Thermal radiation

All objects give out infrared radiation, and the hotter they are, the more they emit. Infrared is a wave, so it needs no particles and can cross the vacuum of space: that is how the Sun warms the Earth. Matt black surfaces are the best absorbers and emitters; shiny, light surfaces are the worst, because they reflect infrared instead.

Key facts

  • Radiation: all objects emit infrared; hotter objects emit more. It needs no particles.
  • Matt black surfaces are the best absorbers and emitters; shiny silver surfaces are the worst (best reflectors).

Worked examples

Example 1

Which object emits infrared radiation?

  1. All objects give out infrared radiation.
  2. The hotter an object is, the more it emits.
  3. So every object emits it, more so when hotter.

Answer: every object, more so the hotter it is

Example 2

Which way of transferring energy can work through the vacuum of space?

  1. Conduction and convection need particles.
  2. A vacuum has no particles.
  3. Radiation (infrared waves) needs no particles, so it can cross a vacuum.

Answer: radiation

Example 3

How does the Sun’s energy reach the Earth?

  1. Space between the Sun and Earth is almost a vacuum.
  2. Conduction and convection need particles, so they cannot work there.
  3. The energy travels as radiation, which needs no particles.

Answer: by infrared (and other) radiation through space

Example 4

Which surface is the best absorber of infrared radiation?

  1. Dark, dull surfaces absorb the most infrared.
  2. Shiny and light surfaces reflect it instead.
  3. So matt black is the best absorber.

Answer: matt black

Example 5

Houses in hot countries are often painted white. Why does this keep them cooler inside?

  1. The Sun heats the walls mainly by infrared radiation.
  2. Light-coloured, shiny surfaces are good reflectors and poor absorbers of infrared.
  3. Dark, matt surfaces would absorb much more and get much hotter.

Answer: white surfaces reflect most of the Sun’s infrared radiation, so the walls absorb less energy and warm up less

Example 6

Two cans hold the same amount of water at 80°C. One is painted matt black and the other is shiny silver. Which cools faster, and why?

  1. Both cans lose heat by giving out infrared radiation (and by conduction and convection to the air).
  2. Matt black surfaces emit infrared radiation much better than shiny silver ones.
  3. So the black can loses heat faster and its temperature falls faster.

Answer: the matt black can: dark, matt surfaces are the best emitters of infrared radiation

Example 7

Why are the fins on the back of a fridge painted black?

  1. The fins must get rid of heat from inside the fridge.
  2. Black surfaces are good emitters of infrared.
  3. So black fins lose heat quickly.

Answer: black surfaces are good emitters, so they lose heat quickly