Waves and space · GCSE Physics
Infrared radiation
Teacher-written GCSE Physics revision on infrared radiation: emission and absorption by surfaces, the required practical, black-body behaviour at GCSE, and how IR sits in the electromagnetic spectrum.
All objects emit infrared. Matt black is a good emitter and absorber; shiny silver is a poor emitter and a good reflector. Hotter objects radiate at a greater rate.
The important bits
What you need to know
- 1
Infrared (IR) is an electromagnetic wave, next to visible red light, used in remote controls, thermal cameras, grills and night sights. It transfers energy by radiation and does not need a medium.
- 2
The rate of infrared emission increases as the temperature of the object increases. The Sun, a fire and a person all emit IR; hotter means more power radiated.
- 3
Matt black surfaces are good absorbers and good emitters of infrared. Shiny, light surfaces are poor absorbers and poor emitters, and are good reflectors.
- 4
A good absorber looks dark because it does not reflect much visible light; the same surface also tends to be a good IR absorber. A space blanket is shiny to reduce radiative loss.
- 5
Required practical: compare how different surfaces absorb or emit IR — for example, Leslie’s cube, or identical cans with different outer surfaces cooling from the same start temperature.
- 6
Control variables: same starting temperature, same volume of water, same environment, lids on to cut evaporation. The best emitter cools fastest; the best absorber warms fastest in the Sun or in front of a heater.
- 7
Infrared is not ionising. The hazard is burns and overheating, not DNA damage of the UV/X-ray kind. Ovens and heaters use that heating on purpose.
- 8
The Earth–atmosphere energy balance uses IR: the ground emits infrared, and greenhouse gases absorb some of it. That is the same wave family as a TV remote, at different wavelengths.
Quotations worth analysing
Short evidence. Real method.
“The hotter an object is, the more infrared radiation it emits in a given time.”
Rate of emission tracks temperature. A thermal camera is a map of that idea, not a photograph of “heat fluid” leaking.
“A black, matt surface is a good absorber and a good emitter of radiation.”
Good at both jobs. Shiny silver is bad at both emission and absorption, which is why teapots and emergency blankets are shiny.
Go deeper
The cooling-can practical is a fair test of surfaces
Two cans, same size, same volume of hot water, same start temperature, one matt black, one shiny. Lids on, same room, thermometers through the lids. Plot temperature against time. The black can’s graph falls faster because it is a better emitter of infrared. Reverse the idea with cold water in front of a radiant heater: the black can warms faster because it is a better absorber. Students compare cans with different volumes or forget the lid, then blame the colour. Write the control variables in the method, and in the conclusion name infrared radiation, not “the black one likes heat”. A Leslie’s cube with a detector at a fixed distance does the emission test more cleanly: same temperature, four faces, four readings. Highest reading from the matt black face.
Go deeper
Link IR to energy transfer without stealing the nuclear word
Radiation as an energy pathway includes infrared and visible light. It is not the same word as nuclear radiation. A hot radiator still warms you across a room partly by IR even if the air is still; convection will also play if air can move. Vacuum flasks use a shiny silvered surface to cut IR, a vacuum to cut conduction and convection, and a stopper to cut evaporation. That three-way design is an insulation question wearing waves clothes. Greenhouse-effect questions on Physics or Combined energy topics: the Earth emits infrared; some gases absorb IR and reduce the rate of energy loss to space. Keep the mechanism: absorb infrared, not “pollution makes heat”. Remote controls use IR LEDs; a camera phone can often see the LED as a purple glow because sensors pick up near-IR. That is a use, not a toy fact, if you attach it to “infrared is next to visible”.
See the idea in action
Two identical 0.20 kg aluminium cans hold water at 80 °C in a 20 °C room. After 10 minutes the matt-black can is at 52 °C (Δθ = 28 °C) and the shiny can is at 61 °C (Δθ = 19 °C). Energy lost by the water in the black can ≈ mcΔθ = 0.20 × 4200 × 28 = 23 500 J; shiny can ≈ 16 000 J. The black surface emitted infrared at a greater rate, so more thermal store left the water in the same time. The shiny surface reflected and emitted poorly. If a third can had no lid, evaporation would swamp the comparison — that is why the lid is a control, not a decoration.
Exam technique
Turn knowledge into marks
Name infrared as an electromagnetic wave. For surfaces, use the pair “good emitter and absorber” versus “poor emitter and good reflector”. In practicals, list control variables and say which can cools or warms faster and why.
Common mistakes
Do not give these marks away
- 01
Saying black surfaces “attract heat” without naming infrared emission or absorption.
- 02
Comparing cans with different volumes or start temperatures, then claiming a colour effect.
- 03
Calling infrared ionising, or mixing it up with ultraviolet sunburn.
Why does a matt-black can of hot water cool faster than an identical shiny can?
ABlack paint conducts heat better through the metal
BA matt-black surface is a better emitter of infrared radiation
CShiny surfaces absorb more infrared from the room
DBlack surfaces stop convection
Show the answer
A matt-black surface is a better emitter of infrared radiation. At the same temperature, a good emitter radiates infrared at a greater rate, so the thermal store of the water falls faster. Shiny surfaces are poor emitters.
Quick questions
If this is the bit you searched
Is infrared ionising?
No. It can burn or overheat tissue, but it does not ionise atoms the way UV, X-rays and gamma rays do.
Why are firefighters’ suits and space blankets shiny?
Shiny surfaces are poor absorbers and poor emitters, and they reflect infrared, so they reduce radiative heating or cooling.
How do thermal imaging cameras work?
They detect infrared emitted by objects. Hotter objects emit more IR, so they appear brighter on the image even in the dark.
Where does infrared sit in the EM spectrum?
Between microwaves and visible red light. Longer wavelength than red, shorter than microwaves, speed 3 × 10⁸ m/s in vacuum.