Lesson video

Hello, my name is Dr.

George.

Welcome to this lesson.

The lesson is called Shadows, and it's part of the unit, Sound, Light, and Vision.

First, here's the outcome for this lesson.

I can describe what happens when light travels and how shadows can form.

Now, I'm sure you've seen shadows plenty of times and you probably have some idea about how they're made, but I hope I can help you understand them even better.

Here are the keywords for this lesson.

There are quite a few of them, but don't worry, 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 the meanings.

The lesson has three parts.

They're called transparent and opaque, what happens when light travels, and shadows.

In the first part, I'm going to introduce some of the important keywords.

Firstly, transparent.

Glass is transparent.

You can see through it.

Water is also transparent, and that's why we can see the stems of these flowers in the vase.

Transparent and objects and materials allow light to pass through.

But if we know the keyword transmit, we can say that in a quicker way.

They transmit light.

They let light pass through.

We can see that in this picture, this photo of a torch, the light from the torch is transmitted by the glass.

The glass lets the light through.

Now, this bathroom window lets light through, but you can't see clearly through it.

So some of the light is transmitted but not in a clear way.

So we don't say transparent for this glass, we say this bathroom glass is translucent, lets light through, but we can't see through it clearly.

Now, wood, obviously, does not transmit light.

It does not let light pass through.

It blocks it.

And we can see in the photo, we can see light from the torch where the torch is not behind the wood, but the part of the torch that's behind wood, we can't see it 'cause the light can't get through.

Keyword, wood is opaque.

So opaque objects, opaque materials don't let light through.

Now, let's try and use those ideas here.

I want you to pick which of the following objects, transparent, translucent, and opaque.

There's one of each.

I'll give you five seconds.

But if you need longer, pause the video, and press Play When you are ready.

Do you have your answers? The storage container is transparent, or at least part of it is, the main part.

The bubble wrap is translucent.

And the brick is opaque.

Now, I'm going to ask you why.

So you're going to explain what makes the storage container transparent.

What is it about it that told you this is transparent? And similarly, for the bubble wrap, what makes you say it's translucent? And why do you say that the brick is opaque? Can you explain that? Take as long as you need.

Press Pause while you're working on this, and press Play when you're ready.

I'm going to show you some example answers now.

Explain what makes the storage container transparent.

It's transparent because light is transmitted, or you could say, "Can pass through the sides.

The contents of the container can be seen clearly." You don't have to say all of those things.

If you say one of those things, "Light is transmitted," "Light can pass through," or, "I can see what's inside," that explains how we know it's transparent.

Explain what makes the bubble wrap translucent.

It's translucent because light can pass through it, but objects behind can't be seen clearly.

Only some light is transmitted through.

Did you get the main ideas? Light comes through, but we can't see clearly through it.

Explain what makes the brick opaque.

It is opaque because it does not transmit light.

Light cannot pass through it.

It blocks light.

That's actually three ways of saying really the same thing.

So if you've said one of those things, well done.

And let's move on to the next part of the lesson.

We'll be using some of those keywords at times.

This part of the lesson is called What happens when light travels.

Let's look at a lamp here in a dark room.

It has a glowing surface.

And every point on that surface is continuously giving out light in all directions.

Light is coming out from every part of the top surface of this lamp, and it's coming out in all directions.

Now, these arrows drawn on, they're called light rays.

They're imaginary.

There aren't really arrows sticking outta this lamp, but they can be useful to draw, here's more of them, because they show some of the directions like travels from some of the points on the glowing part of the lamp.

So there aren't really arrows sticking out of the lamp, but we might draw arrows because we want to show where the light's coming from and which way it's going.

You can't draw every ray of light from every point on a glowing surface, because really, light is coming out in all directions from all parts of this surface, and we can't just keep on drawing arrows forever.

But drawing just a few, like this, gives a good idea of the way light is travelling here.

We can see that it's travelling in different directions.

Light is not really made up of light rays.

As I said, they're not real objects.

There aren't really arrows sticking out this lamp, but these arrows can be drawn to show the direction light takes.

It's like drawing an arrow on a map, or you might draw an arrow saying this way is north.

That arrow isn't there in real life, but it's still useful when we draw it.

Here's a question for you.

Jacob has lost his pet cat in the dark.

He uses a torch to find the cat in the garden.

Tick each letter if there is light at that position in the picture.

