Hello, my name's Dr.

George, and this lesson is called "Seeing in Colour".

It's part of the unit, "Making Images".

The outcome is that by the end of the lesson, you'll be able to do this.

I can state the primary colours of light, describe how they mix to produce other colours, and explain how the eye detects colour.

And I hope you'll enjoy this.

Here are the keywords for the lesson.

I'm not going to read through all these now.

I'm just leaving the slide here, so that you can come back to us any time, and check the meanings, but I'll be explaining them as we go along.

There are three parts in this lesson called the primary colours of light, mixing light of different colours, and the eye and seeing colour.

So let's look at the primary colours of light.

So here we have a phone, and there's a kind of microscope sitting on top of it that's really magnifying what's on the phone screen.

And when we magnify what's on the screen, we can see lots of tiny individual lights, but if we look carefully, we'll see that they're just red, green, and blue.

Those are the only colours of these little lights and three of these different colours together.

A little red, green, blue light close to each other is called a pixel.

And the screen makes many, many different colours, or at least it seems to.

And it just does this by varying the amount of red, green, and blue light that these little lights are showing.

And because you can make every colour appear by mixing different amounts of these three, we call these three the primary colours of light: red, green, and blue.

Now if you have a phone or a tablet or computer, you can see the pixels on the screen.

You need to do this very carefully.

You don't want to get water at all inside your device.

So have a glass of water, and then something you can dry it with a piece of kitchen towel and dip your finger into the top of the water, so that you come out with a drop of water hanging from your finger.

Touch that drop to the middle of the screen, so that it sticks to the screen.

And then the drop will act like a little magnifier.

And if you look closely, it should be able to see the pixels of your screen, and then carefully dab the drop to dry it off.

Here are some more microscope images of screens showing how different colours are actually created by the screen.

So we can see here that this blue and this red, it seems to be there's only one type of light on.

So to make a pure blue like that, just the blue lights come on, and the green and red are off.

Similarly with the red, with the green here, we have a kind of green that's mostly made from green lights and a little bit of red.

You may be able to see white.

All three colours are on.

And then you can see here, purple, magenta, yellow, maybe a surprising one.

And cyan, which is the colour you see there.

Now, if you have a deficiency in your colour vision, which some people call colorblindness, these colours won't look all the same to you as they do to other people.

So bear that in mind as we go through.

So I'll be describing and naming the colours, but it might be interesting for you to talk to someone who doesn't have colour vision deficiency, and compare what you see.

Colours that look very different for people they see in full colour can look quite similar or even the same for people with colour vision deficiency.

Now, some colours are created by mixing all three of the primary colours of light in different amounts, so different levels of brightness of the three colours.

So on the left here, we have a sort of dirty looking brownish yellow, and you can see there's a lot of red in there.

The red's bright, the green's quite bright.

The blue not so much.

And then in the middle, we have a kind of purple, which is mostly made from red and blue.

And on the right, a sort of aqua bit like the colour of the sea, greenish blue, and that is made from green and blue light, and looks like there's a tiny bit of red in there, as well.

So you may have been told that the primary colours of paint are red, yellow, and blue.

And that's because you can make many different colours by mixing different amounts of red, yellow, and blue paint.

They're not actually the proper primary colours of paint, but they're good enough.

If you're doing basic painting, you can make a lot of different colours with those.

The primary colours of light, yellow is not one of them.

It's red, green, and blue.

And if we use the primary colours of light, if we mix these colours of lights as we've been seeing in different amounts, we can make any colour or at least we can make it look like any colour as far as the human eye and brain are concerned.

So a recap.

Which of the following are the primary colours of light? And with these short questions, I'll wait five seconds, but you may need longer.

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

The primary colours of light, red, green, and blue.

And when we mix colours of light, they mix differently to the way paints mix.

So look at the picture on the left, we have red and green paints mixing to make a sort of murky brownish colour, but if we mix red and green light, have a look on the right, we actually get yellow.

That surprises people a lot of the time.

And if you look at this microscope picture of a phone at the bottom, this part of the screen, which is just showing bright red and bright green light in its pixels is actually going to look yellow when you look at it.

That's how yellow is made on a screen.

Now a question, how does a TV screen use light to make a picture? Press pause, and press play when you have your answer ready.

The answer is two of these are correct.

A TV picture is made up of thousands of tiny coloured lights, pixels.

And all the colours on the screen are made by mixing just three colours.

