Radiation: Heat That Travels Without a Medium
You already know that heat can travel by conduction (through a metal rod) and convection (through hot air rising). But there is a third way, and it is the strangest: radiation. A hot object can send heat across empty space, with nothing in between. The Sun heats the Earth through 150 million kilometres of vacuum — that is radiation.
Every object whose temperature is above absolute zero (0 K or −273 ∘C) emits electromagnetic waves. These are the same kind of waves as light, radio, and X-rays, but with wavelengths mostly in the infrared range for everyday temperatures. The hotter the object, the more energy it radiates, and the shorter the peak wavelength. A red-hot iron glows in the visible red; a white-hot filament glows across the visible spectrum.
Radiation does not need a medium. That is why a thermos flask works: the vacuum between the inner and outer walls stops conduction and convection, but radiation still passes — so the walls are silvered to reflect it back.
The Black Body: The Perfect Radiator
To measure how well a real surface radiates, we need a standard. That standard is the black body — an ideal object that absorbs all radiation falling on it (hence "black") and, at any given temperature, emits the maximum possible radiation at every wavelength. No real surface can beat a black body.
A black body does not have to look black. The Sun is very nearly a black body, and it looks blindingly white. The name refers to absorption, not colour.
The power radiated by a black body of surface area A at absolute temperature T is given by the Stefan–Boltzmann law:
P=σAT4
where σ=5.67×10−8 W m−2 K−4 is the Stefan–Boltzmann constant.
The T4 dependence is dramatic. Double the temperature, and the radiated power increases by a factor of 24=16.
Emissivity: How Real Surfaces Compare
No real surface is a perfect black body. Some radiate almost as well; others are terrible radiators. Emissivity (ϵ, a Greek letter epsilon) is the ratio that tells you how close a real surface is to the ideal:
ϵ=energy radiated by a black body at the same temperature per second per unit areaenergy radiated by the real surface per second per unit area
Emissivity is a pure number between 0 and 1. A black body has ϵ=1. A perfect reflector (which radiates nothing) would have ϵ=0.
For a real surface, the Stefan–Boltzmann law becomes:
P=ϵσAT4
A good absorber is also a good emitter. This is Kirchhoff's law of thermal radiation: at a given temperature and wavelength, ϵ=α, where α is the absorptivity (fraction of incident radiation absorbed). A black surface that soaks up sunlight also radiates heat efficiently at night. A shiny surface does neither well — which is why silvered vacuum flasks keep hot things hot and cold things cold.
Typical Emissivity Values
| Surface | Emissivity (ϵ) |
|---|
| Black body (ideal) | 1.00 |
| Lampblack (soot) | 0.95 |
| Water | 0.96 |
| Human skin | 0.97 |
| Brick, concrete | 0.85–0.95 |
| Polished aluminium | 0.04–0.06 |
| Polished silver | 0.02 |
Notice that skin has a very high emissivity — close to a black body. That is why infrared thermometers work: they measure the radiation your skin emits, and with ϵ≈0.97, the temperature calculation is accurate.
Why Emissivity Matters in Exams …