Hello, I'm Dr.

George, and this lesson is called Reflections in Mirrors.

It's part of the unit sound, light and vision and I hope you'll enjoy it.

Here's the outcome for the lesson.

I can explain why reflections, images are seen in mirrors and describe the properties of mirror images.

I'll be helping you achieve this by the end.

Here are the keywords for the lesson.

I'm not going to go through them now because I'll introduce them as we go along, but if you want to check the meanings anytime, you can come back to this slide.

The lesson has two parts called investigating mirror images and conclusions.

Let's start with the first part.

And first, a summary of key things you need to know about light to be able to learn more about reflection.

Light is emitted, given out, from a glowing object in all directions.

Light travels in straight lines at an extremely high speed.

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

Light travelling in one direction represented by a one light ray only reflects in one direction.

So look at the photo.

It has an arrow drawn on it.

It represents a light ray showing the direction of some of the light leaving the torch.

And the other arrow is a ray showing the direction of the light after it has reflected from the mirror.

So we can represent light in this simple way with arrows showing the direction it's travelling.

Light that hits a mirror always reflects obeying the laws of reflection.

So if you don't know these or if you're not sure you remember them, I'll give you a quick summary.

So here's a diagram showing light approaching a mirror represented by an incident ray.

And when we think about the laws of reflection, we like to draw a line called the normal.

It's an imaginary line at right angles to the mirror and it touches the mirror where the incident ray touches the mirror as you can see.

And then we can draw the reflected ray on the opposite side of the normal.

And two important angles, the angle of incidents, symbol i here.

And the angle of reflection with a symbol r.

And these angles are measured to the normal.

That is they're measured between their ray and the normal line, as you can see.

So the laws of reflection are, the angle of reflection is always equal to the angle of incidents.

If you can see that in the diagram, two equal angles and the two angles are always opposite sides of the normal.

One is on one side of the normal, the other is on the other side.

And the two angles and the rays and the normal are all in the same plane.

They can all be shown on a flat screen or a flat piece of paper and that is how they really are.

Now for a question for you to show what you've learned, can you pick out which diagram correctly shows a ray reflecting from a mirror? I'll wait five seconds, but if you need longer, press pause and then press play when you're ready with your answer.

The correct answer is B.

If you didn't choose that one, you might like to go back to the previous slide and remind yourself about the laws of reflection.

B is the only one in which all of the laws of reflection are obeyed, and that's what would happen in real life.

Now let's think about what we see when we look in a mirror.

That's what this lesson is all about.

Here's a picture showing some people in a room or some pretend people in a pretend room and there's a mirror at the back.

And in the mirror we can see a reflection.

So we use the word reflection in two slightly different ways in science.

Reflection means what light does, what happens when it hits a mirror.

Reflection is what happens, light reflects, but it also means what we see when we look in a mirror.

So we look and we see a reflection of the room and there's another word we can use image, which is a key word for this lesson.

And the image is what we see in a mirror.

It's a representation of the real thing, but it's not the real thing.

And how does it happen that we see things in a mirror? So these objects in the room, they are receiving light from light coming through the window or from the lamp, and light hits the objects and it scatters, it reflects from them in all directions.

And some of that light will hit the mirror and the mirror then reflects the light and some of the light enters our eyes.

As you can see in the bottom right picture.

If our eyes are down at the bottom right of the picture there, these rays that are shown are going to enter our eyes after reflecting from the mirror.

But our eyes don't know that the rays had their direction changed partway along as they travelled.

So our eyes in a way trace back where the rays seem to have come from going back in a straight line.

And that means we see the rays as if they've come from behind the mirror and that's what creates the reflection.

So here you can now see the reflection.

It looks as though these things are behind the mirror, even though they're not really.

So let's check if you understood that.

Which of the following causes the images seen in a mirror? Choose one of the options and if you need more than five seconds to think, press pause and press play when you're ready.

Did you choose A, light from a room reflects from a mirror.

That's why we can see things in a mirror.

But what you see in a mirror depends on your position.

So here's a picture of a cat, a person, a chair, and there's a mirror at the back there.

And notice that we can see a reflection of the chair in the mirror.

And the reason is shown by this simple ray diagram.

It shows the direction that light could travel from the chair.

Remember, light is being scattered in all directions from the chair.

So some of it will go the way shown here.

And if that light reflects from the mirror, it can then reach us.

And you can see we are just behind the chair just to the right of it.

And from there we can see the chair's reflection.

