Lesson focus: All objects emit and absorb infrared radiationElectromagnetic waves given off by all objects, more by hotter ones, that transfer energy by heating without needing particles., and net transferThe overall amount moved once you subtract what moved back the other way. depends on temperature and surface properties.
Explain thermal radiation and evaluate surface and insulation evidence.
Build on earlier learning
RefractionLight travelled through space as a wave; infrared radiation is another electromagnetic transfer pathway.
Practical
Radiation and insulation
- Fill a black can and a shiny can with the same volume of hot water.
- Measure the temperature of each every minute for ten minutes.
- The black can cools faster because it emits infrared radiation more quickly.
Record: Cooling curves for both cans on the same axes.
Safety: Take care with hot water.
Cooling curves
Two cans start with the same volume of water at 80 °C. Move the time slider to compare how fast each cools.
Illustrative values. A matt black surface emits infrared radiation faster than a shiny surface at the same temperature.
You will be successful when you can…
- Describe infrared transfer.
- Explain emission and absorption.
- Compare dull and shiny surfaces.
- Interpret cooling evidence cautiously.
Words that carry the science
- infrared radiation
- Electromagnetic radiation involved in thermal energy transfer.
- emitter
- An object that sends out radiation.
- absorber
- A surface that takes in incident radiation.
- reflector
- A surface that redirects incident radiation.
- net transfer
- The overall transfer after opposite-direction transfers are combined.
Radiation needs no particles
01Radiation needs no particles
Every object above absolute zero emits electromagnetic radiation. At everyday temperatures, most thermal radiation is infrared radiationInvisible rays given off by warm objects that carry heat energy, like the heat you feel from a fire..
Radiation can travel through a vacuum, so it is the pathway by which energy reaches Earth from the Sun.
02Objects emit and absorb simultaneously
An object receives radiation from its surroundings while emitting its own. net transferThe overall amount moved once you subtract what moved back the other way. is outward when the object is hotter than its surroundings.
As temperatures approach equality, the opposing transfers balance and net transfer tends toward zero.
03Surface finish changes transfer rate
Dark, matt surfaces are usually strong absorberA surface that takes in heat radiation well, such as a dull black surface. and emitterAn object that gives out radiation, such as heat or light. of infrared. Shiny metallic surfaces are poor absorbers and emitters but good reflectorA surface that bounces heat or light away, such as a shiny silver surface..
Colour in visible light and infrared behaviour are related imperfectly; controlled measurements matter more than labels such as “black”.
Select a vacuum-flask surface
Explain why the walls facing the vacuum are silvered.
- The vacuum suppresses conduction and convection.
- Radiation can still cross the vacuum.
- A shiny metallic surface is a poor infrared emitter and absorber.
- It also reflects much incident infrared, reducing net transfer.
How to make the reasoning visible: Silvering targets the only transfer pathway that readily crosses the vacuum.
Radiation and insulation
Compare equal containers with identical water and surroundings.
| Surface | Start / °C | After 10 min / °C | Drop / °C |
|---|---|---|---|
| matt black | 80 | 59 | 21 |
| dull white | 80 | 64 | 16 |
| shiny metal | 80 | 70 | 10 |
| shiny + lid | 80 | 74 | 6 |
Which surface shows the lowest overall transfer rate?
Why cannot row four prove that shininess alone caused its result?
Check your understanding
Answer from memory, then check against the suggested answer.
Can radiation cross a vacuum?
Best infrared emitters?
What is net transfer?
Why can a foil emergency blanket reduce cooling?
Why does a cold object still emit infrared?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A matt black can and a shiny metal can, filled with the same hot water, cool in the same room.
Both cans transfer energy by infrared radiation. Explain why both lose energy.
(2)Explain why the matt black can cools faster.
(3)Total for this question: 5 marks
Command word: State the two-way radiation process before comparing the net rates.