Lesson video

- Hi folks.

My name's Mrs. Farrow.

For today's session, you're going to need a pen and a piece of paper.

And it's a good idea to switch off your mobile devices or put them all silent to help avoid any distractions.

If you need to take a moment to sort that out, then please do say now and press play when you're ready to resume.

Today's lesson is about radiation.

We're going to explain the process of energy transfer by radiation and how different surfaces can affect the absorption and emission of radiation.

Some of the key words that you'll see during today's session include infrared radiation which is a type of heat energy transfer.

Emit.

Hot or warm objects are able to emit or give out infrared radiation.

And absorb.

Objects can absorb which means to take in infrared radiation which transfers energy to their thermal energy stores.

Keep an eye out for these words as we go through today's session.

We've both broken today's lesson radiation down into three different cycles.

First, what is radiation? Then we'll look at surfaces that absorb and emit and then we'll take a look at some everyday examples.

So let's jump in and take a look at what is radiation? Well, radiation is a kind of energy transfer.

Unlike conduction and convection which requires particles moving or vibrating radiation travels by waves.

This is how the earth is warmed by the sun.

The waves of infrared radiation are able to travel through the vacuum of space, the vacuum being somewhere where there are no particles.

And so warming our earth and maybe making it habitable.

Radiation travels by infrared waves.

These are a lot like light waves and part of the electromagnetic spectrum that have a slightly longer wavelength.

They behave in a similar way to light being able to travel in a vacuum, as we've said, a space with no particles.

They travel at the same speed of light, incredibly fast.

They can be reflected, which means they can bounce off and absorbed, which means taken in by objects.

And they travel in straight lines.

We often refer to 'em as infrared radiation.

Here we can see a picture of a house, but this house has been taken using a special thermal imaging camera.

We can't see for infrared radiation with our eyes and so we use special cameras which are able to add colours to objects to identify warm and cool areas.

Infrared radiation itself has no colour.

These are added by a computer which analyses the amount of infrared radiation being admitted.

So if you look at the image, the blue and green areas are the coolest.

This means they are emitting very little infrared radiation whilst the white and red areas are warmer and so emitting larger amounts of infrared radiation.

Time for a quick knowledge check.

True or false? Infrared radiation appears different colours.

This indicates the heat energy being transferred.

The answer to this question is false.

Infrared radiation does not have a colour.

Colours are added to show the amount of infrared radiation transferred when making thermal images.

Next question.

Which method of energy transfer does not involve particles? Is it A, conduction, B, convection, or C, radiation? The correct answer is C, radiation.

Well done if you've got that one right.

Now it's time for you to have go to practise task.

In this task we would like you to fill in the gaps in order to complete sentences.

You want to pause the video to give this task your full attention and when you're ready to go through and see the answers, you can press play.

Have you got on? Hopefully not too tricky.

Let's take a look.

Sentence A should read radiation is a type of energy transfer.

Energy from the sun travels to the earth by radiation.

Radiation can travel through transparent things and through a vacuum.

Well done if you've got those right.

Let's take a look at a few more to give you a summary paragraph about our learning so far.

Again, you're going to fill the gaps with the most appropriate word and when you're ready to hear the answers, you can press play.

How did you find it? Let's take a look at the correct answers.

Hot objects emit more infrared radiation than cold objects.

Radiation travels via infrared waves.

It travels in straight lines.

And we can see infrared radiation using a thermal imaging camera.

Great job guys.

Well done.

So now we know what radiation is.

Now we need to know how different objects respond to it and what surfaces are able to absorb and emit radiation.

So let's take a look.

All objects emit some infrared radiation, but some surfaces are much better emitters than others.

You will have experienced this but perhaps not understood exactly what's going on.

If we take a look at some some different colours we have black, then all the other colours, white, and silver.

The best emitters of infrared radiation are black objects.

Then the other colours then less good is white and the worst emitter of infrared radiation is silver.

Black surfaces are the best emitters of radiation transferring the most energy from their stores to the surroundings.

Shiny silver surfaces are the worst emitters of radiation.

You may have experienced this yourselves.

Have you ever walked barefoot along the pavement? You find that when work walking on the centre of the pavement on the black tarmac, it's very, very hot underfoot.

However, if you try and balance your way along the curb stones, it's much cooler.

This is to do with how effective the different colours or the surface are at emitting infrared radiation.

We can prove that different surfaces are better or worse emitters of infrared radiation using what's called a Leslie's cube.

The cube has different colours painted on each side shiny silver, white, shiny black and matt black.

We then set up a temperature centre and the cube is filled with hot water.

The temperature sensor measures the temperature change from each of the surfaces and the bigger the temperature change, the more infrared radiation is being emitted by the surface.

The surface that gives off the most infrared radiation is the best emitter.

This would give us some ideas of the most appropriate colours to paint objects depending on on whether we want them to emit lots of infrared radiation or to be able to not emit infrared radiation.

Objects that absorb radiation are heated by them and some substances are better absorbers of radiation than others too.

Again, we look at the spectrum, from black through the other colours to white and silver.

The best absorbers is still black.

