Lesson video

Hello, my name's Dr. George, and this lesson is called "The Visible Spectrum.

" <v ->It's part of the unit "Making Images.

"</v> The outcome for the lesson is, "I can describe what white light is and explain why a visible spectrum can be produced from white light.

" So I'll be showing you how to do that.

Here are the keywords.

I'm not going to go through them all now because I'll introduce their meanings as we go along.

But this slide's here, in case you want to come back any time and check those meanings.

The lesson has three parts, and they're called white light, seeing a visible spectrum, and a second prism.

Let's get started on the first part.

As I'm sure you know, some light is colored and other light is white.

And in the home, you may have colored lights, but mostly you probably have white light, or very close to white light, perhaps a little bit yellow.

A light from the Sun is also white.

Now, if you ask children to draw a picture of the Sun, they'll very often color it yellow, but the Sun isn't yellow, and it's not orange or reddish either.

Although it may look that way sometimes.

If sunlight were yellow, that would mean if it's shone on something white, it would make that look yellow.

So if you put a piece of white paper in the Sun, it should look yellow, but it doesn't it looks white, as shown here.

So the Sun sometimes looks yellow or orange at Sunrise and Sunset.

The Sun's a star, it's not really changing its color every night and day.

It's because the Sun's light is passing through the atmosphere, it's passing through the air to reach us.

And when the angle of the Sun in the sky is low, the sunlight passes through more atmosphere to get to us, and that actually affects the colors that reach us.

But it's not the Sun itself that's changing color.

And when astronauts go into space and they're above the Earth's atmosphere, they can see that the Sun is actually white all the time.

So let's see if you were paying attention.

Two questions.

What color is sunlight?

What color is daylight?

Now, with short questions like these, I'll wait for five seconds, but you may need longer.

And if so, press pause, and then press play when you have your answers ready.

And the color of sunlight is A, it's white.

And the color of daylight, it's white.

So if in the home you want to have lights that make it feel like it's daylight, you're going to have white lights.

Now, some colors can be made by mixing other colors of light.

Have a look at the animation.

When the person receives red and green light, they see yellow.

Green and blue, they see a color called cyan.

And blue and red, they see a color called magenta.

So here we are, red plus green, you see yellow.

And if you see green and blue at the same time, you see cyan, this sort of turquoise, greenish-blue color.

And if you see blue and red light together, then you see magenta, this sort of purple-ish-pink color.

And if you want to see this happening for real with different colors of lights being mixed, you can press the button on the slideshow that says Watch here and you can see a video of that.

Now, if you mix red, green and blue, as the animation is showing now, what you see is white.

So if those three colors enter your eyes with equal brightnesses, you will think to yourself, "I am seeing white light.

" Now, a transparent triangular prism is an object you can use to show that white light from the Sun or from a light bulb is actually made up of all the colors of light that you would see in a rainbow.

A triangular prism is not simply a triangle, it's a 3D shape that has triangular faces on opposite ends.

It's not a pyramid.

And you need a transparent one, so made of glass or plastic.

And if you shine white light into it, what comes out is all of these colors, and it's the colors of the rainbow.

And we call this a color spectrum, this range of colors, or we can call it a visible light spectrum.

It's showing all the colors of visible light, light that we can see.

So when all colors of light that are in that visible spectrum enter your eye at the same time you register that as you are seeing white light.

But there's another way of seeing white light, and that's if just red, green and blue light enter your eyes at the same time.

So it doesn't need to be all colors of the spectrum.

If it's those three in equal brightnesses, you will be seeing white.

So we have these two ways that you can see white light, and you can't really tell the difference.

If you see white light, you don't know if it's made up of all the colors of the spectrum or if it's just red, green and blue, unless you use a triangular prism that's transparent to split that white light up and see what it's made up of.

So here's a question.

When white light passes through a prism, a color spectrum of light appears.

Which statement explains why?

So read the three statements carefully and pause while you're thinking about it and press play when you have your answer ready.

The correct answer is the prism splits the white light up into the different colors it's made from.

So the prism doesn't make the colors or add the colors, it just splits up the colors that are already there.

And rainbows are caused in a similar way.

And what happens with a rainbow is that when there's drops of water in the air, so when it's raining, white light from the Sun hits the raindrops and it gets split into all the colors of the visible spectrum by the raindrops.

There's a diagram here, it shows more detail than I would expect you to need to know, but you might be interested to see it.