Notice we can't see the child in the mirror.

There's nothing mysterious going on here.

It's because if we wanted to see the child's reflection, we'd have to be further over to the right because light ray, they come from the child and reflect from the mirror, reflect over towards the right.

Remember light obeys the laws of reflection.

So this is the way it's going to reflect in real life.

So on the right here you can see a picture of what it would look like if we were further over to the right and we would be able to see the child in the mirror.

And notice we can't see the chair in the mirror anymore.

Now what about the cat? Where would we need to be to see a reflection of the cat? We'd need to be even further over to the right so that light from the cat obeying the laws of reflection when it reflects can reach us and go into our eyes.

And here's a picture on the top right of what that would be like if we were further over to the right.

And you can see we can now see the cat's reflection in the mirror.

So people in different positions, will see different images in the same mirror at the same time.

Now let's check if that's making sense to you.

Here's a picture of four people and a cat and a mirror.

We're looking at this from slightly from above and I'd like you to think about who can see a reflection of the cat in the mirror? So I'll give you five seconds, but if you need longer, pause the video and press play when you're ready.

The person who can see a reflection of the cat in the mirror is Laura and I'll show you why.

His ray of light coming from the cat reflecting off the mirror, obeying the laws of reflection and it reaches Laura's eyes.

And so Laura can see a reflection of the cat when she looks in the mirror.

And remember, when an object scatters light, it scatters light in all directions so there will be some light from the cat that is travelling in that direction and does reach Laura.

Well done if you got that.

I'll show you what some of the other people are seeing when they look in the mirror.

None of them can see the cat, but they can see different things.

Alex can see themselves looking straight on at the mirror.

Andeep, for example, can see Sofia because it's possible for a light from Sofia to reflect off the mirror, obeying the laws of reflection and reach Andeep's eyes.

Now when we look in a mirror, things look quite realistic.

They look similar to real life but not quite the same.

The image in a mirror is laterally inverted compared to the object when it is facing us.

Now that may seem like a strange sounding phrase, I'll explain what it means.

So Laura is looking in a mirror and here's what we would see if Laura was facing us.

She's holding a spanner in her right hand.

Here's what we see when we look at her image in the mirror.

She seems to be holding the spanner in her left hand.

This effect is really noticeable with writing.

So here's a piece of paper with physics written on it and we are looking at it and it looks normal, but then we see its reflection in the mirror and it doesn't look normal.

It looks quite different.

You might have experimented with creating mirror writing like this on paper.

Here's an emergency vehicle, a fire engine, and on the front it has fire written in a normal way, but it also has fire written in a way that is laterally inverted.

It looks the way it would look in a mirror.

And the reason for this is that if the fire engine is coming up behind you and you see it in your car mirror, that word fire looks normal.

So this is to make sure you can very quickly recognise that it's a fire engine.

Here's a question for you about this.

This card says banana with the word top above it, which is the correct reflection of the text on the white card? Which one of these pictures shows what the reflection would look like in a mirror? Pause the video if you need more than five seconds and press play when you're ready.

The picture that shows the correct reflection of the text on the card is B.

That's the one that shows the text laterally inverted correctly.

And if you weren't sure about that, try experimenting by looking writing in a mirror and see what happens.

Now Aisha's been thinking about this and she says, "If mirror images are laterally inverted, why are they not vertically inverted too? Do mirrors somehow know which way is up?" And this is actually a very common question.

Why do mirrors seem to flip things one way horizontally but not the other way, vertically? Aisha's idea isn't right, mirrors can't sense which way is up.

And rotating a mirror on a vertical wall has no effect on the reflection.

So you see here, this mirror is being turned around, it doesn't make any difference.

The reflection is always laterally inverted.

But now let's see what happens when Laura is standing with her spanner on a mirror.

You don't see that sort of thing very often.

So the mirror is now flat on the floor and look at the mirror image.

Now it's still laterally inverted.

Her right hand looks like her left hand in the image, but it's also vertically inverted.

So in real life her head is at the top, but in the reflection her head is at the bottom.

So the thing is mirrors simply reflect what is in front of them.

What's on the right in front of a mirror will be on the right in the reflection.

And what's at the top in front of a mirror will be at the top in the reflection.

And also what's furthest from the mirror will be furthest away in the reflection.

So when Laura was standing on a mirror, her head was furthest from the mirror.

So in the reflection it also looked furthest from the mirror, so it was at the bottom.

Now when we look at text, when we look at writing in a mirror, we flip it around to show it to the mirror in a way.