And the worst absorber is silver.

So we see the pattern of reflected for that which we saw in emitters match black surfaces are the best absorbers of radiation and so shiny silver surfaces are the worst absorbers of radiation.

So good emitters of infrared radiation are also good absorbers of infrared radiation.

We can check to see which surfaces are the best absorbers of infrared radiation using an experiment similar as this.

We wrap different test tubes in the materials we want to look at.

Tube A here has been wrapped in matt black paper.

Tube B in white paper and tube C in silver foil.

We fill each tube with the same volume of water and place a thermometer inside it.

These are then placed next to a heat source.

The tube which absorbs the most infrared radiation will show the greatest increase in temperature.

In which tube do you think the water will get the hottest and why? That's right.

The tube wrapped in black will get the hottest as it's the best absorber of the infrared radiation.

Time for a quick knowledge check.

True OR false.

Shiny surfaces are the best at emitting infrared radiation.

The answer to this question is false.

Shiny surfaces are poor emitters, matt surfaces are the best emitters of radiation.

Next question.

Which surface will best absorb infrared radiation? Is it A, black and shiny B, white and dull, C, black and dull, or D, white and shiny? The correct answer is black and dull surfaces are the best absorbers of infrared radiation.

Let's see how we get on with these.

Radiation is affected by an object's colour.

We are asking you to select a correct answer to the questions.

Question one, which object absorbs radiation very well? Is it dull and black or shiny silver objects? That's right.

It's dull and black objects.

Two, which object does not absorb radiation well? Is it dull black objects or shiny silver object? Correct answer is shiny silver objects.

Which objects emits radiation very well? Is it dull black object or shiny silver object? Correct answer is dull black objects.

And which object does not emit radiation well? Is it dull black objects or shiny silver objects? The correct answer is shiny silver objects.

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

And hopefully with that style of questioning you've been able to spot a bit of a pattern.

Those which are good absorbers are good emitters and are dull black objects.

Those which are poor absorbers and poor emitters are the shiny silver objects.

Now it's over to you for a go to practise task.

In this first part of this task the diagram shows some water being heated with a solar cooker.

The curved mirror is reflecting the sunlight that falls on it and the sunlight is being focused on the cooking pot.

The energy transferred from the sunlight is absorbed by the pot, heating the water inside.

Suggest one reason why a matt black pot has been used.

You'll want to take a moment and pause the video now to give this question your full attention and some thinking time.

Once you've written down your answer press play to have a look at a model answer.

How did you find it? Let's take a look at what should be the correct answer.

Matt black is the best absorber of radiation and so the water will warm faster if the pot is matt black in colour.

The second part of this question says when the water has been heated, equal amounts of the water are poured into two metal pans.

The pans are identical except one has a matt black surface and the other has a shiny metallic surface.

Which pan will keep the water warmer for longer.

Explain your answer.

Again pause the video now to consider your answer and write it down and then press play when you're ready to have a look.

Let's see how you got on.

Now with any answer like this it doesn't have to be word for word the same as mine, but it should follow the same general themes.

The silver pan will keep the water warm for longer, because silver is a poor emitter of infrared radiation.

The water in the matt black pan will cool fastest as matt black is a very good emitter of infrared radiation.

So whilst we wanted the pots that the water was being heated in to be black in order to absorb the infrared radiation once taken away from the solar oven, it's best stored in a silver container.

Question b.

The sun shines onto Easter eggs.

Both eggs are made of dark chocolate.

One egg is wrapped in silver foil the other egg is unwrapped.

Suggest what might happen and explain your answer.

Which of the eggs do you think is going to melt and why? Pause the video to take some time to think about your response and write it down.

When you're ready to have a look at a model answer, press play and we'll go through it together.

How have you got on? Let's take a look again.

Remember, it doesn't have to be word for word the same just have to include the same ideas.

So the chocolate bar in the silver foil will melt more slowly.

This is because the silver foil is a poor absorber of the sun's radiation.

The dark brown of the unwrapped chocolate will absorb the sun's radiation and so will melt more quickly.

Well done if you've got that one right.

This is not easy and you're doing really well at thinking about the answers and having a go at explaining what's happening.

So we now know what radiation is and we've looked at the different types of surfaces that are good absorbers and emitters of infrared radiation.

Now we're going to take a look at some everyday examples of radiation and how we can put our knowledge of absorbers and emitters to good use.

If we think about a nice mug of tea we feel the heat from a warm drink without even touching it because infrared radiation is being emitted from the cup.

Often we'll know something is hot before placing our hand on it and so it helps us to avoid be being burned.

For a cold day it's nice to be able to come and warm your hands over a fire.

Fire emits infrared radiation to the surroundings and we feel it's warmth without having to touch it.

In our toasters, we are able to observe radiation being emitted from the hot elements.

This is what is used to heat and cook the bread.

One situation where we make good use of our knowledge of absorbers and emitters is that of emergency blankets.

These are used to keep people warm after an accident or an emergency.

They are silver and shining.

Heat given off by the person's body is reflected back towards them helping to stay warm and preventing the heat being lost to the surroundings.