So we have white light going into the raindrop, changing direction a bit there.

It's bouncing off the back, reflecting off the back of the raindrop, and the colors are being split, and then all the colors come out.

And with all raindrops doing that together, the effect combines to show you a rainbow.

Now, there's a kind of tradition of saying that there are seven colors in a rainbow, seven colors in the visible light spectrum, but really it just depends on the way you look at it.

You might say, "Well, indigo and violet both look the same to me.

They're just sort of a bit purple, and I can see turquoise in between green and blue.

" So it doesn't really split neatly into seven colors.

It's just a sort of tradition that we name them that way.

In reality, the spectrum is a continuous range of colors.

And a particular area that isn't obviously three separate colors is blue, indigo and violet.

If you look at a rainbow, it can really be hard to tell the difference.

But these colors, they always appear in the same order, as shown here, whether they're in a rainbow or whether you've split them using a triangular prism.

And the reason these colors get split in this order is because of their slightly different properties.

They all travel in slightly different directions when they enter a prism or a raindrop, and that's actually what makes them come out in slightly different directions and split up.

Now, can you remember those traditional colors of the visible light spectrum in the order that they appear?

Start with red, and pause while you think about it, and press play when you're ready.

And here's the answer, red, orange, yellow, green, blue, indigo, violet.

And if you want to memorize those and you didn't remember them, there are some different tricks that you can use.

One is to look at the first letters of these words and see that they spell something that sounds a bit like a person's name, ROY G BIV.

Some people remember it that way.

So as we've said, the visible light spectrum shows the range of colors light can have, but what about these colors that aren't there, but that we do see sometimes?

So white isn't in the rainbow, for example, and nor is magenta, this color shown here.

You won't find that in the rainbow.

And it's because these colors can only be created by mixing different colors of light together from this spectrum.

So blue and red together makes you see magenta, and that just doesn't happen in the spectrum, it doesn't happen in a rainbow.

Red is at one end, blue is at the other end, you see them separately.

So an individual light can't actually be magenta, but we can see magenta if blue and red lights are shone into our eyes at the same time, as shown here.

Now, can you match each of these lamps to the numbers of colors of light it emits?

So each of these lamps, one, two, three, match them with one of A, B, or C.

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

And here are the correct answers.

Magenta is emitting two colors of light.

Magenta is what you see when both blue light and red light enter your eyes at the same time.

Red is just one color of light.

It's there in the rainbow, it doesn't need to be created by mixing other colors.

In fact, red is what we call a primary color of light.

And white is all of the colors mixed together.

Although there is an alternative answer here, this white light could be created by just a mixture of red, green and blue light, as we saw earlier.

And in fact, you can buy lamps like this where you can change the color on them.

And the way it works is that, inside, they have red, green and blue lights.

And as you choose your color, it changes the brightness of those three lights, red, green and blue, to change what your eyes see.

Now, another question, Which is the most accurate description of white light out of these four?

So read them all, and decide which one works best.

Pause when you think about it and press play when you're ready.

And the best description here is that it's a mixture of different colors of light.

Pure light, what does that really mean?

Pure light actually makes it sound like perhaps it's just one single color of light, and it isn't, it's a mixture.

Light that has no color, no, it's a mixture of all the colors, and white really is a color itself.

And it's certainly not an absence of light, it's not light not being there.

It is light, it's just a mixture.

Another task for you.

And for this, you're going to need a ray box, a slit and a transparent triangular prism.

And you're going to use them to create a visible light spectrum, something that looks rather like the rainbow.

The ray box and the slit are just there to give you a beam of white light.

And then you're going to have to try different positions, different angles for the prism, and try to see if you can get a spectrum landing on a screen, or a white wall, or a piece of white paper.

When you've done that, then I'd like you to predict what you think you'll see if red light is shone into the prism instead of white light.

Will there be a spectrum?

And explain why you think that.

Then watch a demonstration of this.

It could be in real life, or it could be on a video, or perhaps you'll even get to try it yourself.

And then decide were your prediction and explanation correct?

And do you need to write an improved explanation?

And if so, do that.

So have a go at that, press pause while you're doing it, and press play when you're ready, and I'll show you some example answers.

And here are some answers.

So a suitable prediction, "The beam of red light will not produce a color spectrum and it won't change color either.

The beam of red light will follow the same path through the prism as the red light from the white beam did.

