Lesson video

Hello, welcome to this lesson called Light and Colour.

My name's Mr. Norris.

This is from the unit called Electromagnetic Waves.

Now colour is a really interesting thing.

Have a look at these three blocks in the front row.

How many of them, if any, do you think are black? Take a moment now to have a good look and decide which one or which ones, if any, do you think are black.

I'm gonna show you now, so make sure you've decided which ones you think or which one you think is black.

There we go, now you might have got that right, or you might not, but the important thing is, I'm sure it was a bit tricky to decide.

Now what about these two remaining blocks? What colours do you think they are? Are they the same colour or are they different colours? Take a moment now to decide which colours, if you had to put a colour for each one, which colour do you think each one is? Okay, make sure you decided, so I want to show you again.

Here we go.

Now if you did get right that right, that's either very impressive or you're very lucky.

But the important thing here is that what colour something is and what colour something looks like actually can depend on a number of different factors.

And that's what we're gonna be exploring this lesson.

The outcome of this lesson is hopefully by the end of the lesson you'll be able to describe what's different about lighter different colours, and you'll be able to explain the apparent colour of objects when it's illuminated with different colours of light.

Here are some key words that will come up this lesson.

Electromagnetic wave, frequency, wavelength, refract, and opaque.

Each word will be explained as it comes up in the lesson.

This lesson is divided into three sections.

In the first section of the lesson, we'll look at the different colours of light, in the second section we'll look at the colour of opaque objects, and in the final section of the lesson we'll look at the effect of colour filters like I showed you in that introduction.

Let's get going with the first section.

So light can be thought of as a wave, and waves occur when something is disturbed or made to oscillate, and then the disturbance or oscillation travels.

So like this rope here is being disturbed and made to oscillate and the disturbance or oscillation travels down the rope.

But light waves are electromagnetic waves, so they're similar to those waves on rope or waves on a string but they're different because they're not oscillations of a rope and they're not oscillations of an object, they're oscillations in or ripples in invisible electric and magnetic fields that are all around us.

So what do light sources, things that give out light? What do they actually do? Well, they disturb nearby electric magnetic fields a bit like what was disturbing that rope, except this is done to invisible electric and magnetic fields.

So the ripples from a light source, the ripples in electric and magnetic fields travel outwards transferring energy, and that's what light is.

Let's just go over some ideas about waves.

So the frequency of a wave is the number of oscillations that are happening per second, and that also gives the number of waves that arrive at a point per second.

So that's the frequency which is measured in hertz, capital H, little Z for hertz.

And then the amplitude of a wave is the maximum displacement of a wave from the undisturbed position and the wavelength of a wave which has given that Greek symbol, lambda, that is the distance between one point on a wave and the same point on the next wave.

So I've gone peak to peak here, but it doesn't have to be peak to peak.

It could be trough or trough or any point on a wave and the distance between the same point on the next wave.

So the waves in that animation were transverse because the oscillations were up and down, but the waves were going along.

So the proper definition of that is you can't just say up and down and along, you've got to give that angle the oscillations at 90 degrees to the direction of energy transfer.

So electromagnetic waves like light waves, they are transverse oscillations in electric and magnetic field.

So the direction of oscillation is 90 degrees to the direction of energy transfer.

And what's important to remember is electromagnetic waves, can have any frequency, and that's what the different colours of light are.

The different colours of light are just electromagnetic waves with different frequencies.

You can see in the diagram there's a visible light spectrum or a rainbow, so if it's called a visible light spectrum with a little frequency scale, that's rough, it might not be quite exact, but it's been drawn on to indicate the different frequencies that the different colours of light have.

So a rainbow pattern of coloured light like this is called a visible light spectrum.

We need to get used to using that word spectrum, but of course the word spectrum just means a continuous range or scale.

If something is on a spectrum or there's like a spectrum, then that just means there's a range to choose from, and it might be a continuous range or scale.

So a visible light spectrum shows the full range of colours, frequencies that visible light waves can have, it shows the full spectrum, all the possible frequencies of visible light.

So let's do a check on that.

