Black Body Radiation
Build confidence in Black Body Radiation for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: a perfect black body absorbs all the radiation that hits it, and the balance of radiation absorbed and emitted controls an object's temperature, including the Earth's.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
A perfect black body is an object that absorbs all of the radiation incident on it. It reflects and transmits no radiation. This is a Triple-only topic.
A good absorber is also a good emitter, so a perfect black body is also the best possible emitter of radiation. The intensity and wavelength distribution of the radiation emitted depends on the temperature of the body.
The temperature of the Earth depends on the balance between radiation absorbed and radiation emitted. If a body absorbs radiation faster than it emits it, its temperature rises. If the two rates are equal the temperature stays constant, which is the condition for a stable climate.
Revision notes
What a black body is
A perfect black body absorbs all of the radiation incident on it.
It reflects and transmits no radiation. Because a good absorber is also a good emitter, a perfect black body is also the best possible emitter.
Temperature and emitted radiation
The intensity and the wavelength distribution of the radiation emitted depend on the temperature of the body.
As temperature increases, the intensity of every wavelength increases, and the peak shifts towards shorter wavelengths.
Earth's temperature balance
The temperature of a body depends on the rate at which it absorbs and emits radiation.
If absorption exceeds emission, the temperature rises. If emission exceeds absorption, it falls. When the two rates are equal the temperature is constant.
Key points
- A perfect black body absorbs all incident radiation.
- It reflects and transmits no radiation.
- A good absorber is also a good emitter.
- A black body is the best possible emitter.
- Emitted radiation depends on temperature.
- Equal absorption and emission means constant temperature.
Worked examples
Example 1
Define a perfect black body. [2 marks]
Model answer
A perfect black body is an object that absorbs all of the radiation incident on it (1 mark), reflecting and transmitting none (1 mark).
Example 2
Explain why a perfect black body is also the best possible emitter of radiation. [2 marks]
Model answer
A good absorber of radiation is also a good emitter of radiation (1 mark), so a body that absorbs all incident radiation must also emit radiation at the maximum possible rate for its temperature (1 mark).
Example 3
Explain what happens to the temperature of a body that absorbs radiation faster than it emits it. [2 marks]
Model answer
Its temperature increases (1 mark), because more energy is arriving than is leaving (1 mark).
Common mistakes
Saying a black body emits nothing.
It is the best possible emitter, not a non-emitter.
Saying black bodies must appear black.
The term describes absorption of all incident radiation, not visual appearance.
Forgetting the emitter half of the definition.
Good absorber and good emitter go together.
Saying constant temperature means no radiation.
It means absorption and emission are equal, not zero.
Exam tips
- Include both absorption and emission in your definition.
- Explain temperature change through the balance of rates.
- State that a good absorber is a good emitter.
- Remember this is a Triple-only topic.
Key terms
- Black body
- An object absorbing all incident radiation.
- Intensity
- The power of radiation per unit area.
- Wavelength distribution
- How emitted radiation is spread across wavelengths.
- Radiation balance
- The comparison of absorption and emission rates.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.