Lesson video

Hello, and welcome to this lesson called The Spectrum of Electromagnetic Waves.

This is from the unit called Electromagnetic Waves, and my name's Mr. Norris.

So people have always been fascinated to see rainbows in the sky or perhaps a rainbow in the mist caused by a waterfall.

And we now know of course, that a rainbow is caused by the dispersion of white light, white light being split up by droplets of water into the different frequencies, the different colours that make up the white light.

But what we're actually going to do today is we're going to start by looking at rainbows, but then we're going to go beyond the rainbow and we're going to look at all the different possible frequencies that light can have.

So let's get going.

Here's the outcome for today's lesson.

Hopefully by the end of today's lesson you'll be able to explain what is meant by the spectrum of electromagnetic waves.

Here are some keywords that will come up this lesson, electromagnetic waves, frequency, spectrum, refraction and radiation.

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

This lesson has two sections.

In the first section, we're going to look at the visible light spectrum, and in the second section of the lesson we're going to go beyond the visible spectrum.

Let's get going with the first part.

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

And that's measured in hertz, the frequency of a wave.

We can also talk about the amplitude of a wave, which is the maximum displacement from the undisturbed position.

And then we've also got the wavelength of a wave, which is given that Greek symbol lambda and wavelength of a wave is the distance between one point on a wave and the same point on the next wave.

And the waves in this animation were transverse.

The oscillation was up and down and the waves were travelling along.

But that's not enough to get marks at GCE.

You've got to talk about the oscillation being 90 degrees to the direction of energy transfer.

That's the definition of a transverse wave where the oscillation is 90 degrees to the direction of the energy transfer.

So light waves are electromagnetic waves, they're transverse oscillations or ripples in the invisible electric and magnetic fields that are all around us.

And in air or a vacuum, all electromagnetic waves have the same speed, which is 300 million metres per second, very, very fast.

That's three times 10 to the eighth metres per second.

And we know that the frequency of a light wave sets the colour.

And we also know that a visible light spectrum shows the full range of colours or frequencies that visible light waves can have.

So there's a estimated or approximate frequency scale that's been put on that visible light spectrum there.

So you can see that red light has the lowest frequency and blue light the highest.

Let's do a check on our understanding so far.

We know that all waves obey the wave equation.

Wave speed equals frequency times wavelength.

Which of the following statements are true? Out of all the colours of visible light, red light has, and then choose which ones are true to complete that sentence.

Pause the video now, off you go.

Okay, I'll go through the answers now.

So out of all the colours of visible light, red light has the lowest frequency and there's a frequency scale just to remind you of that.

What about all the other statements? Now we said that all electromagnetic waves have the same speed in air, but red light has the lowest frequency.

So if the frequency is a lower number, the wavelength must be a bigger number for red light to give the same wave speed.

So that means red light must have the longest wavelength.

And here's a wavelength scale to show that.

You can see that going from red light to blue light, the frequency increases, but the wavelength decreases.

So well done if you've got those two.

And of course, E and F definitely are not true because all electromagnetic waves have the same speed in air.

Well done if you got that right.

So we also know that white light is a mixture of all the different frequencies, all the different colours of visible light.

And we can see that effect in a prism, 'cause a prism can separate all of the different frequencies, all of the different colours that are present within white light, a prism can separate those out and split them up.

And then you get a visible colour spectrum.

And each frequency of visible light is refracted through a slightly different angle by glass.

That causes the different frequencies of white light to spread out from each other.

And that effect for white light separating into the different frequencies is called dispersion.

And a triangular glass prism can display this because it refracts light in the same direction twice.

So the rays leave the block at different angles and spread apart, whereas in a rectangular block, the rays leave the block parallel and close together.

So dispersion is much less noticeable.

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

So in a triangular prism, violet light is refracted more than red light.

Is that true or false? Once you've decided, I then want you to justify your answer.

So which out of A or B best explains why you chose true or why you chose false? Is it because violet light waves have a higher frequency? Or is it because violet light waves have a lower frequency? Pause video now and choose true or false and also A or B.

Off you go.

Okay, I'll give you the answer now.

Violet light does refract more than red light.

So this statement was true.

And the best justification of that is that violet light waves have a higher frequency.

Well done if you got that correct.

Okay, time for a task now on this section of the lesson.

There's four parts to this task.

So give each part your best effort.

You should pause the video now, read through each instruction and complete each part of the task.

Off you go.

Okay, I'll give you some feedback now on each part of this task.

So part one, state what is different about the different colours of light? The different colours of light are all electromagnetic waves with different frequencies and different wavelengths.