Rainbows are said to contain seven colours traditionally, but in reality there's a continuous spectrum, there's a continuous range of colours.

So can you have a go at these two questions just to check your understanding of what we've just said so far? Pause the video now and then I'll see you once you've had a go at answering these two.

Right, I'll give you some feedback now.

So the order of the seven colours of the rainbow kind of traditionally are red, orange, yellow, green, blue, indigo violet.

And which colour of light has the lowest frequency and which has the highest or the lowest frequency is red light and the highest frequency in the spectrum if we go for the seven colours of light in the rainbow is violet light.

Well done if you've got that.

Now let's talk about wavelength briefly.

The wavelength of any wave is set by the wave equation.

Wave speed equals the frequency times the wavelength, but of course in air or a vacuum, all electromagnetic waves, so all light waves have the same speed.

That's 300 million metres per second or three times 10 to the eight metres per second.

So what that means, if frequency times wavelength always gives the same speed, then if waves have a higher frequency, they must have a lower wavelength to multiply to give the same speed.

So the bigger one gets the smaller, the other one gets by the same proportion, they're inversely proportional frequency and wavelength for electromagnetic waves.

So when one increases, the other decreases to the inverse proportion.

So for example, a light wave would double, the frequency would have half the wavelength.

Look at the diagram and make sure you're clear that the frequency scale runs upwards from red to blue, but the wavelength scale gets shorter from red to blue.

Okay, let's do a check of what we've just said.

So starting at red and moving through that spectrum to violet, in what order do the different colours of light appear? Choose the ones you think, off you go.

Okay, I'll give you some feedback now.

So starting at red and moving through the colour spectrum to violet, the colours appear in order of increasing frequency 'cause red light has the lowest frequency and violet light has the highest frequency.

The speed in air is the same for all electromagnetic waves for all colour of lights, so it's not c or d, but it's also order of decreasing wavelength 'cause red light has the longest wavelength and violet light has the shortest wavelength.

Well done if you've got both of those.

So the sun and standard light bulbs give out white light, but white light is really a mixture of all the different frequencies, all the different colours of visible light.

So white light is really all the colours of light combined, and this can be proved by passing white light through a triangular glass prism to produce a visible light spectrum like we've just seen.

So there's some white light going into this prism, and a prism will split up the white light into a visible light spectrum like that.

That effect is called dispersion.

You can say the white light has been dispersed into the different colours.

And just to prove that white light really is made up of the different colours of light, a second prism can then recombine the separated colours back white light like this.

There's a second prism and it combines the colours back into white light.

And that experiment was first performed and explained by Isaac Newton in about the year 1670.

So dispersion occurs because different frequencies of light have different speeds in some materials, and that's why they refract through different angles.

Refraction also changes the wavelength of the waves, but not the frequency.

And the frequency, remember links to the colour.

So this is why we've always been talking about colours of lights having different frequencies because the frequency of a colour is set, the wavelength of that coloured light wave can change in different materials, but the frequency of a certain colour never changes.

So it's the frequency that links to the colour and wavelength of that wave can change depending on the material.

So higher frequency light waves like violet for example, they refract more because they travel slower in a medium like glass, that's what makes them refract more.

Now in a rectangular glass block, this effect of dispersion is barely noticeable.

It does happen, but it's barely noticeable because the rays leave the block parallel and very close together.

When rays are refracted, as they enter the block, they're refracted through a certain angle and when rays leave the block, they're refracted back the same angle.

So whatever angle, each coloured ray or each colour of light is refracted on the way into the block, on the way out to the block is refracted back exactly the right amount, so all the rays are still parallel again when they leave the block.

So that's where the effect is barely noticeable in a rectangular block.

Whereas in a triangular prism rays are refracted twice in the same direction because of the shape of the prism.

So that means the different colours leave the block at different angles and spread apart, and that's how you can end up with colour spectrum on the other side of that triangular glass prism.

Okay, let's do a check on what we've just said, which best explains this effect.

Pause the video now and choose which option you think.

Okay, the option that best explains this effect is option c.

The prism splits up the white light into the different colours that the white light's made from.