" So that would be a reasonable prediction.

And then why might you think that?

Well, "Triangular prisms don't change the color of light.

They can split up a beam of light into the different colors of light that make up the beam, but red light is a single color of light, not a mixture of colors, so no separation occurs.

Prisms change the direction of each color of light by a slightly different angle," by the way.

So you won't have seen a spectrum with red light because the prism splits light into its colors and there is only one color here.

So well done if you correctly predicted that.

And now let's go on to the second part of the lesson, seeing a visible spectrum.

First, a recap of how we see, how our eye works.

So we see when light enters our eye, and that light passes through the pupil at the front, it goes through the lens, and then it lands on the back of the eye, and there's a surface there on the inside called the retina.

And there are cells on the retina that absorb and detect the light.

Some of those cells are a type called rod cells, and they sense brightness, how bright or how dim is it?

And other cells are called cone cells, and they sense color.

We're going to think about these cone cells and there are three types.

One that detects mainly red light, which we can call the R type of cone cell, one that detects mainly green light, which we'll call G, and one that detects mostly blue light, and we'll call it B.

Now, have a think about this.

Does each of these statements apply to rod cells, cone cells, or both?

Choose for each of those.

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

And the answers are these.

Cone cells are the ones that detect color.

Rod cells are the ones that detect just brightness.

There are three types of cone cells.

And which of these cells are found on the retina?

Both, both types.

Now, if the right color of light falls on a cone cell, that would be a color that it's sensitive to, then that cell sends a nerve signal, an electrical signal to the brain along a nerve, and that makes you see color.

And each type of cone cell is actually sensitive to a range of colors, roughly like this.

These colors trigger the R cone cells.

These are the ones that R detects, notices and sends signals to the brain about.

These colors trigger the G cone cells, and these trigger the B cone cells.

So you can see, for example, the G cells, they're not just detecting green, they can detect some blue and yellow as well.

People who have a color vision deficiency, which is often called colorblindness, they either have only two kinds of cone cells, so they're missing one of them on their retina, or it can be that one of the kinds of cone cell doesn't work properly, doesn't quite send the right signals to the brain.

And most commonly, it would be the G cells, that the most common kind of colorblindness is where people's G cells are not functioning normally.

And this would be about one in 24 people.

So colorblindness is fairly common.

Now, if red light comes into your eyes, it mostly only triggers the R cone cells, and green light, the G cone cells, and blue light, the B cone cells.

And what if more than one kind of cone cell fires at once?

Well, then the brain combines the signals to work out what color might be coming in.

So the color we see depends on how much the R and G and B cells are being triggered.

How strong are the signals coming from each of those three types of cell?

And that's how the brain works out what's coming into the eyes.

If pure yellow light comes into your eyes, and that does exist because it's in the visible spectrum, it triggers both the R and the G cone cells at the same time.

They're both more or less sensitive to yellow light.

And this the brain interprets as yellow.

It says, "I know this is yellow because yellow light would trigger the R and the G cells in this way, and not the B Cells.

But we can mimic yellow light, we can fake it if we send to the eye equal amounts of red and green light because that would trigger the R and G cone cells in the same way that yellow light does.

So we would also say to ourselves, this is yellow light.

So there are two ways that we could see or think we're seeing yellow light.

One is actual pure yellow-colored light, as you see in the rainbow, and the other is equal amounts of red and green light.

So we can't see the difference.

And now something for you to do, there's a short paragraph here, and I'd like you to fill the two gaps using any of the letters R, G and B.

Look at the picture on the right.

This person is seeing the color cyan, as shown.

What are the two missing letters here?

Press pause and press play when you've decided.

These are the correct answers.

Pure cyan light, a single color of light from the spectrum, triggers both the G and the B cone cells at the same time.

This causes us to see cyan.

And you can see here that cyan just about overlaps with what the B cells can detect,, and it also is in the range of colors that the G cells can detect.

So our brain recognizes that combination of signals from the G and B cells, and thinks, "I'm seeing cyan light.

" And now, another sentence with a couple of gaps for you to fill.

Press pause and press play when you're ready.

And here are the correct answers.

So it should say the effect is the same as if equal amounts of green and blue light entered our eyes.

So green and blue light trigger the G and B cone cells in the same way that cyan does.

And we can't tell the difference, we think we're seeing cyan light.

So well done if you've got that.

This is something you may find a little bit tricky, but I hope you find it really interesting as well.

