Lesson video

Hello, my name's Dr.

George, and this lesson is called Reflecting Colour.

It's part of the unit, Making Images, and it's all about why objects look different colours.

The outcome for this lesson is I can explain the colour of an object in white light and in coloured light.

There are a few key words in this lesson that I'll introduce.

I'll explain them as we go along, but this slide is here in case you want to come back at any time and check the meanings.

There are two parts to the lesson called Seeing Objects in White Light and Seeing Objects in coloured light.

Let's start.

So a recap about how we see objects at all, and it's because they reflect light.

All surfaces reflect light.

And on the right there's a diagram reminding you about the Law of Reflection, one of the laws of our universe that we've discovered.

And it says that things reflect in a very particular way.

Here we have an incident ray, a ray coming in to the mirror, and then it's reflected, and the reflected ray comes away again.

Remember, real light isn't actually made of rays, but we use rays to represent where light is travelling from and to.

And there are these two angles here, the angle of incident and angle of reflection, the angle between the incident ray and that dash line and the angle between the reflected ray and the dash line.

And the Law of Reflection says these two are equal and also that the two rays are in the same plane and also that the two rays are on opposite sides of the dash line, which is an imaginary line called the normal at right angles to the mirror.

So the way mirrors behave is rather special because if light comes at a mirror all from one direction, after the reflection, all of the reflected rays travel in a new direction, but all in the same direction as each other.

And that's why we can see images in a mirror.

Now, other surfaces that don't behave like mirrors, you can't see an image in the surface.

They also reflect light, but they reflect in a way that we call scattering.

And that means that rays are reflected in many directions, even rays that all came in in the same direction.

The reason that happens is because the surface of these types of objects are not smooth, the surface is bumpy.

And so even rays that come in from the same direction, as shown here, all end up reflecting off in different directions.

And that's what we mean by scattering.

So which objects in this room reflect light? So look at the photo and think about these different eight objects and decide which ones reflect light.

Now, for short questions like these, I'll wait five seconds, but if you need longer, just press pause when you think and press play when you're ready.

So let's check the answers.

It's all of them.

That's not a trick question.

It's important that you realise that any object that you can see, you can see it because it's reflecting light or because it's luminous.

Some objects are luminous, they make their own light, but nothing in this picture is actually making light.

So we see non-luminous objects when some of the light they reflect goes into our eyes.

If an object is scattering, so it's a dull object, it's not shiny like a mirror, if it scatters light, then that light is going to scatter in all directions.

So you'll see it from any angle.

Now imagine you're exploring a cave and it's deep underground.

There is no sunlight reaching this cave.

The only light is coming from your head-torch.

You have a torch on your helmet.

If you turn off the torch, you'll be in total darkness.

There'll be no light anymore and you won't be able to see anything.

And it doesn't matter how long you wait, your eyes will never adjust to that.

Your eyes can adjust to low levels of light but not to no light.

And that's because if there's no light, there's no light entering your eyes.

And the only way we see things is from light entering our eyes.

So here's a question about that scenario.

A person is in a cave deep underground in total darkness, and then they turn their head-torch back on.

Which diagram shows why they can now see the cave wall, the wall on the right.

Now look carefully at these, because some of them look similar, but they're not actually the same.

So press pause if you need to, and press play when you've chosen your answer.

And the correct answer is B.

That shows a ray going from the torch to the wall of the cave and back towards the person's eye.

That's how we see light needs to hit the wall of the cave from the torch and then come back to our eye.

In C, one of those arrows is in the wrong direction.

In E, it seems to show light coming not from the torch, but from our eyes, that doesn't happen.

So well done if you picked out the correct answer there.

Now we're going to look at how we see different coloured objects.

So we know that those objects are scattering light, but how do we actually see in the first place? Here's a quick recap.

We see colour when light enters our eyes and is absorbed by the cone cells on the retina.

So light comes through the pupil, through the lens, as shown here, and then it hits this back surface of the eye called the retina.

And some of those cells can detect the colour of the light that reaches them.

Now there are three kinds of cone cell and each of these absorbs and detects a different colour of light: red, green, or blue.

So they're called the R, G or B cone cells.

And this little picture here shows the range of colours that they can detect.