It's not option d, the red light does refract less and the violet light does refract more, but the white light didn't change colour to red and violet, the red and violet was already there just in the white light ray, and the prism split up the those different colours.

So well done if you said c.

Okay, time to do a task on this section of the lesson.

Part one of the task is to add labels to the diagram to show which rays represent white slides, which rays represent red lights, and which ray represents violet lights.

And then part two is a gap fill task, but you have to choose for each gap between the terms frequency, wavelength, or wave speed.

Pause the video and have a go at both parts of this task.

Here's some feedback now, for part one of the task, white light enters the prism and it's split into red light and violet light with red light being refracted slightly less because it's lower frequency and violet light being refracted slightly more because it's high frequency.

For part two of the task, I'll go through it now.

When light enters a glass block from air, refraction can happen because the wave speed changes, the wavelength of the wave also changes, but the frequency stays the same.

The frequency of a visible light wave is what sets the colour and violet light refracts more than red light because violet light has a greater frequency and the wave speed decreases more when it enters glass.

Well done if you've got most of that right.

So this takes us to the second part of the lesson, which is about the colour of opaque objects.

What is it that makes an object the colour it appears in normal conditions? So when waves are instant on a surface, the waves can be partly or totally reflected, transmitted or absorbed.

Transparent materials transmit most instant light in a regular way, so you can see through with transparent material.

And the diagram shows an example of that.

Translucent materials also transmit light, but there might be significant scattering or absorption, perhaps different surfaces within the material, so you cannot see through a translucent material like you can see through a transparent material, even though it still transmits the light.

The diagram represents what the scattering of light at internal surfaces in a translucent material might look like.

Whereas opaque objects transmit no incident light, all of the lights that is incident on an opaque object is either reflected or absorbed.

So wavelengths or frequencies that are not reflected are absorbed, and the colour of an opaque object depends on which wavelengths of light, or which frequencies of light are most strongly reflected.

So for example, leaves are green because they most strongly reflect the green light, the red light, and the blue and violet light tends to be absorbed by leaves.

So they look green because it's only green light entering your eye from the leaf.

It might be worth thinking about for a moment what colour or colours of light green plants grow best in because it's not green.

Because these plants don't really absorb green frequencies of light, they reflect green frequencies of light, that's why they look green.

These plants leaves absorb red light and blue light, so many green plants actually grow best in red light or blue light rather than green light.

So red surfaces reflect red light and the other wavelengths in the colour spectrum are absorbed, blue surfaces reflect blue light, and all the other wavelengths apart from blue in the visible colour spectrum are absorbed.

And this is going to work for any coloured surface that is a colour that's found in the visible light spectrum.

A surface which is a colour that's found in the visible light spectrum is going to predominantly reflect that wavelength of light and absorb the other colours from the visible light spectrum.

White surfaces reflect all colours of light equally, so when you look at that surface, all frequencies, all wavelength of light are entering your eye at the same time and that will appear as white.

Whereas black surfaces reflect no colours of light, so something looks black if no light from it is coming from it to your eyes.

So black surfaces will absorb all wavelengths of light in the visible spectrum.

Let's do a check on what we've just said.

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

Which of the following statements are correct? Identify, which you think.

Pause the video now and have a go at that.

So I'll give you some feedback now.

Statement a, the shirt adds a blue colour to light from the sun, that's not how colours work.

Statement b is correct though, the shirt reflects blue light from the sun, that's a correct option.

Statement c, the shirt changes the colour of light from the sun to blue, that's not correct.

Light from the sun already contains blue light, and the blue shirt just reflects that blue light and absorbs other wavelengths of light.

The shirt certainly doesn't give out blue light, it's not glowing, it's only reflecting the sun's light, and the shorts also reflect blue light from the sun but of course they don't just reflect blue light, they're white shorts and they reflect all wave lengths of light from the sun including blue.

So well done if you said the b and e were the two correct statements, and all the others are not correct.

So different colours of light can combine to make other colours of light.

Now the colours of light mix in a different way to how paints mix.

So we'll talk through this now.

