Lesson video

Hello, my name's Dr.

George, and this lesson is called "Reflected Images." It's part of the unit, "Making Images." Here's the outcome for this lesson.

I can describe and explain the properties of reflections in a plane mirror, and draw diagrams to show how reflections form.

Here are the key words for the lesson.

I'm not going to read out all these definitions now, because I'll introduce these words as we go along, but this slide is here in case you want to come back any time, and check the meanings for yourself.

The lesson has two parts called, "Reflections in a plane mirror," and "Explaining reflections." Let's get started.

When you look in a mirror, you see an image, and what that means is a representation of real things that aren't actually the real things.

And when we see an image in a mirror, we call it a reflection.

So we use the word reflection in two ways, it means what you see when you look in a mirror, but it also means what a mirror does to light.

So a mirror reflects light, and some of that light enters our eyes.

So if you look at this picture at the bottom, some of the light scattered by these objects hits the mirror, reflects off the mirror, and if our eyes are in the right place, that reflected light is going to enter our eyes, and we'll see the reflections.

The reason we see reflections is because our eyes trace back where the rays from the mirror surface seem to have come from.

So our eyes don't know that the rays haven't been travelling in this way all along.

They don't know that the rays change direction when they hit the mirror.

So what we see it's as if the light has come from behind the mirror.

So which of the following causes the images seen in a mirror? Now for these short questions, I'll wait five seconds, but you may need longer.

And if you do, press pause, and then press play when you've chosen your answer.

(silence) The only correct answer here is A, light from a room reflects from a mirror.

And as you saw, it's that that causes us to see images in a mirror.

Light doesn't actually travel into a mirror, it bounces off it.

And mirrors can't somehow see what's in front of them, and make a copy.

Mirrors can't see anything.

And mirrors don't create light, they're not luminous.

The laws of reflection decide how light actually reflects.

And here's a quick summary.

So here we have an incident ray, a ray that's coming in to a mirror and hitting it.

To think about the laws of reflection, it's helpful to draw an imaginary line, called the normal, and it's drawn at 90 degrees to the mirror, and it meets the mirror where the ray does.

Now here's the reflected ray, and we can think of two angles here, they've been labelled I and R, angle of incidence, angle of reflection, and they're each measured between a ray and the normal.

And the laws of reflection say that the angle of reflection is always equal to the angle of incidence, and it's on the other side of the normal, and in the same plane.

So the two angles and the rays, they're all in one flat plane.

Now, thinking about that, here are some diagrams that are trying to show a ray reflecting from a mirror, and the diagrams are drawn in 3D.

So the mirror is standing up on a table perhaps, and we can see the rays.

Which ray is obeying the laws of reflection? Press pause, and press play when you've chosen your answer.

(silence) And the correct answer is B.

That's the only one that looks as though the angle of incidence could be equal to the angle of reflection, and the two rays are on opposite sides of the normal, and they're in the same plane.

In F you can see an example where the angle of reflection is not in the same plane as the angle of incident, and the ray is coming up off the table.

That doesn't happen.

Now the word plane means a flat surface.

So a flat mirror can be called a plane mirror.

And with a plane mirror, if rays come in that are all travelling in the same direction, so they're parallel, they come in at the same angle, they will all reflect at the same angle as each other, as you can see here, and it turns out that's what produces clear reflections, clear images in the mirror.

Now you can also get curved mirrors, but they make distorted reflections, reflections that really don't look like the real thing.

And that's because parallel rays coming into a curved mirror are not all reflected at the same angle as each other.

Compare the two diagrams to see what I mean.

(silence) Now what you see in a mirror depends on your position.

At the position we're at here, so imagine we are where the camera is that took this photo, we can see the chair's reflection, and that's because it's possible for a ray of light to come from the chair, hit the mirror, and reach our eyes, and so it does.

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

To see the child, we'd need to be further over to the right so that rays from the child that hit the mirror can enter our eyes.

Here's a picture where we have moved over to the right, and now we can see the child's reflection in the mirror, and we can't see the chair's reflection anymore.

Now notice the cat on the left.

If we wanted to see the cat's reflection, we'd need to be even further over to the right, because that's where the reflected rays coming from the cat are ending up.