But the main thing is different frequencies.

It's the frequency of light that sets the colour and the wavelength of light, the wavelength of a particular colour of light can be different depending on the material.

So it's the frequency that sets the colour.

That's what I'm really looking for you to link to the colour of light, the frequency of the light wave.

Part two, explain how white light is different to red light.

White light is a mixture of all the different frequencies of visible light, but red light is just one frequency or a narrow range of frequencies.

Part three, explain what is meant by the visible light spectrum.

The visible light spectrum is the full range of different frequencies that visible light waves can have.

'Cause spectrum just means range.

So the visible light spectrum means the visible light range, the range of frequencies the visible light waves can have.

And part four, explain how a visible light spectrum can be produced using a triangular glass prism.

Your answer should be something along these lines.

When white light is incident on a triangular glass prism, the different frequencies present are refracted by different amounts by the glass.

The shape of the triangular glass prism means that each frequency is refracted in the same direction twice and leaves the present at a different angle.

So all the different frequencies, the different colours are spread apart over a range of angles in order of frequency.

Well done for your effort on that task.

You might want to pause the video now and make any improvements to your answer for part four to improve your work.

So that takes us to the second section of the lesson where we're now going to go beyond the visible spectrum and we're going to look at other possible frequencies that electromagnetic waves can have.

So human eyes can only detect electromagnetic waves in a limited frequency range, only between about 400 terahertz and just under 800 terahertz.

Now one terahertz is a million million hertz, so that's a very high frequency.

So the frequency of visible light waves is very, very high.

But what about all of the frequencies below 400 million million hertz, below 400 terahertz? What about all the possible frequencies that are above 800 million million hertz, 800 terahertz? Because electromagnetic waves can have any frequency.

So electromagnetic waves with higher or lower frequencies, they're not detected by the human eye.

So they can be thought of as the same thing as light, but it's not light because you can't see it.

It's not visible light, it's in the invisible kinds of light.

So let's look at some examples of that.

So electromagnetic waves, the same thing as what light is, but with frequencies in a range just lower than what the eye can detect, so just lower than red light, they're called infrared waves and the word infra means below.

So infrared electromagnetic waves are electromagnetic waves with a frequency that's just below red, the frequency of red light.

And electromagnetic waves with frequencies in a range just higher than violet light, so just higher frequency than the eye can detect, they're called ultraviolet waves, and ultra means beyond.

So these are waves which are just beyond violet, visible light waves in frequency.

So white light from the Sun, we know it contains all of the frequencies of visible light, but it actually contains lots of infrared and ultraviolet frequencies of electromagnetic wave as well as those visible frequencies.

And you've probably heard of the Sun's ultraviolet rays because that's what can cause sunburn if you stay in the Sun for too long.

So we're actually going to look at the infrared waves in sunlight first.

Now they can be detected by putting a sensitive thermometer just outside the red end of a visible light spectrum.

So look in the diagram to see where that thermometer's being placed.

And if you put a thermometer there just outside the red end of a spectrum, it's temperature goes up, but it's not absorbing any light from the Sunlight because the light is split into the different frequencies of light from red to violet.

And we've put the thermometer outside of where any of that visible light goes.

So it must be absorbing an invisible kind of light.

It's actually absorbing infrared.

Now this is because each frequency present in sunlight is refracted a different amount.

And infrared frequencies are lower than the frequency of red light.

So infrared frequencies are refracted less than red light.

So that's why they appear just where they do just before red in the spectrum.

And the thermometer bulb absorbs infrared frequencies quite well.

So its temperature increases when it's placed in the path of the infrared.

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

So sunlight contains ultraviolet light as well as infrared and visible light.

And the ultraviolet light in sunlight would take the path shown through the prism.

So have a look at the diagram, make sure you can see where the ultraviolet light goes from the original beam of sunlight.

Now you all you need to do is decide which of the following statements are true A, B, and C.

So pause the video now and make your decisions.

Okay, I'm going to give you some feedback now.

Statement A ultraviolet waves are higher frequency than violet light.

Well, we should know that that's true because ultraviolet, literally means beyond violet in frequency.

So higher frequencies than violet.

So that's true, well done if you've got that.

Statement B, ultraviolet waves are refracted more than violet light by a glass prism.

Well that's true as well because the ultraviolet waves are refracted beyond violet.

So more than violet.

And statement C, ultraviolet waves are faster in air than violet light.

Well that's not true because in air all electromagnetic waves have the same speed.

So ultra doesn't mean faster, it just means beyond and beyond violet in frequency.

Well done if you've got both of those right, A and B.

