Hello.

My name's Dr.

George and welcome to this lesson, which is part of the unit Sound, light and vision.

This lesson is called How we see things, and it's about explaining and understanding how vision actually works.

I hope you'll enjoy it.

So here's the objective for the lesson: I can explain how we see objects around us using our eyes.

Here are the keywords for the lesson.

Don't worry if you don't know them already.

I'm going to introduce them and explain them as we go along.

But this slide is here in case you want to come back anytime and check these meanings.

The lesson has three parts.

They're called All surfaces reflect light, Light enters our eyes, and Beams of light.

So let's start with the first part, and I'm going to introduce the first keyword.

Objects that give out light are called luminous.

So here are pictures of three luminous objects; a lamp, a phone that looks like it's in torch mode, and the Sun.

And objects that don't give out light are called non-luminous.

So these cups and this tree don't make their own light, nor does the Moon.

It may look very bright at night.

That's actually because the Sun is shining on it and lighting it up.

It doesn't make its own light.

So luminous, non-luminous.

I want to test what you've learned there.

So there are six objects here.

I want you to pick out which ones are luminous.

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

Are you ready? Here are the answers.

The laptop screen, the bulb, and the Bunsen burner are all luminous.

They're all making their own light.

The other three things are non-luminous, they don't make light.

They might look bright if light is shining on them from something else, like the Sun or a torch, but they're not making their own light.

If you got all of those, well done.

Now, if you have a luminous object like this lamp, it emits light, it sends out light in all directions from its glowing parts.

And the arrows in this picture represent light rays and they show a few of the paths that light is travelling along.

So they give you the idea that light is coming out from all parts of the glowing lamp and in all directions.

The light rays don't represent real objects.

They are just representing what light is doing in the picture.

And light travels extremely quickly and it travels in straight lines until it hits the surface of a material.

So light just keeps going until it hits something.

Now, if a light hits a shiny surface like a mirror, it's reflected in one direction, and it obeys the laws of reflection.

I'll just give you a brief recap of this.

So look at the diagram.

It represents a mirror, light coming in represented by an incident ray, an incoming ray to the mirror, and light reflected off represented by the reflected ray.

And then we can see two angles drawn, i, the angle of incidence, and r, the angle of reflection.

And those angles are drawn from a ray to that dashed line down the middle.

The dashed line is an imaginary line at right angles to the mirror and it's called the normal.

So, knowing those things, we're able to understand the laws of reflection, which say this: the angle of reflection is always equal to the angle of incidence.

Those two angles, always the same.

And these two angles are on opposite sides of the normal.

You can see the angle of incidence is to the left of the normal in this picture, angle of reflection is to the right.

So that always happens with reflection.

And the angles are in the same plane.

In fact, the angles, the rays, and the normal, they can all be represented on one flat screen or sheet of paper, which shows they're all in the same plane.

So those are the laws of reflection.

And what happens now if light reflects off something that isn't a mirror, a non-shiny, dull surface like this book? Well, it turns out that dull surfaces also reflect light, but the light goes in many directions.

So we often call this kind of reflection scattering.

That's a keyword for today.

So dull surfaces scatter light.

Let's have a closer look at shiny surfaces and dull surfaces.

Shiny surfaces are actually very smooth and they reflect light in a regular way, so this can form a clear image.

Look at the picture on the left.

Light is coming in, represented by the light rays coming towards the mirror at the bottom.

And then light is reflected off in this very regular, this very predictable way, and this is going to form a nice clear image.

So you can look in a mirror and you can see yourself, for example.

On the right we have a rough surface, bumpy surface, and that scatters light that hits it.

And that makes the surface look duller.

You're not gonna see a reflection in that.

And scattering is a kind of reflection and it actually does obey the laws of reflection.

If you actually look at each incoming ray, each incident ray hitting the rough surface, where it hits and bounces off, it is obeying the laws of reflection.

The angle of incidence is the same as the angle of reflection, but because the surface is bumpy, that leads to reflected rays coming off in all directions.

So if you clean or dust or smooth a surface, you may be able to make it shinier because it behaves more like the picture on the left the smoother it gets.

And two questions for you.

Four objects here on the right.

Which of these objects can reflect light? And in which of these objects might a reflection, an image, perhaps of yourself, be seen? I'll give you five seconds, but pause if you need longer and press play when you're ready.

