Heat does not always need something to travel through. A hot body glows with infrared waves that cross empty space and warm whatever they strike. Follow the waves from emitter to absorber, and see how the black-body spectrum and the Stefan–Boltzmann law describe how much energy is radiated.
Radiation is the transfer of heat by electromagnetic waves. Unlike conduction, which passes energy atom to atom, or convection, which carries it in a moving fluid, radiation needs no material at all — it can cross a perfect vacuum. Everything warmer than absolute zero continually gives off these waves, most of them in the infrared.
Radiation is why you feel the Sun's warmth through 150 million kilometres of empty space, why a fire heats your face from across the room, and why a thermos flask uses a vacuum and a silvered surface to keep drinks hot. It is the only one of the three heat-transfer mechanisms that works without a medium.
Every object above absolute zero emits electromagnetic radiation. The hotter it is, the more energetic the waves and the more power it pours out. A modest rise in temperature produces a large jump, because radiated power depends on the fourth power of temperature.
The waves are electromagnetic, so they travel at the speed of light and pass straight through empty space. That is why the space between the emitter and absorber here is drawn as a vacuum: nothing has to be there for the heat to cross.
When the waves strike the cooler absorber, its surface soaks up their energy and warms up. Dark, matte surfaces absorb far more than shiny ones, which is why a black object heats faster in sunlight than a silvered one.
Each body radiates across a broad range of wavelengths, shaped like a hump. As it gets hotter the hump grows and its peak slides toward shorter wavelengths, which is why heated metal glows first dull red, then orange, then bright white.
Both bodies keep radiating all the time, so what matters is the difference. The hot body emits more than it receives and the cold one the reverse, so the net flow is always from the hotter object to the colder one, until they reach the same temperature.
Conduction and convection both need matter to carry energy, but radiation does not. It travels as electromagnetic waves — mostly infrared — which can cross empty space, which is why we feel the Sun's warmth through space.
It says the power a body radiates is proportional to its surface area and to the fourth power of its absolute temperature, written P = ε σ A T⁴. Because T is raised to the fourth power, a modest rise in temperature gives a large increase in radiated power.
It shows how the radiated energy is spread across wavelengths. Every object emits a broad hump, and as the object gets hotter the hump becomes taller and its peak shifts toward shorter wavelengths — which is why hot metal glows first red, then orange, then white.
Yes. Any object above absolute zero emits radiation. What matters is the balance: a colder body radiates less than the warmer one facing it, so the net flow of energy is always from the hotter object to the colder one.
Good absorbers are also good emitters, and dark, matte surfaces absorb (and emit) far more radiation than shiny, reflective ones. That is why a black car heats up more in the sun, while a silvered surface stays cooler.