3.2.2Waves
Infrared Radiation — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 8 of 13
Section 1 of 6Emission and temperature
All objects emit
- All objects emit Every object emits infrared radiation, and every object also absorbs it from its surroundings.
- This is energy transferred by heating by radiation, which needs no medium and can cross a vacuum.
Hotter means more
- Hotter means more The hotter an object is, the more infrared radiation it emits in a given time.
Net energy flow
- Net energy flow An object that is hotter than its surroundings emits more than it absorbs, so it cools.
- A colder object absorbs more than it emits, so it warms up.
Section 2 of 6Surfaces compared
| Surface | Absorbs infrared | Emits infrared | Reflects infrared |
|---|---|---|---|
| Dull (matt) black | Very well | Very well | Poorly |
| Light-coloured, matt | Fairly poorly | Fairly poorly | Moderately |
| Shiny white or silver | Poorly | Poorly | Very well |
Section 3 of 6Everyday examples
Surface and radiation
Cars in summer
- Cars in summer A black car heats up more than a white car in the Sun because it absorbs more infrared and visible radiation.
Cooling down
- Cooling down A black car also cools faster than a white one at the same starting temperature, because black surfaces are good emitters.
Foil blankets
- Foil blankets Marathon runners wear shiny foil blankets, which reflect their infrared back towards their bodies and reduce the energy they lose.
Solar panels
- Solar panels Water heating panels have dark matt surfaces so that they absorb as much radiation from the Sun as possible.
Section 4 of 6Seeing infrared
1Object radiates
- Object radiates Warm objects emit infrared radiation, which is invisible to the human eye.
2Camera detects
- Camera detects A thermal imaging camera detects the infrared and measures its intensity.
3Image produced
- Image produced The camera displays warmer regions and cooler regions in different colours.
4Surface effect
- Surface effect A black surface looks brighter than a shiny surface at the same temperature because it emits more infrared.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Infrared radiation | Electromagnetic waves just beyond red light in the spectrum, emitted by all objects. |
| Emitter | A surface that gives out radiation. |
| Absorber | A surface that takes in radiation and transfers its energy to the thermal store. |
| Reflector | A surface that sends radiation back rather than absorbing it. |
| Matt surface | A dull, rough surface that scatters light and radiation. |
Section 6 of 6Exam technique
1
Saying only hot objects emit infrared. All objects emit it; hotter ones emit more.
2
Thinking a good absorber is a poor emitter. Dark matt surfaces are good at both.
3
Writing that infrared needs a medium to travel. It is an electromagnetic wave and travels through a vacuum.
4
Forgetting that shiny surfaces are poor emitters as well as poor absorbers.
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