The primary colours of light red, green, blue.

It's not true, the mixing red and green light makes the same colour as mixing red and green paint.

We've seen red and green light makes yellow, red and green paint makes a sort of brown.

And now, I have an investigation for you to do, which I hope you'll enjoy doing.

You're going to try mixing the primary colours of light.

So for this, you'll need two ray lamps, also called ray boxes or light boxes, and they just make white light normally.

But you're going to use filters, red, green, and blue filters.

And though, it should be a slot in the red box that you can put the filter into, so that you just get red or green or blue light and you'll take one of those, and shine it onto a white screen.

And then with the second lamp, shine a colour onto the same spot on the screen, so that they overlap.

And record which two colours you used and what colour you saw.

And also, when you added the second colour, what happened to the brightness? Did it get more less bright or the same? And I want you to try all combinations, even two colours the same.

So red with red or green with green, but also, different ones like red with green, red with blue.

Get your table ready first, and pause the video while you're doing this.

And when you're finished, press play.

Here, we have some example observations, but instead of using white light with coloured filters, these are using coloured lights, type of light called an LED lamp.

And instead of shining a light onto a screen, the lights here are on a table, and you can see the colour of the table where the lights are shining on it.

So if you look at the rectangle that's been drawn, you can see what colour, when there's just one light shining on that or when there are two lights shining on that.

So have a look at this.

It shows all of the different combinations on this one page.

And now, let's look at a table of results.

So your table would be similar to this, I hope, with six different combinations of colours that you could get from your two lights, and the colours made and the brightnesses.

We'll talk more about these in a minute, but notice that, with red and green, you get yellow.

With red and blue, you get a sort of pink colour that we call magenta.

And with blue and green, you get a colour called cyan.

It's not quite just light blue, it's a sort of greenish blue, a little bit like a turquoise.

And let's move on to the second part of the lesson, mixing light of different colours.

So if you mix equal amounts of red and green light, which I hope you did in the investigation, you get yellow and we can see that here, the little square on the table, which is receiving both red and green light shining on it, is now looking yellow.

Green and blue, we get this colour called cyan.

And blue and red, we get this colour called magenta, sort of pinky, purpley colour.

Now, take a look at these.

Four colours here, which of these colours is cyan, and which colour is magenta? Press pause, and press play when you're ready.

And C is cyan and B is magenta.

Now, I'd like to fill in the names of the missing colours.

So in each of these statements, there's a question mark, and there's a missing colour name there.

So press pause, and press play when you're ready with your answers.

And here are the answers, red and green, make yellow.

That's the one that tends to surprise people.

Blue and red makes magenta.

And blue and green makes cyan.

Well done if you've got those.

And as we've seen, adding more light always makes a brighter or lighter colour, and that's because more light is hitting the screen or here, the table, and reflecting into your eye.

It's different from paints where you often get a darker colour from mixing.

So yellow, cyan, and magenta that are created by these mixing of primary colours, they're always brighter than the individual red, green, or blue lights that you started with.

That's not always easy to see, because human eyes are actually better at detecting some colours than others, more sensitive to some colours than others.

But out there in the real world, these colours really are brighter when they're mixed.

Now, these three colours we make by mixing the primary colours, yellow, cyan, and magenta, they're called the secondary colours of light.

And to get them, you mix two different primary colours of light in equal amounts, equal brightnesses of the two lights.

And if you go further and mix all three primary colours of light in equal amounts, you create a colour that we call white.

That's what you see.

So here in this picture, you can see a summary, really, of all of the mixtures that we can get just by adding together equal amounts of the three primary colours.

There's also a video here on the slideshow that you can watch to see the three colours being mixed in different ways, and eventually mixed together all of them to give white.

And now a question, on the left, we have red and green light shining on the same spot on a screen to make yellow.

And then there's a blue light, which we're now going to bring in, and shine it on the same spot.

I'd like to know what happens, what is the new colour? And is the area with the new colour brought in brighter, less bright, or the same brightness? Press pause, and press play when you have your answers ready.

And the answers are you get white, three primary colours add to white, and the screen is now brighter than before.

If you shine more light on the screen, no matter what colour is going to look brighter.

Now look at this animation.

It shows how any colour can be made by varying the amounts of red, blue, and green light by how much, how bright they each are when you combine them.

Right now, we have white all three and equal amounts as you see on the switches on the right.

Now, just red, bring in some green, we go to see yellow, take the red away, we go back to green.

