AQA GCSE Physics Foundation (8463), Foundation tier · Waves › Black body radiation
Practise Perfect black bodies and radiation. 4 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How the intensity and wavelength distribution of the radiation a body emits depend on its temperature. GCSE Physics only, not in Combined Science.
Key facts
All bodies emit radiation
Intensity and wavelength distribution depend on temperature
Hotter → more radiation at every wavelength; the peak moves to a shorter wavelength
Sun (very hot): lots of visible light. Earth (cooler): mainly infrared
Notes
Emission depends on temperature
diagram
All bodies emit radiation. The intensity (how much is emitted) and the wavelength distribution (how it is spread across the wavelengths) depend on the temperature of the body.
As a body gets hotter it emits more radiation at every wavelength, and the intensity at shorter wavelengths increases the most.
So the peak of an intensity–wavelength graph is higher, and at a shorter wavelength, for a hotter body.
As the temperature rises, the peak gets higher and moves to shorter wavelengths.
Example: a metal rod being heated at first emits infrared that you can feel but not see; as it gets hotter it glows dull red, then orange-yellow and finally white.
The Sun is very hot, so it emits a lot of visible light. The Earth is much cooler, so it emits mainly longer-wavelength infrared.
How to answer each type of question
Describe how the radiation emitted changes with temperature
2 marksGrade 5
Say the total radiation (intensity) increases at all wavelengths.
Say the peak moves to shorter wavelengths (a greater proportion at short wavelengths).
Example. A metal rod is heated in a flame. It starts to glow red and later glows yellow-white. Describe how the radiation emitted by the rod changes as its temperature increases.
Show the model answerHide the model answer
The rod emits more radiation (a greater intensity) at all wavelengths (1). A greater proportion is emitted at shorter wavelengths, so the peak wavelength decreases and its colour changes from red towards white (1).
Don’t lose marks
Saying a hotter body emits 'only shorter wavelengths'. It emits more at all wavelengths; the peak moves to shorter wavelengths.
Reading the graph the wrong way round: check which axis is wavelength and which is intensity.
More tips
Memory tricks
Hotter = higher and further left: on an intensity–wavelength graph (wavelength increasing to the right) the hotter curve is higher and peaks at a shorter wavelength.
Red hot to white hot: as an object heats up, more of the shorter wavelengths are added, so the peak wavelength gets shorter.
Exam technique
When a graph is given, quote peak wavelengths or intensities from it to support your answer.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State that all bodies emit radiationEvery object emits radiation; how much, and at which wavelengths, depends on its temperature.
Grade 5
Describe how emission changes with temperatureA hotter body emits more radiation at every wavelength, and relatively more at shorter wavelengths.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Scientists use the idea of a perfect black body to describe how objects absorb and emit radiation. What is a perfect black body?
An object that absorbs all the radiation that falls on it.
State whether a perfect black body is a good or a poor emitter of radiation, for its temperature.
A good emitter (the best possible emitter for its temperature).
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Scientists use the idea of a perfect black body to describe how objects absorb and emit radiation.
(a) What is a perfect black body?[1]
(b) A perfect black body does not reflect any of the radiation that falls on it. Name one other thing that a perfect black body does not do to incident radiation.[1]
(c) Explain why a perfect black body would be the best possible emitter of radiation.[1]
(d) The intensity and the wavelength distribution of the radiation emitted by a body depend on one property of the body. Which property? Tick (✓) one box.[1]
Its mass
Its temperature
Its volume
Its weight
Show the answer and mark scheme
(a)Answer: An object that absorbs all the radiation that falls on it.
an object that absorbs all of the radiation incident on it
(b)Answer: Transmit it
transmit (it)
(c)Answer: Good absorbers are good emitters, and it is the best possible absorber.
a good absorber is also a good emitter, and a perfect black body is the best possible absorber
(d)Answer: Its temperature
Question 2Medium2 marks
A student says: ‘Any object that looks black is a perfect black body.’
Explain why the student is wrong.[2]
Show the answer and mark scheme
Answer: A perfect black body absorbs all incident radiation of every wavelength; a black-looking object only absorbs most visible light and still reflects some radiation (and may reflect or transmit infrared), so it is not a perfect black body.
a perfect black body absorbs all the radiation that falls on it, of every wavelength, and reflects or transmits none
an object that looks black absorbs most visible light, but it still reflects some radiation (e.g. a shiny black surface reflects some light) and may reflect or transmit other wavelengths, such as infrared, so it is not a perfect black body