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- Recognise radiation as the method of heat transfer that does not require a medium to travel through.
Radiation is the emission of energy.
A lot of radiation travels as waves – examples include visible light, UV (Ultraviolet) light, X-rays, gamma radiation, radio waves, etc.
Radiation can travel through a vacuum – that’s how we, on Earth, manage to get the Sun’s warmth and light despite being separated by the large vacuum of space. In other words, radiation doesn’t require a medium.
In the same way that light is radiated, heat can also be radiated. This makes radiation the only form of heat transfer that does not need a medium – both conduction and convection use particles to transfer heat energy from one place to the other.
- Identify infra-red radiation as the part of the electromagnetic spectrum often involved in heat transfer by radiation.
The electromagnetic spectrum is a spectrum of waves created by disturbances in both the electric and magnetic field.
This is the electromagnetic spectrum:
Only a tiny part of the spectrum is visible to the naked human eye, and we call this part visible light. The rest of the light on the spectrum is invisible to us, but can often be seen using different tools.
Infra-red light is the part of the EM spectrum that is most involved in heat transfer.
- Describe experiments to show the properties of good and bad emitters and good and bad absorbers of infra-red radiation.
Good emitters of IR (infra-red) radiation are also good absorbers.
Matt, black surfaces are the best emitters and absorbers; and shiny, light surfaces are the worst emitters and absorbers – the lighter and shinier the surface, the more light it tends to reflect.
Comparing IR emitters:
We use a device called Leslie’s cube.
The four faces on the side of the cube are different – they are usually matt black, gloss black, white and silver.
Leslie’s cube is a metal cube filled with very hot water. The water heats up the surfaces of the cube, causing them to emit IR radiation.
To compare the IR emission of the different faces, we use a thermal radiation detector. When measuring the emissions, we must make sure to hold the detector the same distance from each face we measure, so that the test is fair.
If you don’t have a thermal radiation detector, you can use wax! Stick a coin to a hard glass plane using wax. Do this four times. Place each of these wax-coin objects upright, with the coin side facing Leslie’s cube, the same distance from each of the faces. Be sure not to place the glass planes too far away, because then the experiment will take too long. Don’t use a distance greater than 10cm.
Time how long it takes the coin to fall off each plane. The face that allowed for the shortest time is the best emitter of IR radiation.
You’ll notice that the best emitter is the matt black surface, followed by gloss black, then white, then silver.
Now let’s compare IR absorbers:
Attach a thermometer to the back of four different metal plates of the same dimensions (same width, length and thickness). One will be matt black, one will be gloss black, one will be white, and one will be shiny silver. Or if you wanted to compare the two extremes, you can use just matt black and shiny silver.
Place them all the same distance from a radiant heater and turn on the heater. After a known amount of time, e.g. 5 minutes, turn the heater off. Record the temperature of each plate. The plate of highest temperature is the best absorber.
The image shown right is a picture of a radiant heater.
The best absorber will be the matt black plate, followed by gloss black, followed by white, followed by silver.
Notes submitted by Sarah.
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