diagram we have a laser producing a beam of light that hits the mirror and reflects off.

And there are four angles shown here and they've all got labels, A, B, C, and D.

I'd like you to pick out the one that is the angle of incidence.

Which of these angles is the one that we call the angle of incidence? If you need more than five seconds, pause the video and press play when you're ready.

Do you have the answer? The angle of incidence is C.

It's the angle between the incident ray and the normal.

Well done if you spotted that.

Now there's another angle we're interested in when we investigate reflection and it's called the, "Angle of reflection" And we use the symbol r for it.

It's shown in this diagram.

Can you see that it's the angle between the reflected ray and the normal line? Now you're going to investigate how changing the angle of incidence affects the angle of reflection.

And this is the sort of thing we often do in a science experiment.

Change one thing and see how it affects something else.

Now here are some pupils who are discussing what they think might happen.

They've got some different ideas.

So Alex says, "Each time you test the same i" Angle of incidence, "You might get a different r." So he thinks even if you keep the angle of incidence the same, perhaps the angle of reflection will change.

And he shows that in these diagrams. Aisha thinks perhaps when i increases, r might increase too, but by more.

Have a look at her diagrams to see what she means.

Lucas thinks that when i increases R might increase too, so that they're always equal, as you can see in his diagrams. And Sophia thinks maybe the total angle, i + r will always be the same.

What do you think? And all of these are reasonable ideas because you haven't investigated it yet.

So here's a question before we get on with the investigation.

Which protractor is positioned correctly to measure the angle of reflection, r, in this diagram on the left? A diagram shows a ray reflecting from a mirror.

Only one of the protractors is correctly positioned to measure the angle of reflection.

Which one? If you need more than five seconds, pause the video, press play when you are ready.

Before I show you the answer, I can give you a bit more help.

Here's the angle of reflection.

Here it is on the options.

And it's the angle between the normal line and the reflected ray.

So which protractor is correctly measuring that angle? It's D.

So if you measure the angle between two lines, the baseline of the protractor needs to be along one of those lines.

And in D, the baseline is against the normal.

And the centre point of the protractor should be at the point where your two lines meet.

And again, that's correct in D.

If you set up as shown in D, you'll be measuring off the inner scale of the protractor because that's the one that starts at zero here on one of the lines.

It's zero on a normal line.

So well done if you got this right.

Now I'm going to help you set up for your experiment, and all you need at the moment is a blank piece of paper, a sharp pencil, a ruler and a protractor.

And you start by marking a mirror on your paper using the ruler.

And then mark a point in the middle of the mirror as shown here.

It doesn't have to be exactly the centre.

And carefully place your protractor like this.

So the centre point of the protractor is on the point that you marked, and the 90 degree line of the protractor is along the mirror.

And you're going to make markings every 10 degrees, starting from zero with a pencil like this.

Then remove the protractor and join the point that you marked on the mirror to each of these lines.

First one is a normal line.

It should be at 90 degrees to the mirror.

The others should be every 10 degrees.

And what these lines are for is to show you where to put the beam from your ray box.

So for instance, you might shine the beam along the 10 degree line, and that gives you an angle of incidence of 10 degrees.

So this is just a convenient way of getting all the angles of incidence that you need for the experiment.

When you do this, you'll need to mark where the reflected ray goes.

So you get your pencil and mark the centre of each reflected beam.

The beam might be a little bit wide, a little bit fuzzy, go for the centre.

You might want to use crosses for this.

So mark it in two places, then change your angle of incidence and repeat and so on.

And when you've finished you can complete each reflected beam by drawing a line with a ruler.

And what that does for you is it enables you to measure the reflected angles afterwards with a protractor.

So here is the angle of reflection for the first beam that you saw, and here we've got a larger angle of reflection for the second beam.

Now, just a point about positioning your mirror against the line that you drew.

For a glass mirror, light actually reflects from the back surface, not the front.

So you shouldn't place your mirror like this.

You should place your mirror like this, with its back against the line that you drew.

If you do it the way that's shown on the left, you'll get some angles of reflection or you'll measure some angles of reflection that aren't quite the real angles of reflection.

Now which of the following will help you get accurate results? And accurate means that your measurements are close to the true values of the things that you're measuring.

So think about each option carefully.

And if you need more than five seconds, pause the video and press play when you're ready.

Let's check each of these.

The first statement is correct.

Making sure the mirror is reflecting surfaces in the right place and doesn't move between tests helps you get results that are accurate.

Taking care to position the protractor correctly when marking and measuring angles also helps with accuracy.

If you position the protractor incorrectly, you'll get angle measurements that aren't actually the right measurements.

Always marking the centre of the beam with a sharp pencil also helps you see what's really going on.

And using a narrow a beam as possible helps you see exactly where the beam is going.

