How every object gives out infrared, why dull black and shiny surfaces differ, what a perfect black body is, and how a balance of absorbing and emitting sets a temperature. Separate Physics only.
Objectives
State that every object emits and absorbs infrared radiation, and that hotter objects emit more.
Compare how well different surfaces absorb, emit and reflect infrared radiation.
Describe a perfect black body and how its radiation changes as its temperature rises.
Higher tier: explain how the balance between absorbed and emitted radiation sets the temperature of an object or planet.
Describe a fair test of how surfaces emit radiation.
Black body radiation explained
Start here
Think about the last time you wore a black T-shirt on a sunny day. It felt much hotter than a white one, and you probably noticed it within a few minutes of standing outside. The black cloth soaked up more of the Sun’s energy, while the white cloth bounced more of it away, so the black one warmed you up faster.
Now think about a kettle or a hot cup of tea. You can feel warmth on your hand from a distance, with no touching and no air blowing. That warmth travels as infrared radiation, which is a kind of wave that carries energy through the air and even across empty space, in just the same way that sunlight reaches us.
Here is the surprising part. Every object gives out infrared, even a block of ice, even your own body. The hotter the object, the more it gives out each second, which is why a special infrared camera can see a person glowing in the dark when your eyes see nothing.
The surface matters too. A dull black surface soaks up radiation and gives it out very well, while a shiny silver surface does both badly. This is why a flask is shiny inside and why a solar heating panel is dull black, and you will be able to explain both.
By the end of this lesson you will be able to do three things. You will explain how the temperature of an object affects the radiation it emits, and compare how well different surfaces absorb and emit it. You will also describe a perfect black body, and explain why an object, or even a planet, settles at a steady temperature.
A transverse wave with the wavelength, amplitude, crest and trough labelled. Infrared radiation is a wave like this, and it carries energy from hot objects through the air and through empty space.
Everything gives out infrared
Every object, whatever it is made of, emits infrared radiation. To emit means to give out, and it happens all the time, day and night. Every object also absorbs infrared from the things around it, which means it takes the radiation in and its particles gain energy.
The hotter an object is, the more infrared it emits each second. A mug of boiling water gives out much more than a mug of cold water, and you can feel the difference by holding your hand beside each one, without touching either. An infrared camera makes use of this, because it turns the amounts of infrared into a picture where hotter parts look brighter.
A very hot object does something else as well. As the temperature rises, it starts to give out shorter wavelengths too. The element in a toaster glows red because it is hot enough to emit some red light, as well as infrared, and a hotter one glows orange or yellow.
The Sun is much hotter, with a surface at around 5500 degrees Celsius. It gives out infrared, visible light and ultraviolet. A room at 20 degrees Celsius, by comparison, gives out only infrared, which is why it does not glow and you cannot see the radiation with your eyes.
It helps to remember that emitting and absorbing go on at the same time. You are always giving out infrared to your surroundings, and taking it in from them. Whether you warm up or cool down depends on which of the two is bigger, and we come back to this idea in a later step, because it explains why things settle at a steady temperature.
Good and bad absorbers and emitters
Not all surfaces are equal. A dull black surface is the best absorber of infrared, so it takes in most of the radiation that hits it and turns it into thermal energy. It is also the best emitter, so it gives out the most radiation when it is hot.
A shiny, light-coloured surface is the opposite. It reflects most of the radiation that hits it, so it is a poor absorber. It is also a poor emitter, so it gives out the least radiation at the same temperature, even when it is hot.
A good absorber is a good emitter, and a poor absorber is a poor emitter. That sounds odd, but it is true. Think of it as one rule with two sides, because the same surface features cause both.
You can use this to explain things you see every day. Solar heating panels are dull black so that they absorb as much of the Sun’s radiation as possible, and a flask is shiny so that it gives out little and takes in little. Houses in hot countries are often painted white because white surfaces reflect most of the radiation and stay cooler inside during the summer.
There is a classic experiment for this called the Leslie cube. It is a hollow metal cube with four different faces, which is filled with hot water. An infrared detector held the same distance from each face shows that the dull black face emits the most and the shiny silver face emits the least.
