All bodies emit radiation

1. Every object, whatever its temperature, emits electromagnetic radiation.
2. The particles in every object are constantly moving, and this motion causes the object to give out electromagnetic radiation.
3. Radiation emitted by an object because of its temperature is called thermal radiation.
4. Objects at everyday temperatures, such as you, a cup of tea or a brick wall, emit mainly infrared radiation.
5. Infrared is invisible to the human eye, which is why most objects around you do not appear to glow even though they are emitting radiation all the time.
6. Thermal imaging cameras detect this infrared, which is how they can show people and animals in complete darkness.
7. Even very cold objects, such as an ice cube or a comet far from the Sun, still emit some radiation.

A continuous range of wavelengths

1. A hot object does not emit radiation of just one wavelength.
2. Instead it emits a continuous range of wavelengths and frequencies, with no gaps between them.
3. The different wavelengths carry different amounts of energy, with shorter-wavelength, higher-frequency radiation carrying more energy.
4. The radiation is not spread evenly across this range, because some wavelengths are emitted much more strongly than others.
5. A graph of intensity against wavelength shows a smooth curve that rises to a peak and then falls away.
6. The wavelength at the top of the curve is the peak wavelength, where the object emits most strongly.
7. The curve falls steeply on the short-wavelength side of the peak and more gradually on the long-wavelength side.

Hotter bodies emit more intensely

1. As the temperature of an object rises, it emits more radiation at every wavelength.
2. The whole intensity curve therefore becomes taller, and the area under it, which represents the total energy emitted each second, becomes much larger.
3. A small rise in temperature produces a large rise in the total radiation emitted, so hot objects transfer energy by radiation far faster than cool ones.
4. You can feel this near an electric heater, where the radiation reaching your skin becomes much stronger as the heating element warms up.

Hotter bodies emit shorter wavelengths

1. As the temperature rises, the peak of the curve moves to shorter wavelengths and higher frequencies.
2. A greater share of the radiation is therefore emitted in the visible part of the spectrum, and at very high temperatures in the ultraviolet.
3. This explains the changing colour of a steel bar heated in a forge.
4. At first the bar emits only infrared, so you can feel its warmth but it does not glow.
5. At around $500{}^{\circ }C$ the bar emits enough red light to glow a dull red.
6. As it gets hotter it glows orange and then yellow, and at very high temperatures it looks white because it emits strongly across all the visible colours.
7. Stars follow the same pattern, so the colour of a star tells astronomers about the temperature of its surface.
8. Cool stars, with surfaces at about $3000K$, look red, the Sun at about $5800K$ looks yellow-white, and the hottest stars, at $20000K$ or more, look blue.