So really important idea now.

It turns out that all objects are actually constantly emitting or radiating electromagnetic waves into their surroundings.

That's right.

Everything in the room in front of you right now, every surface, every wall, the ceiling, every object on the desk in front of you, your own body is constantly emitting or radiating electromagnetic waves.

But the range of frequencies emitted and the intensities depends on an object's temperature.

And at room temperatures and below, objects don't emit any visible light or higher frequencies than that.

They mainly actually emit infrared, which is invisible.

So that's why you can't see everyday objects glowing.

As the temperature of an object increases, the intensity of every frequency emitted also increases.

And also as the temperature of the object increases, the range of frequencies emitted increases to include higher and higher frequencies.

So the hotter an object is the more infrared is going to emit in a given time for both of those reasons.

And this is what thermal cameras detect.

You might have guessed that from the image on the previous slide repeated here.

So infrared cameras, what they actually do is they measure the intensity of infrared from each part of the image, which corresponds to the temperature for the reasons given on the previous slide.

Colours are then added to a thermal image to show each temperature.

So a thermal image does not show us infrared itself.

It doesn't show us what infrared looks like.

It shows us which areas emit most infrared because they're the areas that should be painted the brightest colours normally on a thermal image, but the colours are added.

And we should make really clear at this point that infrared frequencies of electromagnetic wave are really easily absorbed by lots of different surfaces, including skin.

And that's going to cause a heating effect when objects absorb infrared from their surroundings or from a hot object nearby.

And that heating effect and the energy transfer is caused without touching because the waves are electromagnetic waves.

They can travel through air and through a vacuum really easily.

Now, at around 600 degrees C, and it's the same temperature for all objects, objects start to emit some visible light as well as infrared.

And that's because increasing the temperature of an object increases the range of frequencies emitted.

So the hotter object gets, the higher the highest frequency is that's emitted.

And eventually an object gets so hot that it starts to emit visible light as well as infrared.

And that's what happens to the heating elements inside a toaster or the filament inside a light bulb and the heating filaments of infrared heaters.

Now they might appear red or orange because they're hot enough to emit some red and orange visible light as well as the infrared that they're emitting.

Okay, time for a check now about what we just said.

Which of the following objects are emitting infrared? Choose from A, B, C, or D.

And also part two of this check, in which of the images can you see the emitted infrared? Pause the video now and make sure you've got answers for part one and part two of this check.

Okay, let's see how you got on.

Which of the following objects are emitting infrared? They all are.

Okay, we said all objects, whatever their temperature are always emitting some infrared frequencies of electromagnetic waves into their surroundings.

And in which of the images can you see the emitted infrared? That's a bit of a trick question because none of them, because the infrared itself cannot be seen.

We can see that objects, the hot metal horseshoe and the bold filament, they are obviously hot enough to be giving out some visible light as well as infrared.

But you cannot see the infrared from any object.

It's not detected by our eyes.

Well done if you've got both of those right.

Let's talk a bit more about ultraviolet now.

Light from the Sun also contains ultraviolet or UV frequencies of electromagnetic wave.

And you might already know that exposure to too much ultraviolet, that's what causes sunburn.

And just like infrared, ultraviolet waves are not visible to humans.

Special lamps that emit ultraviolet, they might look like they're violet, but that's because they emit some violet visible light as well as the ultraviolet they're emitting, which you can't see.

Some insects' eyes can detect ultraviolet and some flowers reflect the Sun ultraviolet to attract those insects.

So the top half of this image is a normal photograph of a dandelion flower.

Those bright yellow flowers you see in grass, and the bottom half of that image is the same dandelion photographed, but it's captured the ultraviolet light.

So it's a false colour image.

You're not seeing the ultraviolet there.

Each part, the intensity of ultraviolet reflected from each part of the daisy has been measured and each number has been turned into a colour.

So we can kind of see which parts of the daisy are emitting most infrared, most ultraviolet, but we can't, we're not seeing the ultraviolet itself in that picture.

You can see how those markings, which are only visible in ultraviolet, make like a nice target for insects to arrive in the very centre of the flower, probably where most of the pollen is.

And a final thing to mention about ultraviolet is that some substances are fluorescent and that means they give out visible light in the moment when they're absorbing ultraviolet light.

And that's how ultraviolet inks work and glow in the dark decorations work in a similar way, but they stay glowing for longer after the exposure to ultraviolet has ended.

So again, objects which are fluorescent, maybe sometimes look violet, but that's maybe violet light they're emitting, you are not seeing any of the ultraviolet there.

Let's do a quick check about some ideas about ultraviolet frequencies of electromagnetic wave.

