GCSE · Physics · AQA · Spec 8463
Black body radiation – emission and absorption (physics only)
Matt black paint or white paint: which soaks up infrared better? Most people say black straight away. Hold that thought, because white paint is about to cause trouble.
Sort the surfaces
Good or poor with infrared?
Some surfaces are good at absorbing and emitting infrared; some are poor at both. Pick a surface, then choose its box. Go with your gut first. The reasons appear as you place each one.
Still to sort
Good absorber and good emitter (0)
Soaks up infrared well, and gives it out well.
Where the line is: It's the surface that decides, not its colour. Dark and dull surfaces land here, and so does white paint.
Poor absorber and poor emitter (0)
Soaks up little infrared, and gives out little.
Where the line is: Shiny metal surfaces land here, whatever colour the metal looks.
Notice there are only two boxes. There is no box for 'good absorber but poor emitter', because a good absorber is also a good emitter.
Two cans, two tests
The cans are identical apart from their outside: one is dark matt, one is shiny metal.
Test 1: both cans sit side by side in the same sunshine. Test 2: both start equally hot and are left to cool. Which prediction is right?
The perfect black body
Reason it through
Why would a perfect black body be the best possible emitter?
First link · your turn
A perfect black body absorbs ALL the radiation that falls on it. So how much of that radiation does it reflect, or let pass through?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Why the surface that soaks up infrared best is also the one that gives it out best.
What you need to know
- All objects emit and absorb infrared radiation, whatever their temperature.
- The hotter an object is, the more infrared it emits in a given time.
- A good absorber of infrared is also a good emitter.
- Dark and dull surfaces are good absorbers and emitters; shiny metal surfaces are poor at both.
- White paint is a good absorber and emitter, so colour is not a reliable guide.
- A perfect black body absorbs all the radiation incident on it, reflects and transmits none, and would be the best possible emitter.
The big picture
Every object, whatever its temperature, emits and absorbs infrared radiation, and the hotter it is, the more infrared it gives out in a given time. Surfaces differ a lot: dark, dull surfaces are good absorbers and good emitters, while shiny metal surfaces are poor at both. White paint is a good absorber and emitter too, so the colour you see is not a reliable guide. A good absorber is also a good emitter. A perfect black body absorbs all the radiation that falls on it and reflects or transmits none, so it would be the best possible absorber and therefore the best possible emitter.
Key points
Worked example
Problem
Three identical containers of hot water start at the same temperature. Their outsides are: matt black paint, white paint and shiny metal. Which one emits infrared most slowly? What can you say about the other two?
⚠ Watch out
Judging a surface by its colour: 'dark absorbs infrared, light-coloured things don't'. White paint is a good absorber and emitter of infrared, and shiny metal is poor whatever colour it looks. Judge the surface (dull or shiny metal), not the colour.
Memory hook
Good in, good out. A surface that's good at soaking up infrared is just as good at giving it out, and a perfect black body, which soaks up everything, would be the best emitter of all. And remember: white paint breaks the colour rule.
Check yourself
A block of ice and a cup of hot tea sit on the same table. Do both emit infrared? Which one emits more infrared in a given time, and why?
Flashcards
(10)Which objects emit and absorb infrared radiation?
How does an object's temperature affect the infrared it emits?
If a surface is a good absorber of infrared, what else is it?
Name a type of surface that is good at absorbing and emitting infrared.
Polished metal: does it soak up and give out infrared well?
Is white paint a good or a poor absorber and emitter of infrared?
Can you judge how well a surface absorbs infrared from its colour?
What is a perfect black body?
How much radiation does a perfect black body reflect or transmit?
Why would a perfect black body be the best possible emitter?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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