5.3.2Light and the Electromagnetic Spectrum
Radiation and Temperature Change
5 min read 5 parts Lesson 10 of 11 Unit quiz
Section 1 of 5Absorbed and Emitted Power
Warming
- If an object absorbs radiation at a higher rate than it emits, its internal energy and temperature increase.
Cooling
- If an object emits radiation at a higher rate than it absorbs, its internal energy and temperature decrease.
Steady temperature
- When the rates are equal, the temperature remains constant even though radiation continues to flow in and out.
Emission grows with temperature
- As an object warms it emits more, which makes it more likely to reach a steady state.
Section 2 of 5Reaching Equilibrium
1
A cool object is placed in sunshine and begins absorbing more radiation than it emits.
2
Its temperature rises, and as it warms it emits more radiation every second.
3
The gap between absorbed and emitted power gets smaller as the temperature climbs.
4
Eventually emission equals absorption and the temperature stops rising.
5
If the incoming radiation later increases or decreases, the object settles at a new equilibrium temperature.
Section 3 of 5Surfaces and Colour
Surfaces and Colour
Matt black
- A dull black surface is the best absorber and the best emitter of infrared radiation.
Shiny silver
- A polished light surface reflects most radiation, so it is the poorest absorber and emitter.
Reflection
- The radiation that is reflected does not warm the object, which is why white houses stay cooler in hot climates.
Thermos flask
- Silvered walls reduce radiation across the vacuum gap, helping the contents keep their temperature.
| Surface | Absorbs | Emits |
|---|---|---|
| Matt black | Very well | Very well |
| Matt white | Moderately | Moderately |
| Shiny silver | Poorly | Poorly |
Section 4 of 5The Earth's Temperature
Daytime
- The Earth absorbs radiation from the Sun, so the surface warms.
Night
- The Earth continues to emit infrared radiation into space while receiving none from the Sun, so it cools.
Atmosphere
- Gases such as carbon dioxide and methane absorb some of the infrared radiation leaving the surface and send part of it back.
Average temperature
- Over a long time the average temperature stays steady only when absorbed and emitted power are balanced.
Change
- More greenhouse gases reduce the rate of emission to space, which pushes the equilibrium temperature higher.
Section 5 of 5Exam Checklist & Tips
5.3.2 – Key Points for the Exam
- Absorbed power The rate at which an object takes in radiation energy.
- Emitted power The rate at which an object gives out radiation energy.
- Equilibrium temperature The steady temperature reached when absorbed and emitted power are equal.
- Reflector A surface that sends most incident radiation back instead of absorbing it.
- Greenhouse gas A gas in the atmosphere that absorbs infrared radiation and re-emits some of it back to the surface.
Common Mistakes
- Saying that the object stops radiating at equilibrium; it still emits, but the same amount that it absorbs.
- Thinking shiny surfaces are the best emitters; they are the worst emitters.
- Assuming the Earth's temperature stays constant because it emits no radiation at night.
- Forgetting that darker surfaces warm up faster in sunshine because they absorb more.