Are you ready? The objects that can reflect light is all of them.

They all reflect light, just in different ways.

And in which of these objects might a reflection, an image, picture of yourself, be seen? Well, of course the mirror, but maybe the coin as well if it's shiny enough, although your reflection might not quite look normal because the surface of the coin is bumpy.

Now, you might be thinking, okay, so I can make anything into a mirror if I can just make it smooth enough.

That doesn't always work.

Here's a piece of white marble, a kind of stone, and however smooth you make the surface, you can't see your reflection in it.

You can make it perhaps a bit shinier, but in marble, light is reflected just under the surface.

It goes through the top surface and it's reflected at points under the surface and scatters in all directions.

So no matter how much you smooth the surface of marble, you'll never see your face in it, and that's true for some other materials too.

Now another question.

Here's a photo of a door and a wall.

Which parts of this image are reflecting light? Where are we getting reflection off these surfaces? Five points to choose from, which points are reflecting? I'll give you five seconds, but pause if you need longer.

And the answer is all of them.

That's not a trick question.

It's important to understand that anything you can see is reflecting light in some way.

So not just those parts that look shiny here, but every part of this is reflecting light, scattering light in this case.

Well done if you got that.

I have a task for you to try now.

Have a look at this photo.

We have an open box and someone is holding a torch in the box, and right now the torch is off but the room lights are on.

But what's going to happen is the room lights will be turned off and then the torch will turn on.

Which sides of the box will light up? I want you to try and predict out of a to e which of those sides will light up when the torch is switched on, and explain why you think that.

Then you're going to watch a demonstration.

You could try it in real life or you could click the link to watch the video.

And then think about whether your prediction and explanation were correct.

And if you want to improve your explanation, go ahead.

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

Let's look at the example answers now.

Prediction: the torch will light all sides of the box, a to e, and that is what happens.

You can see that in the photo here.

Now, how do we explain that? The light from the torch will hit side a of the box first, so there will be a very bright patch on side a.

The light will then be scattered from side a.

This means reflected in many different directions.

So some light hits each of the other sides.

That's why they're all lit, but not as brightly as side a.

So well done if you got some or all of those ideas into your answer.

You can see some light ray now drawn on the photo here representing the idea that light is hitting different parts of side a and scattering off it in many directions.

Now we're ready for the next part of this lesson, Light enters our eyes.

So this is all about how we see.

Think about a luminous object like this lamp.

It's emitting light, it's giving out light, and the reason we can see it is because light from it enters our eyes.

So these three people here, they can all see the lamp because some of it's light is getting into their eyes.

What's not happening is their eyes are not emitting light.

We don't send out light to an object in order to see it, but people have often believed that.

And I'm going to give you a little bit of history now.

Ibn al-Haytham was an astronomer and thinker born in Iraq about 1,000 years ago, and he explained reasons why we see, because light enters our eyes, not because our eyes are giving out light.

So people were unsure about this at the time, they were arguing about it, but he thought about it and could explain it.

He said, "Bright objects dazzle our eyes, which close by themselves." So if something really bright suddenly shines into your eyes, you close them automatically.

It can even be painful.

"So light from an object affects your eyes, not the other way around." It's not your eyes affecting the bright object.

The bright object affects you.

And he also said, "If you look at the night sky, distant stars can be seen as soon as you open your eyes.

Light couldn't travel from our eyes to the stars in that time." He said, "That just can't be how it works.

Instead," he said, "light is constantly arriving at our eyes from the stars.

They're sending out the light.

And so as soon as we open our eyes, some of that starlight enters them." So these were some reasons he gave why we see because light enters our eyes, not because our eyes send out light.

Let's look in more detail at the eye.

So here we have a diagram of the eye looking at it cut through from the side.

So in that diagram, the eye is looking to the left.

No part of the eye emits light.

Your eyes are not luminous.

Let's look at some of the parts that it does have.

We have a pupil in the front of our eye.

It's a hole that lets light into the eye, but it's not completely open.

Nothing can get in it because there's a clear surface like a screen in front of it.

Another important part of the eye is the part that the light hits after it goes through the pupil.

It goes through to the back surface of the eye, and that's because the rest of the eye is transparent, it lets the light through.

So then the light hits the back surface, which is called the retina, and that is covered in cells that detect light.

Your whole body is made of cells, but the ones at the back of the eye are rather special.

They can detect light.