So it's not actually just a single colour.

If you look at the red one, for example, the R cone cell, it can detect the colours of red light, but also it can detect yellow and some green colours.

So if the correct colour of light falls on a cone cell colour that it can detect, then it sends an electrical signal along a nerve to the brain.

And the colour we see depends on the levels of signal coming to the brain from the R, G and B cone cells.

So this person is seeing different colours when combinations of colours reach them.

So for example, if your brain receives some signal from the R cells, but also from the G cells, they're both more or less sensitive to yellow light.

So you are going to experience yellow.

Now, most of the light that's around is white.

The sun emits white light, not yellow.

And most lamps, most lights emit white light as well.

And white light includes light of all colours that there are.

All colours of light you can get are in white light.

That's all the colours of the visible spectrum, the range of colours that we can see.

So if sunlight enters our eyes, that's all the colours entering at once, so that R, G and B cone cells all fire.

They all send off signals at the same time.

And when our brain receives all three signals, this causes us to see white.

We experience white when every colour enters our eyes.

Now let's get on to surfaces and how they look, how different objects look when they're lit by white light.

Now a white surface can reflect all colours of light equally well.

So if there's white light around, if white light shines onto the surface, it reflects all the colours that are in that white light, it scatters them.

And so if you look towards the white surface, light of all colours comes into your eyes from it and you experience white, you recognise that that surface is white.

Now a perfectly black surface, a surface that is a deep black doesn't reflect any light at all.

It doesn't reflect any of the colours of light.

So if you look at it, no light comes to your eyes from it and you recognise that what you're seeing is black.

Black is sometimes described as a colour, but it's not the colour of light.

There is no black light.

If you see something black, it's what you experience when no light enters your eyes.

So a question.

Why do white objects look white in white light? So when white light shines on an object, why would it look white? Press pause and press play when you have your answer ready.

And the correct answer is because they reflect all the colours of the spectrum.

When all of those colours enter your eye, you experience white.

Now, coloured objects.

An object's colour depends on which colours of light it reflects.

Colours of light that aren't reflected by a surface get absorbed, they go in and they never come out again.

And here's an example, a green leaf.

Green objects, like this leaf, only reflect green light.

So they absorb other colours such as red.

So when you look towards this leaf, the only light coming to your eyes from the leaf is green.

So what you experience is the colour green.

Red surfaces only reflect red light.

They absorb all the other colours.

So if you look towards a red surface, only red light's being scattered into your eyes, you see red.

Blue surfaces only reflect blue light and absorb all other colours.

So if you look at that surface, you only receive blue light.

So now a question for you, true or false? Green objects look green because they only absorb green light.

And I want you to think about why.

So why do you think it's true or why do you think it's false? So press pause and press play when you have your answers ready.

The correct answer is false.

And why? Well, green objects look green because they only reflect green light.

If an object absorbs green light, then no green light can reflect from it into your eyes.

So the statement said they look green because they only absorb green light, but actually, it's because they only reflect green light.

Well done if you got that one.

And another question.

A team's football kit has blue shirts and white shorts.

Which of the following statements are correct? Have a look at each of them.

There's more than one correct statement here.

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

And there are two correct statements.

Here they are.

We know the shirts are blue, and that would be because they reflect blue light from the sun and not the other colours that are in that white light.

And we know the shorts are white, so we know they will reflect blue light from the sun.

Now of course, they will reflect all the colours of light from the sun, but it's still true that they do reflect the blue light.

D says the shirt gives out blue light.

Now "gives out" is not really a scientific term, but it makes it sound as though the shirt is making blue light as though it's luminous.

And that's not true.

Well done if you've got both of the correct statements there.

Now I'm sure you already know, white light from the sun and from light bulbs can be represented like this, a spectrum of colours like the colours in the rainbow, because that's what it's made up of.

And that light makes all of our RGB cones fire at once, and we see white.

But any colour we see, it just depends on the levels of signals sent to our brain by the R, G and B cone cells.

So we can make a simplified model of white light, a scientific model where we model it using just a red, a green and a blue ray.

And we're using these rays to represent the three ranges of colours of light that trigger the three kinds of cone cells.

So if we use this RGB model of white light, we could say all yellow surfaces reflect the red and the green light, but absorb the blue light.