So red light and green light actually makes yellow light, green light and blue light makes a kind of light blue colour that's called cyan light.

Blue light and red light makes magenta, the colour magenta looks that kind of deep, kind of vivid pink colour, but it's proper name when it's made from equal intensities of bright blue light and bright red light is called magenta light.

This animation shows that.

So we've just shown green light being mixed into red and we get yellow, turn the red light down and we just got green light, turn the blue light up, we've got green and blue making cyan lights, turn the green light down and that's just the blue light and we're just gonna add some red in, and that's magenta lights.

And if we add the green into the red and the blue, then we get white light.

So it turns out that any colour could be created for mixing red, green and blue light at different intensities.

That's why red light, green light and blue light or red, green and blue are called the primary colours of light.

Now of course that's different to the primary colours of paint, which you might think of as red, blue, and yellow.

Primary colours of light are red, blue and green, or red, green and blue, RGB, the red, green, and blue.

So have a look at this picture which shows a phone screen viewed under a microscope.

And you can see through the microscope that the screen consists of thousands of tiny lights, but they're only red, green and blue tiny lights.

And that's for exactly the reason we just said, it's because any colour can actually be created on the screen by varying the amount of red, green and blue light created by the screen.

And that gives the effect of a whole new colour when that light from the screen enters your eyes kind of all mixed together.

So cyan, magenta and yellow are called the secondary colours of light.

They're made from mixing two of the primary colours in equal proportions.

And if you mix all three primary colours of light together, red, green, and blue in equal proportions, that makes white light like the centre of that diagram where all three spotlights, red, green, and blue overlap, you get white light.

Okay, let's do a check of what we've just gone through about mixing different colours of light.

Give the names of the missing colours.

There's a question mark in each part of this check, which colour replaces each question mark? Pause the video now and make sure you've got an answer for each one.

Right, I'll go through the answers now.

Red and what makes yellow the answer is green.

Red light and green light makes yellow light.

Number two, blue light and red light makes what colour of light? The answer is magenta, that deep pink colour.

And then part three of this question, what colour light and green light makes cyan? The answer is blue light.

Well done if you've got this.

Let's go into a bit more detail now about how these colours are perceived by human eyes.

So human eyes contain three kinds of colour detecting cone cells at the back of the eye, and each kind of cone cell at the back of the eye absorbs and detects not just one wavelength but a different range of wavelengths which roughly correspond to red light or red range of wavelengths, green light or a green range of wavelengths and blue light or a blue range of wavelengths.

So each kind of cell, the red detecting cells, green detecting cells and blue detecting cone cells, they send an electrical signal to the brain when it absorbs light in its wavelength range.

And if more than one kind of cone flies at once, we don't experience both colours together.

The brain interprets this as other colours, which is amazing.

So the colour that we see in our vision depends on the levels of the red, green and blue signals that our brains receives from the red, green, and blue cone cells at the back of our eyes.

So what that means is we can actually represent white light using just red, green and blue rays to represent the three wavelength ranges that stimulate the three kinds of cone cells, that's done on the next slide.

So surfaces can reflect more than one colour of light, for example, a yellow surface, effectively they reflect red light and green light.

So the red range of wavelengths and the green range of wavelengths equally, but they're gonna absorb blue.

So when our red cone cells and our green cone cells get stimulated equally, we see that surface as yellow.

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

So that is gonna stimulate our red cone cells and blue cone cells at the back of our eyes equally, and we see that surface as magenta.

And an application of this is in printers, because printing ink, if you do colour printing, the ink in the printer often isn't red, green and blue.

It's often scion, magenta and yellow ink.

So how can you create any colour you want on a printed page using those colour inks? Well, for example, you can create red by printing yellow and magenta together 'cause the yellow will absorb the blue and the magenta will absorb the green, so only the red is reflected from both.

So if you print magenta ink on top of yellow ink, that could look red if it's done in the right way, which is what printers do.

Now, if light does not contain the colours or frequencies of light that do reflect off that surface, then no light reflects off that surface and the surface looks black.