Another part of the eye that's very important is the lens.

It focuses the light that comes in.

If you've ever used a camera that needs to change its focus to look at distant or nearby objects, your eye is doing that too.

Your lens automatically changes so that you can see clearly the different distances.

So, two keywords here: pupil and retina.

Those are the ones I'd like you to try to remember.

So, question, which of the following statements are about the retina and which are about the pupil? And there may be one or two there that are not about either of them.

So think carefully about that.

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

And here are the answers, a and b are talking about the retina.

It's the back surface of the eye and it's the part of the eye that detects light.

And the pupil is a hole in the front of the eye.

c and e aren't describing them, and in fact, c and e are not describing real parts of the eye at all.

Well done if you got those.

What about non-luminous objects? Well, we see those when light that they scatter goes into our eyes.

Remember, scattering is a kind of reflection.

And these three people can see the book because their eyes are receiving some of the light that it's scattered.

So light from the bulb hits the book, scatters in all directions.

So that's how we can see dull objects, non-shiny objects.

Look around you.

There will be some non-luminous objects around you, I'm sure.

And you'll notice that if you look at one, you can change the position of your head, you can change the angle of your eyes, and wherever you put your eyes, you can see the object, unless there's something in between the object and you.

Shiny surfaces like mirrors are also non-luminous.

Remember, they don't make their own light, but they reflect in a rather special way.

And when a mirror or a mirror-like surface reflects light into our eyes, we see a reflection.

We see that it looks like there are things in the mirror.

And we can only see an object's reflection from certain angles when we look in the mirror.

And that's because rays of light are not scattered by a mirror.

They're only reflected in particular directions.

Question, which of these pictures best shows how the person sees the book? I'll give you five seconds, but you can pause to take longer and press play when you're ready.

And the correct answer is d.

It correctly shows the idea that light comes from the lamp and hits the book and then scatters and some of that enters the person's eyes.

All of the other options either shows something that's not correct or only tell part of the story.

Well done if you got that.

Still thinking about how we see things, I want to ask if you've ever seen a cat at night with its eyes seeming to glow brightly? Cat's eyes don't really glow, they're not luminous, but they happen to be very reflective, very good at reflecting light.

So if there's light around from car headlights or street lamps, it reflects very well of a cat's eyes.

And if we happen to be in the right place to catch that reflected light in our own eyes, then the cat's eyes look very bright.

Now, there's something called a cat's eye that you'll find in some roads, and it's a little reflective object, road reflector.

And these were invented in England in 1934 by someone who got the idea by thinking about the eyes of real cats.

And he thought, how can we make it clear to people who are driving where the edges of the road are without needing street lights everywhere? And so he thought of putting these into the road.

So if you're driving along, your car headlights shine onto these cat's eyes and they reflect back light really well and you see them and that shows you where the edge of the road is.

Road signs, bike reflectors, and high-visibility clothing, as you can see in the pictures at the bottom, are also designed to reflect really well.

So if light shines on them from a car, you can see them clearly because they reflect a lot of light back to you.

Now, human eyes can adjust to very low light levels.

You've probably noticed that if the lights suddenly go down, at first you can't see very much, but then your eyes adjust and you can see a bit better.

So if there's even a tiny amount of light, you may be able to just about see some things, but if there's no light at all, your eyes will never adjust.

You'll never be able to see anything without light.

To see things, light has to be entering your eyes.

Here's a question.

Alex here is clearing out his garage and the only lighting is from an old lamp.

And in the garage there's also an old mirror and Alex's cat is sitting in the corner.

He's at the far back on the right.

Now Alex closes the garage doors from the inside, and the doors fit very tightly so no light can get into the garage and there are no windows, no other openings, no light's coming in from outside.

He then switches off the light in the garage.

After 10 minutes, what can Alex see? So think about that, pause the video if you need longer than five seconds, press play when you're ready.

The correct answer is nothing.

There's nothing in the garage that's making light, there's no light coming in from outside, so his eyes can't adjust.

He's never going to be able to see anything.

Remember, the mirror, the cat's eyes, they're great at reflecting light, but if there's no light there, then they don't make any light of their own.

Okay, a longer task.

Izzy is looking at this book, and I'd like you to pick the correct sentence from each row to explain how Izzy sees the book.

So when you've finished, you'll have six sentences that tell the story of how Izzy is seeing the book.