Perfect black bodies
A perfect black body is an idea, a bit like a perfectly smooth surface that never quite exists. It is an object that absorbs all of the radiation that hits it, whatever the wavelength. It does not reflect any, and it does not let any pass through.
Because it is the best possible absorber, a perfect black body is also the best possible emitter at any given temperature. It gives out more radiation than any other object at the same temperature. Real dull black things such as soot come close, but none is perfect, and the Sun and other stars are also close to being black bodies.
The radiation a black body gives out depends only on its temperature. It does not depend on what the object is made of or what it looks like. So if you know the temperature, you know the whole pattern of radiation that it will give out.
The graph shows how the radiation is spread over different wavelengths. The graph shows two black bodies, and as the temperature of a black body goes up, it emits more radiation at every wavelength. The wavelength at which it emits the most, called the peak wavelength, gets shorter.
This explains the colours of hot things. A metal poker that is just warm gives out only infrared. As it gets hotter it glows dull red, then orange, then yellow, and a very hot object looks white, because the peak has moved towards shorter wavelengths and many colours are mixed.
A graph of intensity against wavelength for two black bodies. The hotter one at 6000 kelvin has a much taller curve with its peak at a shorter wavelength of about 0.5 micrometres. The cooler one at 4000 kelvin has a lower curve with its peak at a longer wavelength of about 0.7 micrometres.
A temperature balance
An object is always taking in radiation and giving it out. If it absorbs radiation faster than it emits it, its temperature rises. If it emits faster than it absorbs, its temperature falls.
If the two rates are equal, the temperature stays steady. A cup of tea in a room is a good example. It cools until it is at room temperature, and then it takes in radiation from the room at the same rate that it gives it out, so it stops cooling.
Here is why it settles. As an object warms up it emits more each second, because hotter objects emit more. So the emission rate climbs until it catches up with the absorption rate, and then the temperature stops changing, even though the radiation keeps flowing in and out.
Higher tier: the Earth works in just the same way. During the day its surface and atmosphere absorb radiation from the Sun, and all the time they emit infrared out into space. Over a long time the two rates are roughly equal, so the average temperature stays roughly steady from one year to the next.
If something changes the balance, the temperature changes until a new balance is reached. If the Earth absorbed more or emitted less, it would warm up, and it would settle at a higher temperature. That is the key to understanding climate change, which we meet in the next step.
A flow with four boxes. The object absorbs more than it emits. Its temperature rises. It emits more each second. Emitting equals absorbing, so the temperature is steady.
The Earth and greenhouse gases
The Earth’s atmosphere is a layer of gases around the planet. Most of the Sun’s radiation passes through it and warms the ground, the sea and the buildings. The warm ground then emits infrared radiation, which has a longer wavelength than the Sun’s light because the ground is much cooler than the Sun.
Some gases in the atmosphere, called greenhouse gases, absorb some of this outgoing infrared. Carbon dioxide, methane and water vapour are examples. They then emit it again in all directions, so some of it goes back down towards the ground, which warms it further.
This is a natural effect, and without it the Earth would be far too cold for most life, so we need some greenhouse gases. The trouble starts when people add extra carbon dioxide and methane to the air, for example by burning fossil fuels. The more greenhouse gas there is, the less infrared escapes to space.
Higher tier: with less radiation escaping, the Earth emits less than it absorbs. So the temperature rises until the emitted radiation again matches the absorbed radiation, at a higher temperature than before. A warmer Earth means melting ice, rising seas and changes in the weather, which affects people everywhere.
It is easy to get the explanation wrong, so be careful. Greenhouse gases do not reflect all of the Sun’s radiation away, and they do not stop the Earth giving out infrared. They absorb some of the outgoing infrared, so that less of it escapes to space, and the surface stays warmer.
A flow with four boxes. The Sun’s radiation warms the surface. The surface emits infrared. Greenhouse gases absorb some of it. Some is sent back down, so the Earth is warmer.
Testing surfaces fairly
You can test how well different surfaces emit radiation using hot water, and it is a very common school practical. Fill a Leslie cube, or four identical containers with different outside surfaces, with water at the same temperature. Then use an infrared detector, held at the same distance from each surface, to measure the radiation it gives out, and you write the readings in a table.