So what we mean in this model is that yellow surfaces reflect colours of light that trigger the R and G cone cells and we'll represent those colours with a red and a green ray.

And the yellow surfaces absorb the colours of light that trigger the B cone cells and we'll represent those colours just using a blue ray.

So using this model, you can see that as we've already said, yellow surfaces reflect red and green light equally, but absorb blue.

Cyan surfaces reflect green and blue light equally, but absorb red.

When green and blue light enter our eye together at equal intensity, we see cyan.

And magenta surfaces reflect red and blue light equally, but absorb green.

So why does an object look magenta under white lighting? Press pause and press play when you've chosen your answer.

The correct answer is it reflects only red and blue light from the white light or it reflects only the colours of light that trigger our R and B cone cells.

And that makes us see magenta.

And now I have a longer task for you.

Guyana is a country in South America and this is the flag of Guyana.

Now imagine the flag is flying outside on a sunny day, so sunlight is shining on it.

And I'd like you to try to explain why each part of the flag looks the colour it does.

So there are actually five different colours on this flag.

We've got the red triangle, the black border around the triangle, the yellow chevron, the kind of arrowhead shape in yellow, the white border around that, and the green background.

So one by one, say how we see each of those colours, but start by saying something about sunlight.

So start your explanation by completing these sentences first.

Sunlight is.

And the red triangle looks red because.

So pause for as long as you need to do the task.

And when you're ready, press play and I'll show you the answers.

So here are ways that you could have written this.

You don't have to have written your answer in exactly the same way, but you need to use the same ideas.

So sunlight is a mixture of all the colours of light in the visible spectrum.

The red triangle looks red because it only reflects red light into your eyes.

The black border looks black because it doesn't reflect any light into your eyes.

The other chevron looks yellow because it reflects only red and green light into your eyes and absorbs blue.

The white border looks white because it reflects all colours of light into your eyes.

The green background only reflects green light into your eyes.

So if you've got most or all of those ideas into your answer, well done.

Now let's go onto the second part of the lesson, seeing objects in coloured light.

Now you don't often see things in coloured light, most probably, because you tend to see things in sunlight, which is white, and lights in buildings, which are usually white.

But in blue light, if blue is the only colour of light shining, a white surface appears blue as shown in these photos.

And that's because white surfaces can reflect all colours of light, but there's only blue light around.

There's only blue light incident on it hitting it.

So it's only reflecting blue light right now.

And that's all that goes into your eyes.

Any black parts of the surface absorb all colours of light.

So they just still look black in coloured light as you can see with the writing on this piece of paper in the pictures.

So what colour would a white surface appear in each of these situations? Take as long as you need.

Press pause to think about your answers and press play when you're ready.

So here are the answers.

In red light, a white surface will appear red.

Because it can reflect any colour of light, so it will just reflect the red light that's around.

In cyan light or cyan light, we can model as being made up of green and blue light.

White surface can reflect any colour.

So it reflects both of those.

And when we look at the surface, we see cyan.

So I think you're getting the idea now that a white surface will appear the same colour as the colour of light that's shining on it.

A perfectly black surface fully absorbs all colours of light that fall on it.

So it will look black in any colour of light.

Whatever light shines on it, it just doesn't reflect, it doesn't scatter any of that light.

In blue light, objects that are red, green, and yellow will all appear black.

And that's because all of these coloured surfaces absorb blue light.

The red can only reflect red light, the green surface can only reflect green light, and the yellow surface only reflects red and green light.

So if there's only blue light around, none of these surfaces can reflect it and they all look black.

In blue light, objects that are blue cyan or magenta, as shown here, will all look blue.

And that's because they can all reflect blue light.

And if blue light's the only colour around, then they're simply going to reflect blue light and that's what you'll see when you look at them.

And now a question.

Think very carefully about this.

Which of these cubes will look red if only red light falls on them? And there's a second question underneath, what colour will all the other cubes look in red light? Pause for as long as you need and press play when you're ready and I'll show you the answers.

I hope you enjoy puzzling your way through these questions and don't worry if you get anything wrong.

I'll explain the answers now.

So the objects that will look red if only red light falls on them are the red, yellow, magenta and white cubes.

