Conduction and convection both need matter to carry the energy along — so how does the Sun’s warmth cross 150 million kilometres of near-empty space to reach us? Through thermal radiation: energy carried by electromagnetic waves that need no medium at all. Every object emits it, and how much comes down to just two things — how hot it is and what its surface is like.
📚 What you need to know
Thermal radiation is energy transferred by electromagnetic waves (mainly infrared) — it needs no medium, so it travels through a vacuum.
It’s how the Sun’s energy reaches Earth across empty space.
Every object above 0 K emits thermal radiation; the hotter it is, the more it emits — and the shorter its peak wavelength.
Surface matters: matt black surfaces are good emitters and absorbers; shiny / light (silver, white) surfaces are poor emitters and absorbers but good reflectors.
A black body is a perfect emitter and absorber (an idealisation); its radiation spectrum depends only on temperature.
The Leslie cube shows that different surfaces at the same temperature emit different amounts.
Energy that needs no medium
Thermal radiation is completely different from the other two transfer methods. It doesn’t hand energy from particle to particle, and it doesn’t ride along on a moving fluid. Instead the energy travels as electromagnetic waves — largely infrared — which need nothing to travel through. That’s why radiation, and only radiation, can cross the vacuum of space.
The Sun’s energy reaches Earth as electromagnetic waves crossing empty space. Conduction and convection need matter, so neither can bridge a vacuum — only radiation can.
A quick way to keep the three methods straight: conduction passes energy through matter without the matter moving; convection moves energy by the matter itself flowing; radiation sends energy as electromagnetic waves, no matter required. Only the last works across empty space — which is the whole reason you can feel the Sun.
Every object radiates — hotter means more
It’s not just hot things: every object above absolute zero is constantly emitting thermal radiation, you included. What changes with temperature is how much and at what wavelengths. Heat something up and it radiates more energy overall, and the peak of its emission shifts to shorter wavelengths.
The black-body radiation curve. A hotter object emits more at every wavelength, has a larger total, and peaks at a shorter wavelength than a cooler one.
This is exactly why a heated poker glows dull red, then orange, then white as it gets hotter — the peak is sliding from infrared through the visible spectrum. A perfect emitter and absorber is called a black body, and its spectrum depends only on temperature. Two upcoming pages put numbers on this curve: its total power (Stefan–Boltzmann) and its peak wavelength (Wien’s law).
It’s all about the surface
At the same temperature, two objects can radiate very differently — it depends on the surface. A matt black surface is a good emitter and a good absorber. A shiny or light-coloured surface (silver, white) is a poor emitter and poor absorber, but a good reflector. And crucially, good absorbers are always good emitters — the two go together.
Same temperature, different surfaces: the matt black object radiates strongly, while the shiny silver one radiates weakly and reflects instead — the result a Leslie cube is built to demonstrate.
This principle is everywhere once you spot it. The cooling fins on the back of a fridge or an old radiator are often matt black to emit heat efficiently. A vacuum flask and the foil on a spacecraft are shiny to reflect radiation and keep heat where it’s wanted. And light-coloured clothing keeps you cooler in summer by reflecting the Sun rather than absorbing it.
Worked examples
WE 1
The space between the Sun and Earth is very nearly a vacuum. Explain how the Sun’s energy still reaches us, and why conduction and convection cannot do it.
Thermal radiation travels as electromagnetic waves, which need no medium.So the radiation crosses the vacuum of space freely and reaches Earth.Conduction needs particles in contact; convection needs a moving fluid — both need matter.A vacuum has no matter, so only radiation can cross itThis is the defining feature of radiation among the three transfer methods.
WE 2
Two identical cans of hot water — one painted matt black, one polished shiny silver — are left to cool. Which cools faster, and why?
A matt black surface is a better emitter of thermal radiation than a shiny one.So the black can radiates its energy away more quickly.The matt black can cools fasterThe same reasoning, run backwards, means the black can would also heat up faster if placed near a radiant source — good absorbers are good emitters.
WE 3
As a metal bar is heated it glows dull red, then orange, then bright white. Explain what’s happening in terms of thermal radiation.
As the temperature rises, the bar emits more radiation at every wavelength — so it looks brighter.The peak wavelength also shifts shorter, moving from infrared into the visible.First it reaches red, then orange, then emits across the whole visible range at once.Hotter → more radiation and a shorter peak wavelength → red → white“White hot” is genuinely hotter than “red hot” — the colour is a thermometer. Wien’s law makes this exact.
🔧 Thinking about thermal radiation
Vacuum in the way? Only radiation crosses it — conduction and convection need matter.
Hotter object? More radiation emitted, and a shorter peak wavelength.
Surface: matt black = good emitter & absorber; shiny / light = poor emitter, good reflector.
Pairing: good absorbers are good emitters (and poor absorbers are poor emitters).
Design goal: to lose heat fast, go matt black; to keep it, go shiny.
temperature ↑
emits
more radiation
peak shifts
shorter wavelength
Quick recap: Thermal radiation is energy carried by electromagnetic (infrared) waves, needing no medium — the only method that crosses a vacuum, and how the Sun warms Earth. Every object above 0 K emits; hotter means more radiation and a shorter peak wavelength. Surfaces matter: matt black is a good emitter and absorber, shiny/light is a poor emitter but good reflector. A perfect emitter and absorber is a black body.
💡 Top tips
Radiation is the vacuum one. If space or a gap with no matter is involved, it must be radiation.
Good absorber = good emitter. Matt black does both well; shiny does both poorly (but reflects well).
Hotter = brighter and bluer. More total radiation and a shorter peak wavelength.
Everything emits. Not just hot objects — anything above absolute zero, including you (in infrared).
Black body = perfect emitter/absorber. An idealisation that stars and the curve on this page approximate.
⚠ Common mistakes
Thinking radiation needs a medium — it doesn’t; that’s its whole point
Saying only hot objects radiate — every object above 0 K emits thermal radiation
Treating shiny surfaces as good emitters — they’re poor emitters but good reflectors
Forgetting that a good absorber is also a good emitter (and vice versa)
Confusing the three methods — only radiation crosses a vacuum
You now have the qualitative picture: what thermal radiation is, that hotter objects emit more at shorter wavelengths, and how surfaces change it. Next we make it quantitative — how much power a hot body radiates in total, how the spectrum’s peak pins down its temperature, and how these ideas let us measure stars. Coming up: luminosity, the Stefan–Boltzmann law and Wien’s displacement law.
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