The UK site for KS3 and KS4 Physics

KS4 Waves: Black body radiation
In this section we find out more about how objects (also known as "bodies") absorb and emit radiation, and we discover that all objects are doing this all of the time, even objects that we would not think of as being particularly warm!

Emission and absorption of Infra Red Radiation

Here is a simple fact - all objects (bodies), no matter what their temperature, absorb AND emit electromagnetic radiation.

By "Objects" we mean, literally any object eg you, me, a book, a bottle etc. Objects are also known as "bodies".

And by "Electromagnetic Radiation", we mean any of the many types that make up the wide electromagnetic spectrum eg Infra Red radiation, Visible light etc.

For example, if you place a book in a closed, window-less room it will absorb radiation from other objects in the room, but at the same time it will emit radiation into the room.

If the book absorbs more radiation than it emits then its temperature will....increase.
If the book emits more radiation than it absorbs then its temperature will....decrease.
If the book absorbs at the same rate as it emits then its temperature will....remain constant.

If the book does become hotter (ie its temperature has increased) then the rate at which it emits or radiates increases; ie hotter objects radiate more than colder objects in a given time.

Notice how this fact is what eventually causes the book to stop getting hotter and for its temperature to become constant.

Look at the following illustration. At first the book absorbs more radiation than it emits so it gets hotter.

But because it has got hotter, it will emit more radiation:

It makes sense, doesn't it, that objects must be emitting and absorbing radiation continuously; if an object didn't then it would either get hotter and hotter endlessly, or get colder and colder. For objects to reach a constant temperature they have to be both emitting and absorbing all the time.

Object surface colour and Radiation

Objects do not emit/absorb radiation equally.
It turns out that it is the surface colour of an object that is the critical factor in making it a good/bad absorber/emitter.
White or shiny surfaced objects are poor absorbers of radiation (they tend to reflect more of it instead). This is why people choose to wear white or at least bright coloured clothing in the summer; they stay cooler if they do.
Black, especially matt black surfaced objects, are good absorbers of radiation.

In the following illustration a radiant heater element (a source of infra red radiation) is placed equi-distant between two identical thin metal pieces to the outside of which are attached, by wax, two identical coins. The only thing different in this "symmetrical" arrangement is the inside surface colour of the metal pieces; one is painted matt black and the other is shiny (or gloss) white.

So what happens after the heater element is turned on?
Very quickly we notice that the wax holding the coin on the metal whose inside surface is matt black, begins to melt and the coin slides down the metal to the bottom ie it falls off!
The other coin, stays firmly in place for a long time afterwards.

This simple experiment proves that:
Black surfaces are good absorbers of infra red radiation, and
White (or shiny metal) surfaces are poor absorbers of infra red radiation.

What about emitters of radiation? Which of the surface types are best/worst for emission?
To answer this question we need to do another simple experiment. For one experiment, have a look at the following diagram:

The idea is - the two cubes are identical in every way except for their outside surfaces; one has a matt black outer surface whilst the other has a shiny metal or a shiny white outer surface.
The cubes are filled with identical amounts of boiling water and thermometers are inserted as shown; temperature readings of both thermometer are taken every 30 seconds or so.
After, say 15 minutes, the temperature values can be plotted against time.

The graph shows that the Matt Black cube cools fastest, meaning that it is the better emitter of infra red radiation.

This simple experiment proves that:
Black surfaces are good emitters of infra red radiation, and
White (or shiny metal) surfaces are poor emitters of infra red radiation.

Or, putting our 2 newly learned statements together, we can learn instead:
Black surfaces are BOTH good absorbers and good emitters of I.R radiation, and
White (or shiny metal) surfaces are BOTH poor absorbers and poor emitters of I.R radiation.

A few quick questions:
1. What colour would you choose for the outside of a kettle if you want it to keep the water inside as hot as possible for as long as possible, give a reason?
Answer: White (or the shiny base metal), because white or shiny metal surfaces are best for making the object a poor emitter of I.R radiation; this will help to keep the water inside hot.

2. What colour car would you choose if you lived in a very hot sunny country and did not have air conditioning, give a reason?
Answer: White (or a shiny silver colour), because these are best for making the object a poor absorber of I.R radiation, keeping it cooler for as long as possible.

3. You live up above the arctic circle; you build a cabin from wood (a good insulator) but what colour would you choose to paint the outside of the wood, give a reason?
Answer: Black (preferably matt black) because this is best for absorbing radiation in order to maximise the heat gained from the Sun.

Now its time for you to have a go at a few questions.

Some objects (or bodies, remember) are such good absorbers and emitters of radiation that they are known as "perfect black body radiators". We will learn more about these in the next section.

Perfect black body radiators and radiation

Some facts about perfect black body radiators

1. A perfect black body is an object that absorbs all of the radiation incident on it.
2. A perfect black body does not reflect any radiation or allow any to pass through (to be transmitted).
3. Since, as we learnt in the previous section, a good absorber is also a good emitter, a perfect black body is a perfect absorber and a perfect emitter.