In row one and two you don't have anything to choose from, but in three to six you'll have to choose the best sentence.

Take as long as you need, press pause, and when you're ready to see the answers, press play.

And here are the answers.

Light travels out in all directions from the Sun.

Sunlight passes through the window and into the room.

Some light from the Sun falls on the book.

Light is scattered by the book.

Some of that light travels from the book to Izzy's eyes.

And Izzy sees the book because this light enters her eyes.

Well done if you got those.

And now for the final part of this lesson, Beams of light.

In this photo there's a torch.

Can you tell if the torch is on or off? I think a lot of people would say it's off, but if I show you the rest of the picture, it's actually on.

So you can only see things when light enters your eyes, and here you can only see the light from the torch when the wall scatters, reflects, some of it into your eyes.

So while the light's actually travelling from the torch to the wall, you can't see that because the light's travelling away from you.

It's not travelling into your eyes.

So usually you can't see light when it's travelling.

But you might say, "Ah, but I've seen beams of light." Yes, but you can see a beam of light if there's dust or water droplets in the air and the light reflects off those and some of it scatters into your eyes.

So if there's a lot of little bits of stuff in the air, then that can scatter some of the light in your eyes and then you can see where the light is travelling.

Now, I don't if you've ever seen a laser.

Schools often have some lasers, or you might have seen laser pointers, little lights that people use to point to things when they're giving a presentation.

A laser produces a very thin beam of very intense light of just one colour.

It's often red.

And a laser beam can harm eyes if we look straight at it.

If laser light goes into our eyes, it damages the light-sensitive cells of the retina and stops them working.

So you wouldn't want all of that light coming into your eyes.

If you shine a laser on a wall and you see some of the scattered light, that's okay, because you're only receiving a bit of that bright light.

It's not harmful.

So never look straight into a laser.

It might not hurt, but it is damaging your eyes.

Now, for each of these sentences, I'd like you to decide, is it about laser light? Is it true for laser light? Is it true for sunlight? Is it true for both? Pause if you need longer than five seconds.

Press play when you're ready.

And the answer for one is both.

Both laser light and sunlight can damage your eyes.

Never look at the Sun.

It might not hurt, but it is damaging your eyes.

Don't worry if you're just looking around and you briefly look past the Sun, but never stare at it.

Only contains one colour of light, that's true for laser light.

It's not true for the Sun.

Contains many colours of light, sunlight.

And only spreads out a tiny bit as it travels, laser light.

So laser light has this very thin beam that goes on like that being thin, so it's not spreading out much as it travels, whereas sunlight spreads out in all directions.

Final task of the lesson.

A red laser, produces a red beam, is set up pointing at a white wall, as you can see in the picture.

Now the laser is switched on.

Predict what you will see in the space between the laser and the wall and on the wall, and explain why you think that.

Watch the demonstration, either in real life or on a video, and then think about whether your prediction and explanation were correct.

And if you want to improve your explanation, you can do that.

So take the time you need, press pause, and press play when you're ready to see example answers.

And here are some answers.

Prediction: there will be nothing visible in the space between the laser and the wall.

A red dot will appear on the wall.

That's what we would expect to see.

And the explanation: red laser light will travel in a straight line from the laser to the wall.

Between the laser in the wall, the laser light has nothing to reflect off, so the beam can't be seen.

Laser light barely spreads out as it travels, so it will hit the wall in one small spot.

The wall will scatter the beam in all directions.

I can see the spot because some light reflected from it travels into my eye.

I might see the line of the laser beam if it reflects off specks of dust in the air.

So well done if you've got those ideas that you can't see the laser beam unless there is dust or something like that in the air, but you can see the spot where it hits the wall because light scatters off the wall and some of it enters your eye.

And then we're at the end of the lesson, and here's a summary of what it was all about.

Luminous objects give out light and non-luminous objects do not.

All surfaces reflect light.

Mirror-like surfaces are smooth and reflect light in a regular way, causing reflections to be seen.

Other surfaces scatter incident light in many directions.

For anything to be seen, light from it, either emitted or reflected, must enter the eye.

Light enters the eye through a hole called the pupil and is detected by the light-sensitive back surface called the retina.

Beams of light can only be seen if the light hits something and reflects or scatters towards an eye.

So, well done for working your way through the whole lesson.

I hope you learned some things about what's really happening when you see things, and I hope to see you again in the future.

Bye!.