Add in the blue, now in cyan.

But notice we're getting in between colours.

The sliders are moving quite quickly, but as we bring in different amounts of a primary colour, we get a range of different shades, of different colours.

And now, we're back to white again.

And here, let's slow things down a bit.

So on the left, we have just red light.

So the RGB standing for red, green, blue.

So we've got full brightness of our red light, but then we start to mix in some green.

We initially just have a dim green light, low level green light, and we increase the green, and we go red into orange, into lighter orange, and eventually, yellow and a sort of pure bright yellow when we have equal red and green.

We could start with green instead, as you see on the bottom, start with just green, and start bringing in more and more red.

We go into a sort of lime green, a yellowish green, and eventually, we'll get into yellow again when we have equal amounts of red and green.

And if we now take that yellow and start mixing in some blue light, what we get is a lighter and lighter yellow.

and we end up with white when we have equal amounts of red, green, and blue.

Now, if you remove light, so go the other way round, you always get a darker colour.

So here on the left, we start with a very pure red colour, just red light.

And if we decrease the amount of red light, we get a darker and darker red, until eventually, we have no red light at all.

And if we have no light, we have black.

And here, darker and darker shades of yellow.

So to make yellow, we need equal amounts of red and green.

And on the left, we have plenty of red and green.

We have a bright yellow, but if we reduce the amount of red and green, we have a sort of yellow, sort of increasingly dirty yellow, until eventually, we end up with black when we switch the lights off completely.

And here, going from white to black.

So again, subtracting the light, removing light, making it get darker and darker.

And if we keep the red, green, blue equal in brightness, we will always have shades of grey.

We'll go from white into darker and darker shades of grey as we reduce the light levels, and eventually, black when there's no light shining at all.

So black is the absence of light.

It's what you see when there's no light.

So if you want to show black on a screen of a phone or a TV, you would simply turn off all of the lights, all of the little pixels.

So these red, green, and blue pixels that we see again here, switch them all off and you get black.

So in reality, black is whatever colour the screen is when nothing is lit.

So it might not actually look a really deep black, it might be more like dark grey, because your phone or your computer screen reflects some of the light from the surroundings.

But things can still look black on the screen, because when the pixels are switched off, that area could look black compared with the brighter areas nearby.

Now, which of the following colours would you get if you mix dim red, so not very bright red, with dim green light? So take a close look at these.

There's word descriptions as well, and there's an illustration on the left showing the sort of thing we're doing.

A little bit of red light, a little bit of green light.

What do we get? Press pause, and when you're ready with your answer, press play.

And what you get is dull yellow.

So remember, if you have a bright green and a bright red light equal brightness together, you get yellow.

We've dropped the brightness here, so we've just got dim lights.

So we're gonna get a darker, dull yellow colour.

Well done if you pick that one out.

And now, I've got six colours here.

I would like you to match each colour to the amounts of red, green, and blue light that would make that colour.

So each of those colours on the left matches with one of these little diagrams on the right showing how red, green, and blue are being mixed.

So for this, take as long as you need, press pause while you're thinking, press play when you're ready, and I'll show you the answers.

So here are the answers, and I'll explain them to you.

So remember that the more light we have, the brighter, the lighter the colour is.

So cyan, first of all, is a secondary colour.

It's made by mixing equal amounts of bright green and bright blue, so it's D.

White is made from equal amounts of bright red, green, and blue, so white is B.

Now, this pale yellow is A, and the reason why, is red and green on their own, if they were bright, would make a sort of intense bright yellow.

But we've added in blue, and as we add more and more blue, we're heading towards making white.

So we've gone from a deep yellow to a light yellow, a pale yellow.

Then number four, we have orange.

And that's made by taking red and adding some green.

If we had no green, we'd just have pure red colour.

If we had equally bright green to the red, we would have yellow.

And we're in between.

So we have a sort of orange colour.

Five is dark grey, and we get greys from equal amounts of red, green, blue.

And this is a dark grey, so there isn't very much.

Red, green, blue, it's C.

And the colour shown in six, which is maybe hard to name.

Some people would call it teal, perhaps, that is E.

It's actually a darker version of cyan.

So it's just green and blue, equal amounts, but not as bright as in D.

If we started to add red, we would start to head into more of a grey colour, and then if we added in an equal amount of red, we would get a pure grey.

Well done if you've got those.

I wasn't expecting you to know exactly the levels that would make these colours, but just to be able to pick out which of them must be which.