So the writing in this image was written with ink that's invisible unless it's exposed to ultraviolet light.

So the question is, how does the exposure to the ultraviolet light cause the writing to become visible? Choose from A, B, and C.

Pause the video, off you go.

Okay, time for some feedback on this one.

The correct answer was B.

The ink emits visible light when it's exposed to ultraviolet light.

So the ultraviolet light gets absorbed and that causes the ink to give out visible light.

So ultraviolet light is not pinky purple, it's invisible.

You can't see ultraviolet.

And that's also why it's not A, because if the ink reflected the ultraviolet that hits it, you still wouldn't be able to see that.

So the only reason why ink glows is 'cause it emits visible light when it's exposed to ultraviolet light.

Well done if you've got that one.

So remember that we said that electromagnetic waves can have any frequency.

Well, so far we've only been talking about infrared, visible and ultraviolet frequencies of electromagnetic wave.

But what about electromagnetic waves which have as low frequency as say 100 hertz? Or what about electromagnetic waves which have an incredibly high frequency, up to say 10 to the 22 hertz, that's a one with 22 zeros after it, number of oscillations per second frequency of 10 to the 22 hertz? Well actually all of these are possible and everything in between 'cause electromagnetic waves can have any frequency.

However, we give electromagnetic waves in different frequency ranges, different names because they interact differently with different materials.

So the lowest frequency electromagnetic waves, we call them radio waves.

And that is exactly what carries radio signal and TV signal, signal to a walkie-talkie, a Wi-Fi signal is carried by radio waves.

Of course we can't see them.

And then in between radio waves and infrared waves, that frequency range which is missing, we call those microwaves for electromagnetic waves with those frequencies.

And don't confuse the microwave radiation, the microwave electromagnetic waves with a microwave oven, a microwave oven cooks food using microwave electromagnetic waves, microwave frequency, electromagnetic waves.

What about the very high frequency electromagnetic waves? Well, they're called X-rays and gamma rays if they're extremely high frequency electromagnetic waves.

So this is what I meant at the start of the lesson when we said electromagnetic waves can have any frequency, literally any frequency in this continuous spectrum of possible frequencies.

And we've said waves, electromagnetic waves in the seven different frequency ranges, they're given different names because they interact differently with different materials.

And just as an example of that, glass transmits visible light but absorbs infrared.

So if you want to hide from a thermal camera, you could hide behind a sheet of glass.

I mean anyone, any human could see you behind the sheet of glass because visible light is transmitted through glass, but you'd be invisible to a thermal camera 'cause the thermal camera would absorb the infrared that your body gives off and not let it get to the camera.

Whereas some kinds of opaque plastics can absorb visible light whilst transmitting some infrared.

So stars including the Sun, are so hot and large that they produce electromagnetic waves in all seven frequency ranges at detectable intensities.

And you can use that to produce different kinds of images of the Sun by using different kinds of telescopes that detects different frequencies of electromagnetic wave.

For example, that is a radio telescope that detects very long wavelength electromagnetic waves from objects in space.

And remember that the colours in images like this are going to be false.

They're added by computers just so we can see which parts of the Sun emit each frequency most intensely.

So we'll talk a bit about each of the kinds of, each of the frequencies of electromagnetic wave that we've mentioned.

So the lowest frequencies of electromagnetic wave are called radio waves.

And we've learned how to produce radio waves to transmit encoded information over long distances.

Radio, TV, phone, phone signal and Wi-Fi.

Microwave frequencies of electromagnetic wave, they're produced in our homes to cook food in microwave ovens and they're also used for communications.

And then the highest frequency electromagnetic waves are called X-rays and gamma rays.

They're produced from changes in atoms in lots of materials.

An X-ray machine in a hospital can produce controlled bursts of X-rays to create an X-ray photograph.

So X-rays can pass through skin and flesh, but they're absorbed by bone.

That's how you can create a X-ray photograph.

And of course that's different to visible light and other kinds of electromagnetic waves which are, which can't pass through skin and flesh.

Worth mentioning at this point, just a reminder that all frequencies of electromagnetic wave have the same speed in air in a vacuum, which is 300 million metres per second, three times 10 to the eighth metres per second.

And what that means is that frequency and wavelength in air are inversely proportional for the entire spectrum of electromagnetic waves.

So when one of frequency and wavelength increases, the other must decrease to the inverse proportion so that the wave equation still works, and frequency times wavelength still gives the same wave speed in air.

So here's an example wavelength scale.

You can see that radio waves are the lowest frequency and longest wavelength, electromagnetic wave.