So the torch is switched on, where is there light? I'll give you five seconds, but press Pause if you need longer, and press Play when you have your answer ready.

And the answer is all of them.

There is light where the torch is.

There is obviously light on the cat.

We can see it's in the light of the torch there.

But there's also light at position B, in between.

If the light is going from the torch to the cat, it must be passing through the space in between, even though it might not look obvious.

We can draw a light rays on the diagram, just a few arrows showing the way the light is travelling.

It's travelling from A through B to the cat at C.

Well done if you got that right.

Another question, which of the following statements about light rays is true? Read the three statements carefully.

Think about which one is right.

I'll give you five seconds, but press Pause if you need longer.

Are you ready for the answer? It's light rays show the direction light travels.

The other two answers are not right.

Well done if you got that.

When we draw light rays, we should always draw them as straight lines because that's actually how light travels.

It travels in straight lines.

And we can see in these two pic, where if we look at the left-hand picture, again, we've got an opaque block of wood.

Light doesn't travel through it, so we can't see part of the torch.

And if we try to draw that situation, perhaps from above, as here using light rays, we can see that some of these light rays hit the wood, and that represents the light that doesn't get through to us.

And some of the light rays go to the side of the wood, and that represents the light that does go past and reach our eyes.

Because light rays travel in straight lines, you can't see around corners.

If there's something around a corner, the light doesn't bend around for you to see it.

Which image shows some paths taken by some of the light from this torch? Use what you've learned so far.

And if you need more than five seconds, press Pause, and then play when you're ready.

And did you get to the answer A? Light travels in straight lines.

Light rays are drawn as straight lines.

Here's that photo of the lamp in the room again.

And you may notice that the room is more brightly lit where it's close to the lamp, and further away from the lamp, the room is darker.

And that's because light spreads out as it travels.

You could almost think of it getting thinner or more diluted as it spreads out further and further.

And that's also why glowing objects look dimmer, less bright, from further away.

Now, also remember, light travels just as well in daylight as it does in darkness.

In daylight, this lamp might not look very bright.

That's not because there's less light coming from the lamp, it's just because we are seeing it compared with brighter surroundings.

Okay, if the Sun was shining into this room, the lamp just wouldn't look as bright in comparison, but it would be giving it off the same amount of light.

And light actually continues travelling in straight lines until it hits the surface of a material, until it hits something.

So light doesn't run out.

It doesn't sort of reach a natural stop.

It just keeps going until something gets in its way.

And most surfaces, when light hits them, absorb some of the light and reflect the rest.

And when the light is reflected, it's scattered in many directions.

So light that's absorbed by a surface never comes back out.

But light that's reflected bounces back off the surface.

And most surfaces do some of both of those things.

But this all happens very, very quickly because light is extremely fast.

It travels incredibly quickly.

And that's why light given out into a room is absorbed very quickly.

So light comes outta this lamp, and it very, very soon is going to hit the walls of the door or you, it's going to hit surfaces, and a lot of it's going to get absorbed, or scattered and then absorbed.

So the light coming out the lamp, it doesn't last very long.

So if you want to keep the room light, you have to keep the lamp on.

It gives out some light, but it's quickly gone, absorbed.

Then it gives out some more light, then you need it to give out some more.

So if you keep the lamp on, it can keep a steady light level in a room because it's continuously giving out light.

Here's a question for you.

On a dark night, a car is driving away from you.

Why do the lights seem to get dimmer when the car is further away? As you can see in this picture on the right.

If you need more than five seconds to think, press Pause, and then press Play when you are ready with your answer.

So did you choose answer B? Light spreads out more the further it travels.

Well done if you did.

That's why the lights of the car look dimmer, less bright, when the car is further away.

A longer question for you.

Something for you to discuss.

Four statements.

I'd like you to think about whether each one is correct or incorrect.

And if you decided that, you can also choose whether you're sure of it or whether you just think so.

So for each statement, choose one of these four boxes.

Press pause while you're thinking about this and discussing it, and press Play when you're ready with your answers.

Okay, so here are the answers.

I'm sure about all of these.

So light travels out from the lamp in straight lines.

That is true.

There's a set distance that light from the lamp can travel.

Not true.

Remember, it keeps going until it hits something.

Light comes outta the lamp until it fills the room, then it stops.

No, that's not true.

But the next statement is, light keeps coming outta the lamp and bouncing off things.

That's true.

Reflects off things.

It also gets absorbed.

That's what happens.

