Thermal radiation is infrared radiation, and every object emits it
Definition
Thermal radiation
The transfer of thermal energy as infrared radiation, which every object emits and which needs no material to travel through.
- Infrared is part of the electromagnetic spectrum, sitting just beyond red light at wavelengths slightly too long for the eye to detect.
- Every object emits thermal radiation all of the time, whether it is a radiator, a bowl of soup, an ice cube or you.
- Objects also absorb the radiation that lands on them, so emission and absorption are going on together at every moment.
- The hotter something is, the more radiation it emits each second, which is why you can feel the warmth of an electric fire from across the room but not the warmth of the wall behind it.
- A thermal imaging camera simply detects that infrared, which is how a survey can pick out where a house is losing energy through a poorly insulated roof.
- Emitting radiation is not the same as glowing: an object only becomes visibly red hot at very high temperatures, but it has been radiating infrared the whole time.
Radiation is the only transfer that needs no medium
Definition
Medium
Any material through which energy is transferred, such as the metal of a spoon or the air in a room.
- Conduction needs particles touching one another, and convection needs a fluid that can flow, so both need matter.
- Thermal radiation carries energy as an electromagnetic wave, so it travels perfectly well through a vacuum where there are no particles at all.
- That is how the Sun's energy reaches us across about 1.5×1011 m1.5 \times 10^{11}\ \text{m}1.5×1011 m of empty space, which no amount of conduction or convection could ever bridge.
- Like all electromagnetic waves it travels at 3×108 m/s3 \times 10^{8}\ \text{m/s}3×108 m/s in a vacuum, which is why you feel the warmth of a fire the instant it is uncovered rather than waiting for warm air to arrive.
- Radiation travels in straight lines and in all directions from a source, so stepping behind a screen cuts it off at once.
- It also passes through transparent materials such as air and glass, and this is exactly why a vacuum flask still needs silvered walls: the vacuum stops conduction and convection, but only a reflective surface deals with radiation.
Example
- Stand in front of an electric bar fire and one side of you warms almost immediately.
- The warmth is not carried by the air, because the air in the room takes far longer to warm and would rise rather than travel sideways.
- Infrared radiation from the glowing bar travels straight to your skin and is absorbed there.
- Put a sheet of card between you and the fire and the warmth stops at once, which shows the transfer is by radiation travelling in straight lines.
Common Mistake
- Do not think only hot objects radiate, because every object emits thermal radiation; a cold one simply emits less.
- Do not confuse thermal radiation with the radiation from a radioactive source, because thermal radiation is infrared and is nothing to do with nuclear decay.
- Do not say radiation "travels better" through air than through a vacuum, because it needs no medium at all.
- Do not describe an ordinary household radiator as heating the room mainly by radiation, because most of its output goes into the room by convection.
Exam technique
- If a question involves energy crossing a vacuum or empty space, the answer has to be radiation, so say so and say why.
- Use the word infrared rather than "heat rays", since the precise term is what is credited.
- When you are asked which process is at work, check the material in between first: no material means radiation, a solid means conduction, and a fluid free to circulate means convection.
- For a vacuum flask question, deal with all three processes separately rather than writing one general sentence about insulation.
Self review
- Which part of the electromagnetic spectrum is thermal radiation?
- Which objects emit thermal radiation?
- Why can thermal radiation reach the Earth from the Sun when conduction and convection cannot?
- Give the speed at which thermal radiation travels through a vacuum.
- Why are the walls of a vacuum flask silvered as well as evacuated?