You use shiny silver in order to prevent infrared radiation being transferred to the surroundings.

You may have seen hot water solar panels on the roofs of houses.

Many people use them in order to heat their hot water to use when washing up or in showers and bath.

The solar panels that are installed on people's homes are usually painted black.

This is because black is the best absorber of thermal radiation and so will mean that the water is going to heat up fastest.

Time for a quick knowledge check.

True or false.

Firefighters suits are made of a shiny material to help them absorb infrared radiation.

Again this question is false.

Firefighters would not want to absorb infrared radiation which is being emitted by a fire.

This would make them very hot indeed.

Firefighter suits are shiny to reduce infrared radiation being absorbed because it is a poor absorber and it helps to keep them cool.

The back of a fridge is painted black, but why? Is it A, to emit infrared radiation, cooling the fridge.

B, to absorb infrared radiation, cooling the fridge.

Or C, to reflect infrared radiation cooling the fridge.

The correct answer is A, to emit infrared radiation.

This removes infrared radiation from the back of the fridge helping to cool it down.

Now it's time for a practise task.

This first part of the task, we'd like you to look at each of the objects and then tick to suggest whether it is trying to promote radiation or prevent it.

Pause the video now and have a think about each of the scenarios.

And when you're ready to go through the answers, press play.

Let's see what you've put.

So the inside of a thermos flask has a silvered surface.

Is this because we are trying to increase the amount of radiation or decrease the amount of radiation.

It is to prevent radiation from occurring.

And so to keep the inside of the thermos flask warmer.

Window frames are often white or silver.

Is this promoting radiation or preventing it? White or silver surfaces are very poor emitters and so this would prevent infrared radiation being emitted.

Barbecues are black or dark colours.

We are using the barbecue to cook our food and so we want the barbecue to emit infrared radiation into the food.

And so this would be to promote radiation.

Often summer clothes are light in colour.

Objects which are lighter in colour are poor absorbers of infrared radiation and so this would be preventing radiation from occurring.

And solar panels that we find on the roofs of our houses are black.

This is promoting the absorption of infrared radiation and so heating the house more quickly.

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

Now we're going to take a look at some questions which apply to different scenarios where different surfaces have been painted different colours for different reasons.

You're going to read each of the questions and then have a go answering it.

On this slide we have question one.

A car's radiator cools hot water from the engine.

Explain why the radiator is painted black.

And then a car can get too hot when the sun shines on it.

What would be the best colour to have in a hot, sunny country? A black car or a white car and why? Pause the video now and take a few moments to have a go at answering these questions.

And when you're ready to hear a model answer, press play.

Let's see how you got on.

Car's radiator cools hot water from the engine.

Explain why the radiator is painted black.

The car's radiator is painted black because black is a good emitter of infrared radiation and so will cool the water faster.

A car can get too hot when the sun is shining on it.

What would be the best colour car to have in a hot and sunny country? A black coloured car or a white coloured car? If you're trying to prevent the car from getting too hot, we will want it to be a poor absorber of infrared radiation.

And so a white car is better, it will stay cooler as it's a poor absorber of infrared radiation.

Well done if you've got those right.

Now let's take a look at some more examples.

Question three asks the hot element in a toaster heats the bread.

The air between the element and the bread is heated and rises away from the toast without heating it.

Explain how the toast therefore gets hot.

And then secondly, a house has a solar panel.

It heats the water when the sun shines on it.

Explain why black is the best colour for a solar panel.

Pause the video now and jot down your ideas.

And when you're ready to hear a model answer, press play.

Ready to have a look at the answers? Let's see.

The hot elements are toaster heats the bread the air between the element and the bread is heated and rises away from the toast without heating it.

So explain how the toast gets hot.

I often, particularly on cold mornings, like to hold my hands over the toaster 'cause you will feel the warm air rising off of the toaster and it helps to heat my hands up whilst I'm waiting for my toaster cook.

So it's not the air that's heating and cooking the bread.

So what is it? Well, the toast is heated by the infrared radiation which is being emitted by the elements in the toaster.

Question four.

A house has a solar panel.

It heats the water when the sun shines on it.

Explain why black is the best colour for a solar panel.

Now black, as we know, is a good absorber of infrared radiation and so black solar panels will heat the water faster.

It's a fantastic effort guys.

And you've done really well.

We've now reached the end of our learning on radiation today.

So what are our key takeaway points? Well, radiation is a type of energy transfer.

When radiation hits an object the infrared radiation is absorbed.

Matt black objects are very good absorbers of infrared radiation.

But shiny silver surfaces are poor absorbers.

Hot objects emit or give out infrared radiation.

And matt black objects are very good emitters of infrared radiation whereas shiny silver surfaces are poor emitters.

We use our knowledge of absorbers and emitters of infrared radiation to make products that can either increase or decrease the infrared radiation.

Well, thank you so much for all your hard work today.

I hope you've enjoyed this session as much as I have.

Why not try have a go at our exit quiz before you log out and see how much you can recall.

We'd like to thank you for choosing to use Oak National Academy today.

And we hope to see you again soon.

Bye for now.