And we can see more easily what happens if we write on something that's transparent.

So we've written the word physics here, but if we want to look at it in the mirror, what we normally do is turn it around and when we turn it around, the P is on the right and it's on the right in the reflection as well.

Now a question, what happens if you turn this mirror 90 degrees? So we have the same piece of card, same banana with top above it.

We have a mirror there and we're going to turn it the way that's shown by the arrows.

We're gonna turn it a right angle, which picture shows what we would see? Pause the video if you need more than five seconds to think and press play when you're ready.

The correct answer is A, the reflection looks the same as it did before.

Rotating the mirror makes no difference.

And if you're not sure, you believe me, try it out in real life and you'll see.

Now let's look at something else about what reflections look like.

And to do that I'm going to talk to you about an illusion.

So we're going to use a piece of glass.

And glass is transparent, it transmits light, it lets light pass through it, but actually it doesn't transmit all of the light.

Some light is also reflected from the surface of a glass.

You might have noticed that you can see your reflection in glass, especially if you are indoors at night with the lights on and the curtains open, you can often see your reflection quite clearly in the glass.

So glass reflects some light and it transmits some light and we can use that to create illusions.

In this photo there's a candle and it looks as though it is lit.

There seems to be a flame on the candle, but is it really? No, you're actually seeing a reflection of a flame on top of a real unlit candle.

I'll show you the rest of the picture.

So here we have two candles and a piece of glass in between them and they both look as though they're burning.

But here's a picture from a different angle from the side.

In fact, the front candle is burning and the back one isn't, even though it looks as though it is.

So how does this work? Well, the back candle, the unlit candle reflects light, scatters light that's come into the room and some of that light passes through the glass and reaches our eyes.

At the same time, light from the flame goes out in all directions.

Some of it hits the glass and some of that reflects and reaches our eyes.

And so our eyes receive light from the unlit candle together with light that's been reflected from the flame of the lit candle.

It makes it look as though the unlit candle is actually lit.

If you'd like to see that on a video from different angles, click the button here and take a look.

So if you set up this illusion, you have to position the candles and the glass carefully.

So to make the reflection of the flame be in just the right place so the candle looks lit.

So we make the image of the flame look as though it's here just on top of the unlit candle.

And it always happens, the images in a mirror look as though they're behind the mirror.

And in fact, we say that a reflected image is behind the mirror.

But we can call reflections virtual images because when we see an image in a mirror, the light reaching us from it isn't really coming from behind the mirror.

Often there's a wall behind the mirror, it only looks as though it's coming from behind the mirror.

And now here's a question, on the left you can see a cat is looking in a mirror.

Now looking from above, can you say where is the cat's reflection? Is it , A, B, C, or D? If you need longer than five seconds to think, press pause and press play when you're ready.

The answer is A, the cat's reflection is as far behind the mirror as the cat is in front of the mirror.

Here's a picture of what the cat sees.

Now compare the cat's reflection with the real cat on the left.

So the image of the cat looks as though it's as far away as that cat.

Now's a task, you're going to investigate how the location, the position of a mirror image changes when you move an object away from the reflecting surface.

So you're going to put the real object in different positions and see what happens to where the image is.

So first you'll set up a transparent screen.

It could be glass or probably plastic.

It needs to be stable so that it's not going to fall over.

So you might have a screen that has feet that hold it up, or you might need to clamp it to the desk.

And then you'll need two candles and two rulers as shown in the picture.

You're going to light the nearer candle and move it close to the screen and then measure and record the distance of its flame from the screen.

Don't try to hold your ruler up to the flame, just keep your ruler flat on the table.

Then move the candle that's behind the screen until it looks as if it's lit.

In these three pictures, it's right in the middle picture.

In the picture on the left, the candle's a bit too close and the flame looks like it's hovering above.

And in the picture on the right, the candle's too far and the flame looks as though it's below where it should be.

So when you position the candle correctly, measure and record its distance from the screen.

Then you're going to repeat this, taking more measurements with the lit candle at five other distances from the mirror.

And you'll need a table like this one where you can write your results for the distance of the flame, the real flame from the screen, and the distance of the reflected flame behind the screen.

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

I hope the investigation went well and you now have a table of results.

Here's an example set of results.

Yours may be slightly different.

You might have experimented with a different range of distances depending on your equipment.

Now let's look at conclusions from this experiment.

First of all, what is a conclusion? Is when you write what you found out in an experiment and how you know this.