This sort of thing is the reason why screens on televisions, phones, computers are able to make us see all the different colors.

They simply have tiny red, green and blue lights in them.

And by sending out different amounts of those three colors, they can make us think we're seeing light of any different color from the spectrum, and some colors that aren't in the spectrum too.

So I have a longer task for you.

It's a written task this time.

We could produce a visible spectrum on a screen using sunlight.

So you used a ray box to make a beam of white light, but you could take a beam of sunlight and do a similar thing.

Have a look at this spectrum.

It's got some letter labels on it.

I want you to fill in the table, describing some aspects of what is happening at each of those letters.

And then a question for you afterwards to explain why no part of the screen appears magenta, that purplish-pink color shown here.

Take as long as you need, press pause while you're doing this and press play when you're ready.

Let's take a look at the answers.

First of all, it asks you to describe the color of light that's hitting the screen and reflects back into your eyes so you can see it.

And from A to E, those are red, yellow, green, cyan and blue.

If you said orange for B, well, it's close enough.

And if you said indigo or violet for E, that's close enough.

Blue, indigo and violet do all look a bit similar.

Now the cone cells that are triggered.

Red is just triggering the R cone cells.

Yellow, the R and the G.

Green, just the G.

Cyan, the G and the B.

And blue, just the B.

And the color the point appears to be is the color that you described that's hitting the screen.

These are the colors we see.

Now, why do we not see magenta in the spectrum?

Well, a way that you could explain that is to say, "No part of the screen is reflecting both red and blue light into your eyes to trigger the R plus B cone cells together, which is how magenta is seen.

" So the left-hand side of the screen is reflecting red light.

The right-hand side of the screen, or close to the right-hand side, is reflecting blue light.

So we're receiving those from separate places.

There's nowhere here that is reflecting both blue and red light to make us see magenta.

So well done if you've got a lot of this right.

And now for the final part of this lesson, this is quite a short part, called a second prism.

Imagine a beam of red light incident, that's coming into a transparent triangular prism.

And when that happens, it actually changes direction, as shown here.

And what if we have a second triangular prism and we place it upside down like this?

Well, it actually can reverse the changes in direction.

So we get something like this.

So the light that's coming out is traveling in the same direction.

And this would happen for any beam of any single color of light that you put in if you have two prisms like this.

But with different colors, there'd be slightly different angles.

Now, which of these diagrams shows how two prisms would redirect, change the direction of, a beam of violet light?

So think about that.

Press pause if you need longer than five seconds and press play when you are ready.

And the correct one is A, did you realize that that's the one where the beam is coming out in the same direction as when it went in?

So that's what would happen.

Now, I'd like you to answer a few questions, writing down your answers.

So predict what you think you'll see when the second prism is added as shown.

Now we have white light going into the first prism.

You can see it split into its colors.

What do you think will happen when it goes through the second prism?

And then explain why that's what you think.

Then I'd like to watch a demonstration of this either in real life or it could be on a video.

And then think about whether your prediction was correct and was your explanation correct?

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

And pause this video for as long as you need while you're doing that, press play when you're ready and I'll show you some example answers.

Now let's take a look at some answers.

So a sensible prediction would be something like, "The second prism will make the visible light spectrum disappear and turn into a beam of white light traveling in the original direction again.

" And why might you expect that to happen?

Well, here's an explanation.

"The second prism reverses the changes in direction that happens to each color of light.

So all the colors of light are no longer separated out.

They all end up traveling in the same direction together again.

A mixture of all the colors of light looks white, and that's why it turned back into a white beam.

" So you may have not written your answer in exactly the same way as that, but well done if you predicted that the light would come out white and had an idea of why.

And now we're at the end of the lesson.

So here's a summary for you.

White light from the Sun and many light bulbs consists of light of all the colors in the visible light spectrum or rainbow.

The transparent triangular prism can split up sunlight into a visible light spectrum.

The visible light spectrum shows the range of colors that light can have.

There are three kinds of cone cells in the eye that detect color, R, G and B, and each is sensitive to a range of colors of light.

The colors we see depends on which cone cells are triggered to send a signal to the brain.

So well done for working through the lesson.

I hope you found it interesting.

And next time you watch TV, perhaps think about the fact that your television is fooling you into thinking that it's making many different colors of light.

Really, it's only producing red, green and blue.

I hope to see you again in a future lesson.

So bye for now.