A practical attempt to make a perfect black body radiator might look like the following:

ie it is a large enclosed space with a very small opening.
Any light (radiation) entering the hole is unlikely to be reflected back out since it will be absorbed at the internal surfaces of the body even if some internal reflection takes place as shown.
So, the object will be a nearly perfect absorber, and as such, it will also be a nearly perfect radiator.

The radiation emitted

Have you ever wondered how Astronomers are able to tell us that "...this star has a surface temperature of..X degrees celsius" and "this star has a surface temperature of ...Y degrees.." ?
Furthermore, they tell us these facts even though they have never been anywhere close to the star to measure its surface temperature.
How do they know these temperatures?

It is because they know that there is a link between the Temperature of an object and the Wavelength of the radiation that it emits.

Put simply - as the Temperature of an object increases, the Wavelength of its emitted radiation decreases.

Astronomers know the exact link between Temperature and Wavelength and so by measuring the wavelength of the radiation emeitted by the object, they can tell us for example, that the surface temperature of our Sun is about 6,000 degrees celsius, whilst the surface temperature of the distant star Rigel is over 12,000 degrees.

Put a little less simply - objects don't just emit one wavelength of radiation; they emit a range of wavelengths but the peak of this range shifts as stated above.
So a more accurate statement would be: as the Temperature of an object increases, the PEAK of its emitted range of Wavelengths decreases.

It should also not be a surprise that the intensity of the radiation emitted by an object should increase as the temperature of the object increases (ie they are proportional to each other) eg hotter objects emit more intense radiation. The intensity of the radiation from the star Rigel is far greater than from our Sun.

The following graph illustrates how the peak Intensity increases with Temperature, but the peak Wavelength decreases with Temperature, and it illustrates how objects always emit a range of wavelengths.

Temperature of Radiation and Colour

At ambient temperatures (ie normal, everyday temperatures) the range of the emitted radiation from an object will be in the infra red region of the electromagnetic spectrum, so it is invisible and hence has no colour.
But as the temperature of objects rise, their wavelengths shift towards shorter wavelengths, towards the range of the visible spectrum. Eventually at around 1000K (1000 kelvin or 1273 degrees celsius) the wavelength of "red light" is reached and so the object emits radiation of the colour red.

You have probably heard the term "red hot" being used to indicate that an object is extremely hot. Well it is, but not as hot as objects that emit orange or yellow or white light! To produce, for example, yellow light an object needs to be at a temperature of around 2800K which is almost 3 times the temperature to produce red light!

To return to our star example: the super hot star Rigel has a bluish tint since its range of wavelengths have reached well into the UV region of the EM spectrum; by comparison our relatively cool Sun has a yellowish-white colour (though its "range of wavelengths" will still extend into the UV region).

The following diagram illustrates the simple relationship between the Temperature of an object and the "colour" of the radiation emitted at its peak wavelength

Constant Temperature

We have already explained and illustrated the idea of "constant temperature" above in the previous section, but the following is a quote from the Specification for this section:
"A body at constant temperature is absorbing radiation at the same rate as it is emitting radiation. The temperature of a body increases when the body absorbs radiation faster than it emits radiation."

Solar Radiation, the Earth's Atmosphere and its Temperature

The atmosphere is a layer of gases, which we call "air", that surround the Earth. It is held in place by the Earth's gravity.
Without the atmosphere there would be no life on Earth (or at least not life like we know it):

It is the final point about the atmosphere and the warming of the Earth that we want to focus on here.

The following diagram shows what happens when typical solar radiation reaches the Earth's atmosphere.

As you can see, the solar radiation is a combination of short wavelength IR, visible and a range of UV electromagnetic radiation.
The atmosphere effectively blocks, by absorbing, much of the dangerous UV radiation (not all of it, otherwise we wouldn't need sun creams)
The short wavelength IR and the visible radiation passes through the atmosphere and reaches the earth's surface causing it to warm up.
As we know by know, all objects emitt as well as absorb radiation, so we are not surprised to learn that the warmed earth radiates, but since it is so much cooler than the Sun, its IR radiation is of a much longer wavelength which is not able to pass back up through the atmosphere; instead it gets absorbed by certain atmospheric gases (known as "greenhouse gases", water vapour, carbon dioxide and methane) which, in turn, re-radiate some of the heat back towards the earth.

This whole process, known as the earth's natural greenhouse effect, is a "good thing" because in its natural balanced state it has enabled the earth to warm up to a steady temperature establishing the conditions for life as we know it.

Human activities, however, mainly the burning of fossil fuels (eg. in cars and power stations) and clearcutting of forests, have accelerated the greenhouse effect due to the release of additional carbon dioxide, causing global temperatures to increase, leading to climate change. The release of additional methane due to the cultivation of farming livestock eg cows, is also believed to have contributed to climate change.

So, to summarise the factors affecting the Temperature of the Earth:

There are many factors, all interlinking, including -
The rate at which radiation from the Sun is absorbed by the Earth's surface and its atmosphere.
The rate at which some of this absorbed radiation is re-emitted by the Earth's surface and its atmosphere.
The concentration of the greenhouse gases, water vapour, methane and carbon dioxide ("additional" amounts increase their ability to absorb radiation and so will tend to raise the temp of the Earth).

Now its time for you to have a go at the final set of questions on the whole of the Waves topic.