4.6.3.1Emission and absorption of infrared radiation
AQA GCSE Physics Foundation (8463), Foundation tier · Waves › Black body radiation
Practise Emission and absorption of infrared radiation. 10 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
All objects emit and absorb infrared radiation, and hotter objects emit more of it. You need to know what a perfect black body is and why it would be the best possible emitter. GCSE Physics only, not in Combined Science. This links closely to the required practical on infrared radiation from different surfaces.
Key facts
All bodies emit and absorb infrared, at any temperature
Hotter body → more infrared radiated in a given time
Perfect black body: absorbs all of the radiation incident on it
A perfect black body does not reflect or transmit any radiation
Good absorber = good emitter
So a perfect black body would be the best possible emitter
Matt black: best emitter and absorber; shiny silver: worst (good reflector)
Notes
Everything emits and absorbs infrared
diagram
All bodies (objects) emit and absorb infrared radiation, whatever their temperature. Even an ice cube emits infrared.
The hotter a body is, the more infrared radiation it radiates in a given time.
The surface also matters. Dark, matt surfaces are good emitters and good absorbers of infrared. Shiny surfaces, such as polished metal, are poor emitters and poor absorbers, because they reflect most of the radiation that hits them.
A perfect black body would absorb all of it and reflect none.
Perfect black bodies
A perfect black body is an object that absorbs all of the radiation incident on it (all the radiation that hits it).
A black body does not reflect or transmit any radiation.
A good absorber is also a good emitter. So a perfect black body, the best possible absorber, would also be the best possible emitter.
A perfect black body is an ideal: some real objects come close, but none is perfect.
Everyday examples
Solar water-heating panels are matt black so that they absorb as much of the Sun's radiation as possible.
Buildings in hot countries are often painted white so they reflect more of the Sun's radiation and stay cooler.
A shiny foil blanket helps to keep a person warm: its shiny surface emits little infrared and reflects the infrared emitted by the body back towards it.
In the required practical, the matt black face of a Leslie cube emits the most infrared and the shiny silver face the least, even though they are at the same temperature.
How to answer each type of question
Compare the infrared emitted by two objects
2 marksGrade 4
Identify which object is hotter, or which has the better emitting surface.
Give the rule that explains it.
Example. Two identical metal cans contain water. One is at 80 °C and the other is at 40 °C. Which can emits more infrared radiation each second? Give a reason for your answer.
Show the model answerHide the model answer
The can at 80 °C (1). The hotter a body is, the more infrared radiation it emits in a given time (1).
Explain an everyday design choice
2 marksGrade 5
Name the surface and say whether it is a good absorber, emitter or reflector.
Link this to the effect (more energy absorbed, less emitted, stays cooler).
Example. Explain why a solar panel used to heat water is painted matt black.
Show the model answerHide the model answer
Matt black surfaces are the best absorbers of radiation (1). So the panel absorbs more of the Sun's radiation, transferring more energy to heat the water (1).
Don’t lose marks
Saying cold objects do not emit infrared. All objects do.
Saying a black body is simply 'an object that is black'. It absorbs all of the radiation incident on it.
Saying shiny surfaces are good emitters because they 'shine'. They are poor emitters and good reflectors.
Saying a perfect black body reflects some radiation. It reflects none and transmits none.
More tips
Memory tricks
'Good at taking in, good at giving out': good absorbers are good emitters.
A black body is like a perfect sponge for radiation: it soaks up all of it and bounces none back.
Hotter means more infrared, every time, and even cold objects emit some.
Exam technique
Use 'in a given time' or 'each second' when comparing how much infrared is emitted.
When explaining a design, name the surface property, say whether it is a good absorber, emitter or reflector, and link it to the effect.
Learn the definition of a perfect black body exactly: absorbs all of the radiation incident on it.
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 objects emit and absorb infraredAll bodies, whatever their temperature, emit and absorb infrared radiation.
Grade 4
Link temperature to infrared emittedThe hotter a body is, the more infrared radiation it radiates in a given time.
Grade 5
Identify good and poor emitters and absorbersDark, matt surfaces are good emitters and absorbers; shiny surfaces are poor emitters and absorbers because they reflect radiation.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Two identical metal blocks are at temperatures of 20 °C and 80 °C. Which block emits more infrared radiation each second?
The block at 80 °C
Give two factors that affect how much infrared radiation a surface emits each second.
Temperature and the type of surface (colour/texture).
State whether a matt black surface is a good or a poor emitter of infrared radiation.
A good emitter.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
All objects emit and absorb infrared radiation.
(a) Which statement is correct? Tick (✓) one box.[1]
Only hot objects emit infrared radiation.
All objects emit infrared radiation, whatever their temperature.
Cold objects absorb infrared radiation but never emit it.
Objects only emit infrared radiation when they glow.
(b) Two identical metal blocks are at temperatures of 20 °C and 80 °C. Which block emits more infrared radiation each second?[1]
(c) Give one example of an object that emits infrared radiation but does not glow.[1]
(d) Name one device that detects infrared radiation.[1]
Show the answer and mark scheme
(a)Answer: All objects emit infrared radiation, whatever their temperature.
(b)Answer: The block at 80 °C
the block at 80 °C / the hotter block
(c)Answer: A person (or a hot drink, or a radiator).
a person / a hot drink / a warm radiator / an animal / an iron
(d)Answer: An infrared (thermal imaging) camera.
infrared camera / infrared thermometer / infrared detector / thermal imaging camera
Question 2Medium5 marks
A student is investigating how the type of surface affects the rate of emission of infrared radiation, using a Leslie cube and a detector.
(a) Give two variables the student should keep the same for each surface, to make this a fair test.[2]
(b) The student fills the cube with hot water once, and then tests the four surfaces at 5-minute intervals. Explain why this could give unfair results, and suggest how the student could avoid the problem.[3]
Show the answer and mark scheme
(a)Answer: Any two: the water temperature when reading, the detector’s distance from the surface, and the timing.
the temperature of the water in the cube each time a reading is taken
the distance between the detector and the surface
the length of time the reading is taken over
the volume of water in the cube
(b)Answer: The cube cools between tests, so later surfaces give unfairly low readings; take all four readings quickly one after another (or refill with water at the same temperature before each test).
the water (and the cube) cools down during the investigation, so later surfaces are tested at a lower temperature
a surface tested later would give a lower reading, not because it is a poorer emitter but because the cube is cooler by then
take the readings for all four surfaces quickly, one straight after another (while the temperature is the same) / refill with hot water at the same starting temperature before testing each surface