For example, here we've got blue light hitting a green surface, green surface only reflects the green range of wavelengths, so we've only got blue light, so there's no green light there to reflect, so no light reflects from that surface in this situation if it's only illuminated with blue light, so it's gonna look black.

And the same for this situation.

We've got a red surface that's only gonna reflect the red range of wavelength, but it's illuminated with cyan lights, which only contains green and blue wavelengths, there's no red wavelengths to reflect, so that surface will look black.

And say for this situation, a magenta surface would only reflect the red and blue range of ranges of wavelengths, but it's only been illuminated with green light so none of the light that is on it gets reflected and it's gonna look black to this observer.

Let's do a check on what we've just been talking about.

A blue cube is illuminated in only red light, what colour does the cube appear? Blue, red, magenta or black, a, b, c, or d.

Make a decision now.

And though you've made your choice, choose which one of these gives the best reason for your choice.

Pause video now and choose.

Make sure you've chosen a, b, c, or d for part one and a, b, c, or d for part two as well.

Right, I'll go through the correct answer now.

That cube would appear black, and the best reason it will appear black is because blue surfaces do not reflect red light, and there's only red light hitting the cube, there's no blue light hitting the cubes, there's no light that could reflect from that cube, therefore it will appear black.

Well then if you've got those right.

Okay, here is Alex's costume for the school show as seen in white light.

So kind of the true colours of each part of the costume.

He's got a white hat, a red T-shirt with a green circle and magenta sleeves and a blue robe.

But this is what his costume would look like under different coloured spotlights on the school stage.

Under a red spotlight, only the parts of his costume that reflect red light will look red, and there's no other colours of light to reflect so all of the surfaces that don't reflect red light will look black.

And this is what he would look like under green light.

All of the parts of his costume that reflect green light will appear green, and all of the parts of his costume that don't reflect green lights will look black.

And this is what Alex's costume would look like under a cyan spotlight.

Cyan light is made up of blue light and green light.

So the hat of his costume reflects both the blue light and the green light so it looks cyan.

But the magenta sleeves only reflect the blue light out of blue and green 'cause magenta reflects blue and red, but there is no red, so there's only the blue that's reflected from his sleeves.

The circle on his shirt only reflects the green range of wavelengths, so that's still gonna look green.

And the robe reflects the blue range of wavelengths.

So you can get the idea of the appearance of colours can change if it's illuminated with different coloured lights.

You have a go at this one now.

What colours will Alex's costume appear under blue stage lighting? Choose which you think out of a, b, c, or d.

The correct answer here is c.

The white hat reflects the blue light that he's illuminated with as do the magenta sleeves and the blue robe.

But the green circle and the red T-shirt don't reflect blue light so they look black, that's why the answer is c.

And there's one more of these for you to do.

What colours will Alex's costume appear under magenta stage lighting? Choose from a, b, c, or d.

Okay, here the correct answer is d.

The hat is the giveaway really because white reflects all colours of light so the hat must look magenta because it reflects the red light and the blue light that make up magenta.

The sleeves reflect both the red lights and the blue lights as well and the red light and the blue light is reflected by the red T-shirt and the blue robe, and the green circle doesn't reflect the red light or the blue light so looks black.

Well done if you said D.

Okay, time for a task.

Izzy's costume for the school show is shown in white light.

She's got a blank hat, blue T-shirts with white triangle and green sleeves and a cyan robe.

Put ticks in the table to show which parts reflect red light, green light and blue light using the columns headed R, G and B for each colour of light.

And part two of the task then predict the colour that each part will appear in blue light and then in yellow light.

Pause the video now, have a good go both parts of that task.

Right, well done for your effort, I'll go through the answers now.

So for each part of Izzy's costume, what colours of light does it reflect? Well, the hat is black, so it reflects no colours of light in any situation.

The T-shirt body is blue, so that only reflects the blue light, the triangle is white, so that reflects red, green and blue light, the sleeves are green, they only reflect green light, the robe is cyan, so it reflects green light and blue light.

So that table will be really helpful for part two of the task.

So in part two of the task, firstly, Izzy is illuminated in blue light, so only the parts of her costume, which reflect blue light will look blue.