Now that's partly to do with the width of the slit and you probably won't have a choice about that, but it may also help to have the ray box reasonably close to the mirror so that the beam hasn't spread too much by the time it hits the mirror.

And now it's time for the investigation.

I'll run through the instructions and then you can pause the video and do the experiment.

So you're going to collect mirror and a ray box with a slit and some ray boxes need a separate power supply as well.

And then with a sharp pencil, ruler and protractor, mark out a mirror the normal and the angles of incidence as I showed you earlier.

And then place the mirror with its reflecting surface, its back surface on the mirror line you drew.

And use the ray box to direct a beam of light along each incident ray and mark the centre of each reflected ray on the paper in two places with a sharp pencil.

When all the reflected beams are marked, you can move the mirror away and use a ruler to complete each reflected ray to draw those lines on the paper.

And then use your protractor to measure the angle of reflection of each reflected ray.

And I recommend that you draw your table of results first so that when you make your measurements, you can fill it in as you go.

So pause the video, take your measurements, and then press play when you're ready.

If you finished your experiment and collected and recorded all your results, have a look at this example results table.

Your results may not be exactly the same as these, but I would expect them to be similar if you've done the experiment correctly.

And let's see what we can learn from these results.

Let's look at the laws of reflection.

So the first law of reflection states that the angle of reflection always equals the angle of incidence.

Here we see a laser shining a beam onto a mirror.

And can you see that as the angle of incidence changes, the angle of reflection always changes so that the two stay the same.

The second law of reflection says that the reflected ray is always on the opposite side of the normal to the incident ray.

In the left hand diagram here, that's not happening and it's not what we see.

And the right hand diagram is correct.

The two rays appear on opposite sides of the normal, and you will have seen that in your experiment.

The third law of reflection says that the reflected ray lies on the same plane, the same flat surface, as the incident ray and the normal.

So if you look at the left hand picture, the incident ray is skimming along the paper and yet the reflected ray is rising up above the paper.

That's not what happens.

What happens is what we see in the right hand diagram.

If the incident ray is along the paper, then so is the reflected ray.

And here's a question.

Which of the following correctly shows the light ray reflecting off a mirror? Think about the laws of reflection.

If you need more than five seconds, press pause, press play when you're ready.

And the correct answer here is A.

Perhaps can see in A that the angle of incidence and the angle of reflection are the same, which isn't true in the other pictures.

Now, we talked about accuracy earlier, getting results that are close to the true values of what you're trying to measure.

And the truth is, however hard you try, there are always going to be some small errors in your experimental measurements.

Now when I say errors, I don't mean ways in which you messed up the experiment, I just mean inaccuracies.

Small differences between your measurements and what the true values are.

In this experiment, the mirror, light rays and protractors all have to be aligned perfectly, which isn't always easy.

And it's difficult to mark the centre of each ray exactly.

And a protractor can only measure to the nearest one degree.

So all of these things mean that there are probably small inaccuracies in your results.

These two pupils have looked at Andeep's results table, here on the right, and they have some thoughts about these results.

Andeep says, "In my results, the angle of reflection" That's r, "Isn't always equal to the angle of incidence (i).

The law of reflection isn't true." But Izzy says, "Andeep's results do support the law of reflection.

In this experiment, you'd expect there to be errors of one to two degrees." Which pupil do you agree with? Who do you think is correct? Press pause if you need longer than five seconds to think.

And the person who has the right idea here is Izzy.

Andeep's results are close to being the same as each other, and where they're different, they're only different by one or two degrees.

And you could expect that from the little errors, the little inaccuracies that you could get in this experiment.

And a final task for you for this lesson.

Firstly, complete and label the diagram to show the laws of reflection, and then fill in the gaps in the sentences.

Take as long as you need.

Press pause, and press play when you're ready.

Okay, let's look at one.

Here's a suitable diagram.

So the reflected ray is drawn in, the normal line is drawn in, the angles are there.

It's easier to label the angles just with letters 'cause they fit on the diagram.

And then on the side you can write what you mean by those letters, i and r.

And then filling in the missing words.

The angle of reflection is always equal to the angle of incidence.

It's on the opposite side of the normal.

If you said always the same as the angle of incidence, that's fine.

It means the same thing.

The two rays and the normal are all in the same plane.

So well done if you've got those or most of them.

And we're at the end of this lesson now.

Here's a summary of what we've done.

If light hits a mirror, it reflects, it bounces off.

Light travelling in one direction, one light ray, only reflects in one direction.

The normal is an imaginary line drawn at 90 degrees to the point of reflection.

The angles of incidence and reflection are measured to the normal.

The angle of reflection is always equal to the angle of incidence at the other side of the normal.

So well done for working through the whole lesson and I hope you enjoyed the investigation and I hope to see you again in a future lesson.

Bye.