Now, let's move on to the third part of the lesson, the eye and seeing in colour.

You may know some of these things about the eye, but I'll give you a recap.

So we see things because light enters our eye, and here have a diagram of the eye in the middle, and it's from the side cut through.

So we have an eye that's looking to the left in that diagram.

And in the front of the eye, that dark circle is called the pupil.

And it's actually a hole that light goes through.

There is a covering, a transparent covering over that hole, as well.

And light comes in and it hits the lens, which is an adjustable part of your eye that makes the light focus on the retina.

Don't worry about knowing in detail how the lens works.

You don't need to for this, but it makes sure that a clear picture appears at the back of the eye.

So that back of the eye surface on the inside is called the retina.

And it's covered in special cells that detect light.

And these cells come in different types.

There are cells on the retina that detect colour, and they're called cone cells.

And there's three kinds of cone cells normally, one detects red light, one green, and one blue.

So we could call them the RG and B cone cells.

And this little diagram is showing roughly what they can actually detect.

In fact, there's a range of colours that each type of cone cell can detect, and they're a little bit overlapping.

So in fact, the cone cells that we call green detectors, they can actually detect yellow, green, and some blues.

And if the right colour falls onto a cone cell, it recognises that, and send a signal to the brain.

It's an electrical signal that goes along a nerve.

And if the wrong colour falls on a cone cell, it doesn't notice, it doesn't send a signal.

And if two or three different types of cone cell fire, they're switched on the send messages at once.

The brain combines the signals to make other colours.

So the colour we see depends on the levels of the RG and B signals our brain receives from the three cone cells.

It's able to say, "Oh, I'm receiving this much red and this much green." So this is a dark yellow for example.

And that's why we say that red, green, and blue are the primary colours of light.

They are the primary colours of light more or less for the human eye, because these are the colours our eyes detect, and we see all other colours as combinations of those.

The other ones are called rod cells, and they're also sensitive to light.

They detect light, but they detect the amount of light, the light level, also called the intensity that falls on them.

And they don't detect colour.

So they just tell you, this is bright or this is dim.

And cone cells, the colour sensitive cells, they don't detect colour very well in dim light.

They're great in bright light, but this means in dim light.

So at night for example, we often see mainly in shades of grey.

If you notice when it's quite dark and you look around, it's hard to tell what colour anything is anymore.

By the way, the reason they're called cone cells and rod cells is because of their shapes.

And now, a few short questions.

Where in the eye is light detected? Name the type of cell that detects brightness.

Name the type of cell that detects colour.

Press pause while you're thinking about those, and press play when you have your answers ready.

And it's at the retina, the back surface of the eye on the inside, that detects light.

The rod cells detect the brightness, and the cone cells detect colour.

Now, I've got a longer task for you to try.

Alex is watching a cartoon film on his phone, and suddenly, the whole phone screen goes a single, very bright colour, and it's emitting just red and blue light.

And there's a close-up on the right of what the pixels are doing on the screen.

I'd like you to say what colour the phone screen is showing now, and explain why Alex sees this colour, including what you now know about light sensitive cells in his eye and the signals they send to his brain.

So have a go at writing your answers to those.

Press pause while you're doing it, and press play when you're ready.

And here are example answers.

The colour on his screen is magenta, or at least it is, if the blue and red are a equal brightness.

Now, here are some things you could have said about why he sees this colour.

The screen gives out red and blue light that enters Alex's eye through the pupil, and hits the retina at the back of his eye.

The red light is absorbed by one kind of cone cell, and the blue light is absorbed by another.

Nerve signals for red and blue light are sent to his brain at the same time, his brain combines these, so he sees the colour magenta.

And because the red and blue lights are bright, the strong signals make it appear very bright.

So well done if you included most of those points, and especially if you used the scientific words, such as pupil and retina, absorbed, cone cell.

And now, we've come to the end of the lesson.

So here's a summary of what we've done.

All colours can be created using red, green, and blue light, the primary colours of light.

Mixing equal amounts of two primary colours creates a secondary colour.

All three in equal amounts make white.

When there is no light, we see black.

The retina on the back of the eye detects light.

Three types of cone cell detect either red, green, or blue light, and rod cells detect brightness.

Display screens are made of tiny red, green, and blue lights called pixels, which vary in brightness to make different colours.

So I hope you've really enjoyed learning about colours and how we see them, and I hope to see you again in a future lesson.

So bye for now.