Absorption & Emission of Radiation
- As an object absorbs thermal radiation it will become hotter
- As it gets hotter it will also emit more thermal radiation
- The temperature of a body increases when the body absorbs radiation faster than it emits radiation
- Eventually, an object will reach a point of constant temperature where it is absorbing radiation at the same rate as it is emitting radiation
- At this point, the object will be in equilibrium
An object will remain at a constant temperature if it absorbs heat at the same rate as it loses heat
- If, however, the object starts to absorb radiation again at a higher rate than it radiates it, then the object will heat up again
- Likewise, if it loses radiation at a greater rate than it absorbs it, then the object will cool down again
- The temperature of a body can be regulated by balancing how much incoming radiation is absorbed and emitted (or reflected)
- This is used for emergency blankets, to keep a trauma victim warm:
- Rescue teams use light-coloured, shiny emergency blankets to keep accident survivors warm
- A light, shiny outer surface emits a lot less radiation than a dark, matt (non-glossy) surface
- This keeps the patient warm, as less infrared radiation is emitted than if an ordinary blanket had been used
Emergency blankets keep a patient warm by emitting less of their body heat through it
When I used to teach absorption and emission of radiation to my GCSE students, I would model the concept of equilibrium by doing a practical. The students had to keep a beaker of water at a constant temperature of 50 degrees Celsius. They had a thermometer to monitor the temperature, and a Bunsen burner to heat the water as needed. The trick was to either put the flame at the exact right distance from the beaker or to intermittently move the flame to and from the beaker. The water has to absorb the same amount of thermal energy as it emits to find that constant temperature, to be in thermal equilibrium. Even though it's a very simple experiment, my students really enjoyed this hands-on approach to conceptual learning.