Every object emits and absorbs infrared, perfect black bodies, how temperature changes the radiation, and the balance that sets the Earth's temperature.
What you will learn
- Explain that all bodies emit and absorb infrared radiation
- Describe how the radiation emitted depends on temperature
- Explain how the balance of absorbing and emitting sets a temperature, including the Earth's (Higher)
Everything glows a little
All objects emit and absorb infrared radiation, whatever their temperature: a block of ice does too.
A perfect black body absorbs all the radiation that falls on it. It reflects and transmits none. A good absorber is also a good emitter, so a perfect black body is also the best possible emitter.
You can compare real surfaces with a Leslie cube: a metal cube of hot water whose four sides are matt black, shiny black, matt white and shiny silver. An infrared detector held the same distance from each side shows matt black emits most and shiny silver least. The water temperature and the distance are control variables. A detector gives a measured reading, not an opinion (as a hand held near each side would), so the results can be compared.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Everything glows a little
- Hotter means more, and shorter
- Staying at one temperature (Higher)
- Try it: The Leslie cube
10 practice questions, marked as you go; about 25 minutes in all.
Key terms
Perfect black body, Earth
In this unit: Lenses, visible light, sound and black bodies
Reflection and the glass block practical, total internal reflection and optical fibres, colour and filters, lenses and magnification, sound and echoes, ultrasound and seismic waves, and black body radiation (Physics only).