And here's a picture of what we would see if we were far enough over to the right.

We would see the cat in the mirror.

If different people in different positions are looking in the same mirror at the same time, they'll see different images.

And here's a puzzle about that.

So there are four people, and a cat in front of a mirror here, I'd like you to think about who can see a reflection of the cat in the mirror.

Press pause while you're thinking about it, and press play when you've decided.

(silence) Here's the answer, it's Laura.

Why is that? It's because light from the cat that hits the mirror can reach Laura's eyes.

Here's what everyone else sees.

So Andeep can see Sophia, Sophia can see Andeep, and Alex can see themselves, because light from Alex can go to the mirror straight along that normal, and is going to bounce straight back again.

Now here's Laura.

She's facing a plane mirror, a flat mirror, and she's holding a spanner in her right hand.

But look at her image in the mirror.

The image looks like she's holding a spanner in her left hand.

So we're going to think about what images look like in mirrors, and we find that they're the same size as the real object, and the right way up, upright, but they're back to front.

Left is right, right is left.

And we have a technical name for that, we say that images in mirrors are laterally inverted.

Lateral means side, and inverted means flipped around.

So laterally inverted is flipped from side to side.

So here's Laura, if she turned around and faced us, and here's that image in the mirror again.

And that's what we mean by lateral inversion.

And the same applies if we hold up text, if we hold up writing to a mirror.

Here we have a piece of paper that says, "Physics," and the image in the mirror in the right hand photo, it's upright, it is the right way up, but it's laterally inverted.

It doesn't look like normal writing anymore.

And here's an example on a fire engine of some text that is actually written in a laterally inverted way deliberately, so that if it's seen in a car mirror, it will look the right way round, and that's if the fire engine comes up behind a car, someone looking in their mirror can very quickly and easily recognise the word "Fire." So which of these is the correct reflection of the text on the white card? Press pause and press play when you've decided.

(silence) And here's the correct answer, it's B.

The image is upright, it's not upside down, but it is laterally inverted.

Now Jacob's been thinking about this and he says, "Mirrors must somehow turn images around, or flip them to make reflections laterally inverted." And a lot of people think that, and are quite confused about why a mirror would do that.

This idea is actually not right, mirrors simply reflect what is in front of them, and I'll show you what I mean.

Have a look at this photo of a scene made of children's toys, and its reflection.

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

There's a cat in a basket on the right here, and it's on the right in the reflection as well.

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

The people's heads here are above their feet in real life, and they are in the reflection as well.

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

So the lamp in this picture is furthest from the mirror, and its reflection is also furthest away from the mirror.

And we can use this to actually explain why reflections are laterally inverted, but not vertically inverted.

So images and mirrors don't seem to be flipped vertically, but they do seem to be flipped laterally.

Reflections actually aren't laterally inverted, compared to what's in front of the mirror.

Here's a picture of the scene from above, and here's what its reflection would look like.

It's not flipped left to right.

Things that are on the right in real life, are also on the right in the mirror.

So what's going on? When you want to look at yourself in the mirror, you turn to face mirror.

Reflections are only laterally inverted compared to what they look like if the object is facing you.

Here's Laura facing us.

And to look in the mirror, she has to turn around, and that's what causes the lateral inversion.

She has flipped around to be able to look in the mirror.

Let's look at that with text.

If you want to see a reflection of some writing, you usually have to turn it to face away from you, you need to make it face the mirror, and then the mirror just reflects what's in front of it.

That's normally not obvious when text is written on paper.

But here it's written on a transparent sheet, and so you can see that when we want to sort of show the text to the mirror, we flip it around as in the picture on the top right.

And we flipped it around, so the P on the right, and the P on the right in the reflection as well.

So the mirror is just showing us what we've showed it.

If we don't turn the text around, as in the bottom picture, then the reflection isn't turned around either.

So here's a question, in a mirror writing is laterally inverted.

Which statements are true? So read all the statements, decide which ones are true, and press pause while you're thinking, and play when you are ready.

(silence) And there are two true statements.

A mirror just reflects back what is in front of it.

And if you could look through the back of some writing, that you show to a mirror, you would see the same as the reflection.