To make it a fair test, you only change one thing, which is the surface. Everything else stays the same, such as the water temperature, the size of the container and the distance to the detector. These are called the control variables, and keeping them the same makes the comparison fair.
In another version you fill two cups with hot tea. One cup is painted dull black and the other is shiny silver, and you time how fast each cools. The dull black cup cools faster, because it is a better emitter, so it loses energy more quickly.
The results from a test like this might look like the chart. The dull black face gives the biggest reading and the shiny silver face gives the smallest. These readings are only an example, so use the pattern and not the exact numbers.
Be careful about what the test shows. It shows that the better emitters send out more infrared each second. A good scientist also repeats each reading, then finds the mean, so that one odd reading does not spoil the answer, and the result is more reliable.
A bar chart of example infrared detector readings for three surfaces at the same temperature. The dull black surface reads 9, the white surface reads 6 and the shiny silver surface reads 2.
Recap
Every object emits and absorbs infrared radiation. The hotter an object is, the more it emits each second. Very hot objects also emit shorter wavelengths, such as visible light, as in a glowing toaster.
Dull black surfaces are the best absorbers and the best emitters. Shiny, light surfaces are poor at both, because they reflect most radiation. This explains solar panels, flasks and white houses, and you should be ready to use these three examples.
A perfect black body absorbs all of the radiation that hits it, and it is the best emitter at its temperature. What it emits depends only on its temperature. When it gets hotter it emits more at every wavelength, and its peak moves to a shorter wavelength.
The temperature of an object is steady when it absorbs and emits radiation at the same rate. It rises if it absorbs faster than it emits, and falls if it emits faster. Greenhouse gases absorb outgoing infrared, so the Earth warms until the rates match again, at a higher temperature.
In an exam you might be asked to explain why a surface is matt black or shiny, or to say what happens to a black body as it gets hotter. Another favourite is to explain why the temperature of the Earth is roughly steady, and what extra greenhouse gases do. Use the words absorb, emit and rate, and say which one is bigger.
Key points
All objects emit infrared, and hotter objects emit more each second.
Dull black surfaces absorb and emit best. Shiny surfaces are poor at both.
A perfect black body absorbs everything and is the best emitter.
Hotter black body: more at every wavelength, and a shorter peak wavelength.
Temperature is steady when absorbed radiation equals emitted radiation.
Greenhouse gases absorb outgoing infrared, so the Earth warms to a new balance.
Worked examples
Example 1
Which type of radiation do all objects emit because of their temperature?
All objects have thermal energy, because their particles move.
They give some of it out as radiation.
The radiation that objects at everyday temperatures give out is infrared.
Answer: Infrared radiation.
Common slips
Slip: Only hot objects have thermal energy. All objects have thermal energy, even cold ones.
Slip: Only objects that are very hot give out radiation. All objects emit radiation. Hotter objects emit more.
Slip: The radiation is ultraviolet. Ultraviolet comes from much hotter objects. Everyday objects emit infrared.
Example 2
Explain why solar heating panels are painted dull black.
A dull black surface is the best absorber of infrared radiation.
So it absorbs more radiation from the Sun.
This transfers more energy to heat the water in the panel.
Answer: A dull black surface absorbs the most radiation, so more energy is transferred to heat the water.
Common slips
Slip: A dull black surface reflects most of the radiation. Shiny surfaces reflect. Dull black surfaces absorb the most.
Slip: So it emits less radiation than other colours. A dull black surface is also the best emitter. The aim here is to absorb.
Slip: This stops the water from boiling. The colour is chosen to increase absorption, so that more energy heats the water.
Example 3
Explain why the temperature of a planet becomes steady.
The planet absorbs radiation from its star.
It also emits radiation into space.
The temperature is steady when the rate of absorbing equals the rate of emitting.
Answer: The temperature is steady when the rate at which the planet absorbs radiation equals the rate at which it emits radiation.
Common slips
Slip: The planet stops absorbing radiation. It keeps absorbing. The two rates become equal, so it does not stop.