Whereas gamma rays are very, very, very high frequency, but also very, very, very short wavelength.

So when you do frequency times wavelength, you still get the speed of light in air and a vacuum.

Let's do a check on some of the things we've just said.

Fill each blank with either the terms wave speed, wavelength or frequency.

Pause video now and have a go at that.

There's just three blanks to fill in, off you go.

Right, I'll talk you through this one now.

So radio waves, infrared and ultraviolet are all the same thing as light electromagnetic waves, which is oscillations or ripples in electric and magnetic fields.

But the only difference between each kind of electromagnetic wave is their frequency and wavelength and they could be either way round and that causes each kind, each frequency range of electromagnetic wave to interact differently with different materials.

And all electromagnetic waves have the same wave speed in air and a vacuum.

Well done if you've got all of those.

Final thing we should just mention is that electromagnetic waves can also be called electromagnetic radiation.

Radiation just means something that's emitted and transfers energy away from a system.

So sound waves are acoustic radiation.

You can have nuclear radiation and this is electromagnetic radiation, things which are emitted and transfer energy away from a system.

Electromagnetic waves counts as that.

And all kinds of electromagnetic radiation can be produced in nature.

For example, by stars, lightning and hot objects.

Some living things actually produce electromagnetic radiation.

These jellyfish are emitting some visible light.

Electromagnetic radiation from the Sun is naturally present at earth's surface.

So radiation's a completely natural thing, radiation from the Sun is completely natural, but of course humans have also found ways of making each kind of electromagnetic radiation.

And the final thing we'll say is that the lower frequencies of electromagnetic radiation, talking about radio waves and microwaves and infrared are not harmful unless the intensity is very high.

The higher frequencies of electromagnetic radiation, particularly ultraviolet x-rays, gamma rays, they can present a greater risk but the danger is reduced if the exposure is minimised.

Let's do a check of some of the ideas we just talked about, which two of the following statements are true? So you're looking for two of these, which are true and the rest are all false.

Pause the video, have a read through, make your decision, off you go.

Right, I'll tell you which ones are true.

A was true.

Light is a kind of electromagnetic radiation.

Radiation just means something which is emitted and transfers energy and light certainly is emitted from some objects and transfers energy away from those objects.

The other statement that's true is F, the Sun does emit some harmful radiation.

Radiation is naturally occurring.

Radiation is not always dangerous and harmful.

There is potentially harmful radiation in the countryside such as ultraviolet from the Sun, for example, and very, very few electronic devices are going to emit harmful radiation because if they did, they wouldn't be allowed to be put on sale.

Okay, well done.

Lots of information about different kinds of electromagnetic waves with the different frequencies.

So there are four parts to this task to complete this lesson.

Most of the parts of this task are fairly straightforward, so you should pause the video now and have a go at each part, off you go.

Right, well done for your effort on that task.

I'll give you some feedback now.

So what is meant by the spectrum of electromagnetic waves? It means the full or continuous range of frequencies that electromagnetic waves can have.

The spectrum of frequencies that electromagnetic waves have, which is all possible frequencies.

Part two was just adding the names of the seven frequency ranges of electromagnetic waves to the diagram, so should have gone starting from the lowest frequency electromagnetic waves, that's radio waves, then microwaves, then infrared, then visible light, then ultraviolet, then x-rays, then gamma rays as shown in this diagram here.

Part three of this task, describe some properties that all electromagnetic waves have.

Hopefully you've included some of these.

So all electromagnetic waves consist of oscillations or ripples in electric and magnetic fields.

That's what they are.

Electromagnetic waves are transverse waves and in air or a vacuum they all travel at the speed of light, 300 million metres per second, three times 10 to the eighth metres per second, and all electromagnetic waves transfer energy.

Well done if you've got most or all of those.

And part four of this task, state the only difference between each kind of electromagnetic wave that causes them to interact differently with different materials.

What we're looking for here is their frequency.

You might have mentioned their wavelength, but really they're defined by their frequency rather than their wavelength.

Very well done if your answers were along those lines.

Here's a summary of the lesson.

Electromagnetic waves or electromagnetic radiation are transverse oscillations or ripples in electric and magnetic fields.

Light is electromagnetic radiation that we can see.

Colours in the visible spectrum have different frequencies.

Violet light has the highest frequency and refract more than red light in glass.

Electromagnetic waves in other frequency ranges interact differently with different materials.

In order of frequency, starting with the lowest frequency, the spectrum of electromagnetic waves is radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma rays.

And all electromagnetic waves can transfer energy and they all travel at the same speed in air.