Well done if you got those right.

Now, reflections don't appear to be on the mirror surface, they appear to be behind the surface, and the same distance behind the mirror as the object is in front.

Watch the person move this toy cat.

As they change the distance between the cat and the mirror, the distance between the image and the mirror changes in the same way.

But reflections aren't really behind the mirror, because they're not real objects, they're not really there.

Often what's behind the mirror is a wall.

However, we can still describe the location of where the image appears to be.

We could say it's the apparent distance of the image behind the mirror surface.

Let's look at that.

So here we have a cat looking in a mirror, it's 10 centimetres away from the mirror.

Now in the second picture, the larger picture, we're looking from above, where is the cat's reflection or where does it seem to be? So it's at one of positions A, B, C, or D.

So press pause if you need longer than five seconds to think, and press play when you're ready.

(silence) The correct answer is A.

A is the same distance behind the mirror as the cat is in front of the mirror.

Here's a picture of what we would see if we were behind the cat.

So we can see the reflection there, and there are two real cats.

One is sitting next to the mirror, and one is sitting further back.

If we just look at the reflection, we can now see that it looks the same distance as that cat on the left.

So how far behind the mirror does the cat's reflection appear to be? Press pause if you need more than five seconds to think.

(silence) And the answer is 10 centimetres, the same distance behind the mirror as the real cat is in front of the mirror.

And now a longer task for you.

Write the word "Fire" in capital letters, and then try writing what the reflection of that word would look like if you wrote it on a card, and then turned it to face a plane mirror, a flat mirror.

So you're going to write "Fire" in what could be called mirror writing.

And then in question two, I'd like you to look at each row, there's three rows here, and choose one option from each row, from each set, that correctly describes reflections in plane mirrors.

And then for question three, I'd like you to describe what a mirror does when it reflects the word "Fire." So use what you've learned so far.

So take as long as you need, press pause while you're doing it, and press play when you're ready, and I'll show you some example answers.

(silence) So here at the top you can see the word "Fire" and its reflection.

If you're not sure whether yours is right, you could always show it to a mirror, and see what you can see in the mirror.

And the words and phrases that describe reflections in plane mirrors are upright, laterally inverted compared to the object when it's facing you, located behind the mirror surface, and the same size as real objects.

And now what does a mirror do when it reflects the word "Fire"? A mirror simply reflects back what is in front of it.

The word "Fire" would be turned to face the mirror, so the F would be on the right and laterally inverted, because it's been turned and is being viewed from behind.

This is what the mirror reflects.

So it's not easy to explain that, but well done if you did.

Now let's go onto the second part, explaining reflections.

The reason we see images in mirrors is because our eyes trace back where the rays that are reflected from the mirror surface seem to have come from, as I said at the beginning.

So we see this reflection, because it looks to us as though these rays have come from behind the mirror, even though they haven't really.

Now look at this diagram, in this reflection the light travels three metres to the mirror, and then two metres from the mirror to Sam's eyes.

Sam's eyes detect light that's arrived at a certain angle from five metres away.

So Sam's eyes recognise, this light has travelled five metres.

And they don't know that the light reflected on the way, they don't know the light changed direction.

So they send exactly the same signals to Sam's brain that they would if there really was a lamp five metres away at that angle.

So what she sees is what you see on the right here, she sees a reflection that's five metres away from her.

So why do we see images in mirrors? Press pause if you need more than five seconds to think, and press play when you're ready.

(silence) And the answer is that light that enters our eyes from a mirror seems to be coming from real objects behind the mirror's surface.

Now, how can we draw how images, reflections are made? Well, we can draw dashed lines from the image to show where light rays appear to be travelling from.

Here's an example.

So see the ray that's entering Sam's eye, it looks like it's coming from the image, so that dash line isn't real, but it's representing where light seems to have come from.

So we can call this dashed line a virtual ray, because the light didn't really travel along that path.

And notice that the distance that the image, the reflection appears to be behind the mirror, distance D1, is equal to the distance of the object that's actually in front of the mirror, which is D2.

Now, fill in the blanks to explain the mistake that's been made in this reflection diagram here.