Slip: The planet stops emitting radiation. It keeps emitting. A warmer planet emits more, until the rates match.
Slip: The temperature is steady when the star stops giving out radiation. The balance is between the rates of absorbing and emitting, not the star switching off.
Questions and answers
Black body radiation practice questions for GCSE Combined Science Physics. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
Name the type of radiation that all objects emit because of their temperature.
Show answer
Answer: infrared
Everything has thermal energy.
The radiation is infrared.
Question 2 Easy
What happens to the radiation an object emits when it gets hotter?
Show answer
Answer: It emits more.
A hotter object gives out more energy.
It emits more radiation.
Question 3 Easy
Name the type of surface that is the best absorber of infrared radiation.
Show answer
Answer: matt black
Dark surfaces absorb well.
Matt black is the best absorber.
Question 4 Easy
Name the type of surface that is the worst emitter of infrared radiation.
Show answer
Answer: shiny silver
Light, shiny surfaces emit poorly.
Shiny silver is the worst emitter.
Question 5 Medium
Explain why a solar panel for heating water is painted matt black.
Show answer
Answer: A matt black surface absorbs more radiation, so more energy heats the water.
Matt black absorbs the most radiation.
More of the Sun’s energy heats the water.
Question 6 Medium
Describe a perfect black body.
Show answer
Answer: It absorbs all the radiation that hits it and is the best emitter at its temperature.
It absorbs everything that hits it.
It is also the best emitter at its temperature.
Question 7 Medium
State what happens to the wavelength at which a black body emits most strongly as it gets hotter.
Show answer
Answer: It gets shorter.
A hotter body has a peak at shorter wavelengths.
The wavelength gets shorter.
Question 8 Medium
An object absorbs radiation more slowly than it emits it. State what happens to its temperature.
Show answer
Answer: It falls.
More energy leaves than arrives.
The temperature falls.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
Two identical cups of hot tea are left to cool. One cup is painted matt black and the other is shiny silver.
(a) State which cup cools faster. [1 mark]
(b) Explain your answer. [2 marks]
(c) State what happens to the temperature of a cup if it absorbs radiation at the same rate as it emits it. [1 mark]
Show answer and marking points
Answer: (a) The matt black cup. (b) A matt black surface is a better emitter of infrared radiation, so it loses energy faster than a shiny one. (c) It stays steady.
(a) the matt black cup (1)
(b) matt black surfaces are better emitters of infrared (1)
(b) so it radiates energy away faster (1)
(c) it stays the same (1)
Exam-style question 24 marks
The Earth is warmed by the Sun.
(a) Explain why the temperature of the Earth is roughly steady. [2 marks]
(b) Explain how an increase in greenhouse gases changes this. [2 marks]
Show answer and marking points
Answer: (a) The Earth absorbs radiation from the Sun and emits infrared, and the temperature is steady when the two rates are equal. (b) Greenhouse gases absorb some of the outgoing infrared, so less escapes and the Earth warms until the rates match again.
(a) the Earth absorbs radiation and emits infrared (1)
(a) steady when the rates are equal (1)
(b) greenhouse gases absorb outgoing infrared (1)
(b) the Earth warms until the rates match again (1)
Common mistakes
Saying only hot objects emit radiation
Every object emits infrared, even a cold one. Hotter objects emit more.
Saying shiny surfaces are good emitters
Shiny surfaces are poor emitters and poor absorbers. Dull black surfaces are best at both.
Saying a black body emits no radiation
A perfect black body is the best emitter as well as the best absorber.
Saying an object cools whenever it absorbs radiation
It warms up if it absorbs more than it emits. It cools only if it emits more than it absorbs.
Saying greenhouse gases reflect all the Sun’s radiation away
They absorb some of the infrared that the Earth emits, so less escapes to space.
Questions students ask
Is Black body radiation on the Foundation or Higher tier?
Black body radiation is on both Foundation and Higher papers for GCSE Combined Science Physics, aimed at around grade 6.
What is a common mistake on Black body radiation?
Saying only hot objects emit radiation. Every object emits infrared, even a cold one. Hotter objects emit more.
Where are the answers to the Black body radiation questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.