So well done if you've got at least some of those correct.

Now we're ready to talk about shadows.

When a bright light is shone onto an opaque object, remember, an object that doesn't let light through, a shadow forms, as you can see in this picture.

A shadow is an area where the light can't reach because light only travels in straight lines.

So shadows happen because light travels in straight lines and because some objects, like this bottle here, are opaque.

So light cannot curve around an object to light the area behind.

Doesn't bend around the bottle.

It just hits it and doesn't make it through.

So here's a representation of what the light is doing using the light rays.

The rays hit the opaque objects, the wood and the bottle, and so they're blocked.

So the wall directly behind is not lit.

Whereas these rays, there, ones that are passing by the side of the objects.

So they do hit the wall and they light the wall where they hit.

So we can use the idea of rays to understand why there's a shadow on part of the wall and not on another part of the wall.

Here's a 2D diagram, two-dimensional diagram, showing how a shadow forms. Don't worry if it looks complicated, really, there's just three objects here, lamp on the left, an object in the middle that's opaque, and a wall on the right.

And then we have arrows, light rays, showing what the light is doing.

It's travelling out from the lamp.

And some of the light is not blocked by the object.

And we show that with these light rays that are going past the object, they're reaching the wall and lighting it.

But some of the rays hit the object, they don't make it through.

And so that part of the light, sorry, that part of the wall doesn't get light from the lamp.

It's the shadow.

Here's a question about shadows.

Which of the following options shows where the tree shadow will be? Pay attention to where the Sun is, and pick which shadow would be in the right place.

If you need more than five seconds, press Pause, and then play when you're ready.

And the answer is D, it's on the other side of the tree from the Sun.

So the Sun's light is shining on the tree, but some of it's not making it through to the left-hand side of this picture.

And so there's a shadow there.

Something to make sure you're very clear about.

Shadows are not reflections.

So a reminder about reflections.

Reflections happen when light bounces off shiny surfaces, like mirrors.

Here's a picture of a toy cow and it's reflection in a mirror.

That's not a shadow.

Shadows happen when light is blocked.

Here's the toy cow blocking light, which makes a shadow.

That's not a reflection.

So these are different things.

Reflection on the left.

Shadow on the right.

Shadows are also not objects.

They're not a thing that you could pick up or hold.

They're just an unlit part of a surface.

They're just a place that isn't lit up and they're there in the shape of the object that blocks the light.

Here's another object making a shadow.

It's a toy giraffe.

And you can see the shadow behind is in the shape of the giraffe.

The shadow of an opaque object, like this giraffe, can't show any of the object's pattern or colours.

It just shows the shape in a single solid colour.

There's a word for that, which is a silhouette, but you don't have to know that word.

When the torch is turned on in this picture, nearby surfaces will become much brighter because of the torch light hitting them, but the area in shadow does not get any extra light.

Let's see.

Okay.

So the area where there's a shadow directly underneath the object now seems very dark, but actually, it's the same brightness that it was before we switched on the torch.

It just looks darker now compared with the surroundings.

So around the shadow, we now have very bright surface, and so that makes the shadow look dark, but that area is no darker than it was before we switched on the torch.

We can use light rays again to represent what's happening here.

We're just showing two light rays that are just skimming the edge of the object, two light rays are just making it onto the surface, showing us where the edges of the shadow are going to be.

Here's a picture again of a bottle casting a shadow.

And this is to show you that shadows often aren't actually black, they're the colour of the surface that they're on.

So here, the shadow is landing on some paper, it's on green, yellow, and red paper.

So the shadow itself in those places is green, yellow, red, or grey on the table.

So the shadow is just whatever colour is the surface that it's on, although it will be darker than the rest of the surface, as you can see here.

Now, here we're on the second picture, we have a torch on the left that's switched on is creating a shadow of the bottle.

Now, we're going to switch on the torch on the right.

It's off at the moment, but now it's on.

Then what happened when we turned the second torch on? We got another shadow, but something else happened as well.

The first shadow got a bit lighter.

Just about see that here.

So you can see in the first picture, we can see a little square representing the colour of the shadow there.

And in the second picture, we can see a square, its the same shadow, but it's brighter because some of the light from the right-hand torch is shining there.

Here's a question, which of the following is the correct description of how the tree's shadow is made? Read these carefully.

Three of them are wrong, one of them is right.

Press Pause if you need more time, and press Play when you're ready.

And the correct answer is A, the light can't pass through the tree so the grass is not lit up.

