AQA Physics 4.6.3.1–4.6.3.2 · Physics only
Infrared emission, absorption and black bodies
Everything around you is giving off infrared, even the ice in your freezer. Find out how temperature sets how much a body emits and at what wavelengths.
Triple Science · Higher tier
Start here
Switch on the toaster.
Look down into a toaster as you switch it on. The wires are dark at first, and they get hotter and hotter.
If you had to guess, as they get hotter, do the wires start to glow, or do they stay dark?
Temperature and emission
Turn up the temperature. Watch the curve.
All bodies, no matter what temperature, emit and absorb infrared radiation. The hotter the body, the more infrared radiation it radiates in a given time. The intensity and wavelength distribution of the emission both depend on the temperature.
TEMPERATURE
1000 °C
PEAK
2.3 µm · infrared
Orange. More of the visible band is emitted, and more at shorter wavelengths. The peak is still in the infrared.
Earlier temperatures stay on the graph as thin lines. Every cooler curve sits inside the hotter one: more at every wavelength, with the peak further left.
Perfect black bodies
The best absorber is the best emitter.
A perfect black body absorbs all of the radiation that falls on it. It does not reflect or transmit any. Because a good absorber is also a good emitter, a perfect black body is the best possible emitter.
A hollow box with a tiny hole behaves almost like one: radiation that gets in bounces around inside until it is absorbed.
Real surfaces, from the infrared practical: matt black is the best emitter and absorber; light, shiny surfaces reflect infrared and are the poorest emitters and absorbers.
Higher · absorbing and emitting
Triple HigherConstant temperature means the rates are equal.
Absorbs faster than it emits
Its temperature rises.
Emits faster than it absorbs
Its temperature falls.
Same rate both ways
Its temperature stays constant. It is still absorbing and emitting.
The temperature of the Earth depends on many factors, including the rates of absorption and emission of radiation, and the reflection of radiation into space.
Some of the Sun's radiation is reflected straight back into space. The rest is absorbed and warms the Earth, which emits infrared. Some of that infrared is absorbed by gases in the atmosphere and emitted back towards the surface. If the Earth absorbs radiation faster than it loses it, its temperature rises until the two rates match again.
Think again · cold objects
“Cold things don't give off any radiation.”
Your call
ChooseAn ice cube at −10 °C sits in a freezer. Does it emit infrared radiation?
Sort
When a body gets hotter: true or false?
Place each statement.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- Say how the amount and the wavelengths change. Both, every time.
- Explain
- Link the temperature change to the emission, then to what you see.
- Interpret
- Read a curve: which is hotter, where the peak is, whether it is in the visible band.
Key fact
All bodies emit and absorb infrared. The hotter the body, the more it radiates in a given time, with the distribution shifted to shorter wavelengths. A perfect black body absorbs all radiation and is the best possible emitter.
Key note · AQA 4.6.3.1–4.6.3.2
Infrared emission, absorption and black bodies
- All bodies, whatever their temperature, emit and absorb infrared radiation.
- The hotter the body, the more infrared it radiates in a given time.
- Intensity and wavelength distribution depend on temperature: hotter shifts the distribution to shorter wavelengths.
- A glowing metal’s peak is still in the infrared. The Sun (about 5500 °C) peaks in the visible.
- Perfect black body: absorbs all incident radiation; good absorber = good emitter; best possible emitter.
- Higher: constant temperature means absorbing and emitting at the same rate. Earth’s temperature depends on absorption, emission and reflection into space.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask MrBadmus
Muddled about why white-hot isn't the peak?
The emission curves are calculated for a perfect black body at each temperature and scaled so the hottest curve shown fills the graph. Real objects emit less than a black body at the same temperature.