So that's the T-shirt body, the triangle and the robe, everything else will look black.

Now in yellow light, Izzy's hat is still gonna look black because that doesn't reflect any of the lights that's instant on it.

Now the yellow light is made up of red light and green light, but the T-shirt only reflects blue light and there is no blue light, so the T-shirt is also gonna look black.

The triangle is white, so it's gonna reflect all the light that lands on it, so that is red light and green light so it's gonna look yellow the same as the light.

The sleeves will reflect the green light and the robe, will also reflect green light, but there's no blue light on it and it won't reflect the red light, so it is going to look green.

Very well done if you got those right.

So that takes us to the last section of the lesson, which is on colour filters.

So objects that transmit light often only transmit some wavelengths, some colours of light, the other wavelength are absorbed, and that's how coloured filters work.

So you can see in the diagram, the red filter will only transmit the red wavelength of light, and it will absorb the green and the blue.

And the colour of a filter depends on the wavelengths of light that it transmits through to our eyes.

A secondary colour filter that could be yellow, magenta, or cyan, that could transmit two wavelength ranges.

For example, a magenta filter can transmit both red and blue light and only absorb the green range of wavelengths.

Now, perception of colour by the human brain is actually really complex, it can be affected by many factors including our expectations.

So look at the snooker table and now I've covered it with a red filter, only wavelength and a range close to red are transmitted.

Does the yellow ball still appear a shade of yellow or is it now a shade of red? Take a moment to decide.

Now if I take away the filter, those lines linking to each ball show the colour that each ball really was through that filter.

So that yellow ball really wasn't yellow anymore, but you might have felt it still looked yellow because that's what your brain felt it should be because that's our expectation about that ball in that position on a snooker table.

But it was actually that red colour when viewed through the red filter.

And colour illusions can be really interesting.

Are the stripes in the middle, black or gold? What about now? So there's the first option, there's the second option.

So actually perception of colour is a very complicated thing.

Let's do a quick check about colour filters, this should be fairly straightforward.

A white spotlight is covered with a red filter and shown towards Alex, which of the following explains the effect of the red filter? Pause the video and choose a, b, c, or d.

How does the red filter work? What does it do? Which option? Pause the video now, off you go.

Okay, the correct answer is c.

A red filter only transmits red light from the white spotlight.

That's all it does, simple.

Okay, here is the final task of this lesson, there are two parts.

Part one, a white spotlight is covered with a red filter and shone towards Alex.

Explain what colour each part of Alex costume will appear to be, and part two, the red filter is replaced with a magenta filter.

And Jun says a magenta filter will have the same effect as a red filter on top of a blue filter, is Jun correct? You need to decide yes or no and then explain your answer.

Pause the video now and complete both parts of that task with your best effort.

Off you go.

Okay, I'm gonna give you some feedback now, part one, your answer should include these ideas.

A red filter only transmits red light to Alex, the hat, t-shirt and sleeves all reflect red light so they will appear red, the circle and the robe will appear black as these do reflect red light, they absorb it.

And for part two of the task, actually Jun is not correct.

A magenta filter transmits both red and blue light, but when a blue filter is placed over a white light, only blue light would be transmitted through that.

So as a red filter can only transmit red light, if you placed a red filter, then on top of the blue one, no light will be transmitted.

Because a red filter can only transmit red light, a blue filter only transmits blue light through to the red filter, which then won't transmit that blue light, therefore absorbing all light.

Whereas the magenta filter, of course, would transmit both red and blue light.

Well done if you work that out.

Here's a summary of the lesson.

Light waves are oscillations or ripples in invisible electric and magnetic fields that are all around us called electromagnetic waves.

The different colours of light are just electromagnetic waves with different frequencies.

Violet light has the highest frequency, and the shortest wavelength.

Violet light is slower in glass, so refracts through greater angles in glass.

White light is a mixture of all wavelengths in equal intensities.

Transparent and translucent objects transmit most instant light, whereas opaque objects transmit no incident light.

And the colour of an opaque object depends on which wavelengths of the instant lights are reflected.

And colour filters only transmit certain wavelengths absorbing the others.