So that area of the grass where the shadow is, the light isn't getting to it from the Sun because the tree is blocking it.

Well done if you got that one right.

Now, let's think about the different shapes and sizes of shadow that you can get even from the same object.

So here, we have a lamp, we have an object, we have a wall.

And if we move the object closer to the lamp, as in the lower picture, we get a larger shadow.

So same lamp, same object, same wall, but moving the object closer to the lamp gives a larger shadow.

And the light rays in these diagrams are trying to show us how that's happening.

And here we have photos of the same object, the bottle, and it's arranged in three different ways, and we're getting some quite different shadows.

Same torch, same bottle, same wall, but look at the different shapes of those shadows.

The shadow depends on where the light can reach, and that's affected by the size and the position of the object.

So here, the bottle has been repositioned in the different pictures.

And it also depends on the angles and distances involved.

So we could have different distances between the torch and the object, between the object and the wall, different angle of the torch, different rotation of the object.

All of these things are affecting the shadow.

And we can draw these particularly useful light rays here, and they're drawn just from the edges of the torch, just skimming past the edges of the object.

And that helps us see where the shadow's going to be, why the shadow's going to be the way it is.

In this picture, we have the object standing up the same way every time, but the torch is being moved, it's at different angles.

And again, we have here, well, we have one light ray here showing a ray that can just get past the bottle, and that helps us see where the edge of the shadow is going to be.

Now, in this question, we have a penguin toy, and it cast a shadow on a screen behind it.

Which of these images shows what that shadow will look like? Pause the video if you need more than five seconds.

And the correct answer is A, it's the same way round.

It's facing the same way as the penguin.

It's not a reflection.

And it's not coloured or patterned in the same way as the penguin.

It's just dark or the same colour.

Well done if you've got that.

Now for the size, which image shows the correct shadow size.

Think about light rays.

Imagine light rays that come from the lamp, just make it past the head of the penguin.

Where will it get to on the screen? Five seconds, or pause the video if you need longer.

And the correct answer, C, a light ray from the lamp just skimming over the pigeon, the pigeon, the penguin's head will make it to the top of the shadow.

And there you are.

So the first two answers are wrong because the shadow should go up to where that top ray reaches.

Only C does that.

Well done if you've got those.

And here's the final task of this lesson.

Here we have an empty picture frame.

It's got the glass in it and it's got a wooden frame, there's just no picture.

And there's a sticker on the glass, and we have a torch at the front of the picture.

The torch is shining at this empty picture frame.

Two questions.

Explain why the frame and the sticker cast shadows on a wall behind, and explain why the glass does not cast a shadow.

And if you can, use keywords from the lesson in your answers.

So think about what those keywords might be.

Take as long as you need to write down your answer.

And pause the video for that, and press play when you're ready.

And here's an example answer.

Explain why the frame and the sticker cast shadows on the wall behind.

The frame and the sticker are opaque, so they do not transmit light.

Light that hits them will be absorbed or reflected, so blocked from travelling to the wall behind them.

Light can only travel in straight lines, so can't bend around a frame or sticker to light the wall behind.

This leaves unlit areas of wall in the shape of the frame and sticker.

Don't worry if your answer wasn't exactly like that or if it wasn't as long as that.

But did you get the key ideas? The frame and sticker are opaque so they block light.

The glass is transparent so it lets light through.

So the frame and sticker stop light getting to the wall because light can only travel in straight lines.

It can't bend around them.

Then question two, explain why the glass does not cast a shadow.

Glass is transparent.

Light that hits the glass can pass through it to light the wall behind.

So the wall behind the glass is lit rather than in shadow.

Did you get that idea into your answer? And did you manage to use these keywords, opaque, transmit, transparent? If you did, well done.

So that's the end of the lesson.

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

Transparent objects transmit light.

Translucent objects transmit some light, but not in a clear way.

Opaque objects do not transmit light.

You can't see through them at all.

Light travels in straight lines at an extremely fast speed.

Light rays straight arrows added to diagrams to show the direction that some light is travelling.

Light spreads out as it travels.

Light will always continue travelling in straight lines until it hits the surface of a material.

Some light is absorbed and some is reflected, bounced back.

A shadow is an unlit area in the shape of an object that is blocking light.

So well done for working through the whole lesson, and I hope you'll think about some of these ideas next time you see shadows.

I hope to see you again in another lesson.

Bye.