There's something wrong with it.

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

(silence) And the answer is that the virtual ray should be a dashed line in this diagram.

Now images in a mirror, reflections, are very different to images produced by projectors and pinhole cameras.

Here's a projector, it sends light out and that hits a screen, and that light scatters off the screen, and some of it enters our eyes.

So the image is on the screen, and we can see that when we look at the screen.

With a pinhole camera, the light ends up on a screen as well.

That's where the image is.

So in both of these, the image forms where rays of light meet, or hit a screen, and some of that light then travels from the image on the screen into your eyes.

And we call images that form in this way, real images.

But with reflections, the image forms where rays of light appear to have come from, not where they've actually come from.

No actual rays of light travel from the image to your eyes, unlike with those other two examples.

Images formed in this way are called virtual images, and a reflection is one of the ways that a virtual image can form.

So if you take a screen, there's no way that you can put it, behind the mirror, on the mirror, where you'll actually capture that image.

Now which of the following statements explains why reflections are virtual images? Press pause while you think, and press play when you've decided.

(silence) The correct answer is that a reflection appears where light rays seem to come from, not where they actually come from.

That's the reason why we call a reflection a virtual image.

Now, I'll show you how to draw a ray diagram for reflection.

First draw the mirror, draw an eye and draw the object.

And we'll keep this simple by drawing the object as just a single point.

Next, you're going to draw the reflection, and you know by now that it's behind the mirror, and equal distance from the mirror as the object.

So use a ruler, and it can also be useful to use a set square to make sure you draw these distances correctly.

Next, you draw a ray from the image to one side of the eye.

And this needs to be a dashed line where it's behind the mirror, because there it's virtual, but it needs to be solid where it's in front of the mirror, because there it's a real ray, the light is really travelling there.

Like this.

Now draw the incident ray from the object to the mirror.

So look at the solid ray here, that's the reflected ray.

So draw the incident ray so that you obey the laws of reflection.

And that's solid because it's real.

Now draw another ray from the image, that same point, to the other side of the eye, and again, make the right parts dashed and solid, like this.

And draw the incident ray from the object to the mirror to complete the diagram.

So you draw a solid ray to make that reflection.

And this is a correct ray diagram showing how an image forms by reflection in a mirror.

So now which of the diagrams here has the first set of rays drawn correctly? So each diagram here only has one set of rays.

We drew two before.

Which one of these is right so far? Press pause while you're thinking, press play when you're ready.

(silence) Here's the correct answer, it's B.

If you compare the images, the solid rays are the same in all of them, but only B shows the virtual ray correctly.

The reflected ray reaches Sam's eyes, and to her it looks as though it has come straight through the mirror from the image, as shown in B and D.

But only B shows the image the same distance behind the mirror as the object is in front.

Well done if you pick that one.

And now, I have four diagrams for you, and I'd like you to add rays to show how the image, the reflection of each object is produced.

In one of the diagrams there are two eyes, and in one of the diagrams there's a longer object where you'll have to think about what happens to light from both ends of the object.

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

(silence) So let's take a look at the answers.

Here they are.

Take some time to look carefully at these, you might want to pause the video while you do.

But I'll just show you that in diagram three you can see that we have rays reaching both of the eyes, and those are different rays coming from the object, and each eye projects back to see the image behind the mirror.

And in four, to make it clearer, the ray from the top are shown in red, and the ray from the bottom is shown in yellow.

Well done if you've got some of those right, and very well done if you've got them all right.

And now we're at the end of the lesson, so here's a summary for you.

In a plane mirror, reflections appear the same size as the object, upright, and the same distance behind the mirror as the object is in front.

In a plane mirror, reflections appear laterally inverted compared to the object when it is facing us.

Mirrors reflect back what is in front of them.

Reflections are laterally inverted, because objects are turned to face a mirror.

Reflections are virtual images, as they appear where light seems to have come from, not where light actually comes from.

And virtual rays show where light seems to have come from, they're drawn as dashed lines.

So well done for working through this lesson.

Some of this is not easy, but I hope you found it interesting, and maybe you'll think about it next time you're looking in a mirror.

I hope to see you again in a future lesson.

Bye for now.