And that's because they do reflect the red light.

Red, of course, red surfaces only reflect red light.

Yellow is able to reflect green, but there's none around.

So it just reflects the red.

Magenta is able to reflect blue and red, so it just reflects the red that's there.

And white, of course, can reflect any colour of light.

So it does reflect the red.

So all of these objects will look red.

Now what about the other cubes? Well, those are all the ones that are not able to reflect red light, and there is no other light around.

So did you realise that these cubes will look black because they're not reflecting light into your eyes? Now let's think about cyan light.

Cyan light consists of green light and blue light together.

And in cyan light, the cubes that reflect both green light and blue light will look cyan because that's the colour you see.

Green and blue enter your eyes.

The white cube can do that.

The white cube can reflect any light.

The cyan cube can too, but the other cubes can't reflect both of these colours.

The magenta can only reflect blue, not green.

So it's going to look blue.

Now what about cubes that can reflect green but not blue? Well, they will reflect the green light and they will look green.

So that's the green cube, but also the yellow cube.

Yellow surfaces can reflect green and red, but there's no red light around.

So they just reflect the green.

They can't reflect blue.

And the cubes that reflect neither blue nor green, so they don't reflect any of this light, they're going to look black.

And that would be the red cube, which can only reflect red light and also the black cube, which doesn't reflect any kind of light.

So this is what we would see if we shone cyan light on these cubes.

So a similar question for you now.

Yellow light consists of red light and green light.

So I'd like you to think about what colour each of these cubes would look in yellow light.

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

Let's look at the answers.

So here are the colours that they would look.

So the red cube can reflect the red light that there is, but not the green.

So it just looks red as normal.

The yellow cube can reflect red light and green light.

That's why it normally looks yellow.

So it does reflect both of those and it still looks yellow.

The green cube doesn't reflect the red, but it does reflect the green.

So it looks green as normal.

Cyan can reflect green and blue, but there's no blue light around.

So it just reflects the green and it looks green.

The blue cube.

Blue surfaces don't reflect red or green light, but only blue.

So when there's no blue light around, they reflect no light and they look black.

The magenta cube.

Magenta surfaces can reflect red but not green, so it will now look red.

The white cube will simply reflect all of the light that's around.

That's the red light and the green light.

So when you see that, it's going to look yellow.

And the black cube, as always, reflects no light and looks black.

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

And if you've got any wrong, have a think about it and perhaps go back and rewatch the previous example.

Now, back to this flag of Guyana.

I'd like you to predict what colour each section this flag will look if green light, only green light is shown on it.

And I'd like you to explain why you think that you'll see each of those colours.

So you could put your answers into a table like this.

Then you're going to watch a demonstration either in real life or a video and see if your prediction was correct and do you think your explanation was correct and if you think you need to improve the explanations that you wrote.

And when you've done all that, press play again and I will show you the answers.

So here are the answers.

The flag will look a bit like what you see here on the bottom right, although that triangle should really look black.

So the red triangle will look black because it reflects only red light and absorbs other colours.

The green light is absorbed so it looks black.

The black border will look black because black doesn't reflect any colour of light.

The yellow chevron will look green.

Because yellow reflects red and green light, there is no red light to reflect, it's just going to reflect the green.

The white border will look green because it reflects all colours of light.

So it reflects the green light and appears green.

And the green background will look green because it simply reflects the incident green light.

So well done if you got most of that or all of that right, and you might like to practise this by thinking about other country's flags and what colours their parts would look in different colours of light.

We're at the end of the lesson now, so I'll give you a summary of what it was all about.

We see colour when light entering our eyes is absorbed by the red, green, and blue cone cells on the back of our eye.

The colour and object appears to us depends on the amounts of red, green and blue light that reflect off it into our eyes.

White objects reflect all colours of light, so look the same colour as the light that is incident on them.

Red, green and blue objects reflect only one primary colour of light.

Cyan, magenta and yellow objects reflect two primary colours of light.

The colours of light that an object does not reflect are absorbed.

Black objects absorb all colours of light.

So well done for working through this lesson.

I hope you enjoyed it and I hope you'll think about it a little bit when you are seeing things of different colours and remember why they look the way that they do.

I hope to see you again in a future lesson.

So bye for now.