So here's a question.

Here's that table of results again.

True or false? The results in the table support the conclusion that reflections appear to be the same distance behind a mirror as the object is in front.

Decide whether you think that statement is true or false, and then choose a justification.

An explanation of why you think that.

Press pause if you need longer than five seconds.

And the statement is actually true and the justification is it's difficult to measure exact distances.

So you'd expect some small errors.

So if you look at these results, the distances aren't always exactly the same, but they are close.

And the small differences between them could easily be explained by small errors, small inaccuracies that happen when you do an experiment.

You might not have been able to measure the position of the candle flame exactly right.

So reflections appear to be the same distance behind a mirror as the object is in front.

That would be a good conclusion or part of the conclusion from this experiment, it's what the experiment shows.

And in this video we can just see that happening.

So person moving the cat to different distances from the mirror.

And as the real cat moves it's reflection, it's image moves too.

And their distances from the mirror always match.

They're always the same.

Reflections aren't really behind the mirror because they're not real objects.

There wasn't really a flame on the other candle behind the glass.

Reflections are images.

They look like the real thing, but they're not.

The space wear a reflection appears to be often does not really exist.

There could be a brick wall behind the mirror in this video.

So the cat reflection isn't really coming from inside the brick wall or outside beyond the brick wall.

Now have a look at this diagram.

There's a reflection happening.

There's light coming from the lamp and in this reflection travelling three metres to the mirror, then reflecting and then travelling two metres to reach Sam's eyes.

So the light's travelled five metres altogether.

And when Sam sees it, it looks to Sam as though the light is from five metres away.

But what it actually looks like is that it's five metres away in a straight line.

So when Sam sees the reflection, it looks like what you see on the right.

So she sees a lamp that looks five metres away, which matches the distance that the light has actually travelled to reach her eyes.

And that's why a reflection always appears the same distance behind a mirror as the object is in front.

When light obeys the laws of reflection, this is what happens.

And light always obeys the laws of reflection.

However far light travels before it hits a mirror, in this example, three metres, that's the distance at which the reflection appears behind the mirror.

Three metres again.

And by the way, the direct distances between the object in the mirror and the image in the mirror labelled D here, they're also equal to each other.

Now here's a cat looking in a mirror again.

The cat's 20 centimetres away from the mirror.

How far away from the cat does the cat's reflection appear to be? Pause the video if you need longer than five seconds.

And the answer is not 20 centimetres, it's 40 because the cat is 20 centimetres in front of the mirror and it's reflection looks 20 centimetres behind the mirror.

So the reflection seems to be 40 centimetres from the cat.

And again, we can see this by comparing the reflection of the cat with the real cat that's behind the mirror.

They look the same distance away, 40 centimetres.

Well done if you got that.

And now for a couple of longer questions, explain why reflections are seen in mirrors and describe and explain the properties of reflections, the properties of mirror images, what are they like? So take as long as you need to write down your answers and then I'll show you a couple of example answers.

Press pause and press play when you have your answers ready.

So here are the example answers.

Explain why reflections are seen in mirrors.

Mirrors are very shiny, so reflect light that hits them.

Some light is reflected into our eyes, causing us to see an image in the mirror.

The image is located where the light appears to have come from, which is behind the mirror.

Describe and explain the properties of reflections, mirror images.

Reflections, mirror images, appear the same distance behind a mirror as the object is in front of the mirror and the right way up.

Whatever is on the right in front of a mirror will be on the right in the reflection as you look at it.

This causes text facing a mirror to be laterally inverted.

Objects on your right are on your reflections left if there were a person opposite you.

You don't have to have written your answer exactly like that.

But have you included the idea of objects and images being the same distance from the mirror and the idea of lateral inversion? If you have, well done.

And that's the end of this lesson.

So here's a summary of what it was all about.

Light reflects from mirrors and other shiny surfaces, obeying the laws of reflection.

Some light is reflected into our eyes, causing us to see an image in the mirror.

The image appears to be where the light appears to have come from, which is the same distance behind the mirror as the object is in front of the mirror.

The image in a mirror is the right way up.

Whatever is on the right in front of a mirror will be on the right in the reflection.

This causes text facing a mirror to be laterally inverted and objects on your right are on your reflections left if there were a person opposite you.

Well done for working through this whole lesson.

I hope you enjoyed it and I hope you feel you know a little bit more about why things look the way they do in mirrors.

I hope to see you again in a future lesson.

Bye.