Key Terms
- Black body: A theoretical perfect absorber and perfect emitter of all radiation
- Black body radiation: The continuous spectrum of EM radiation emitted by all objects due to their temperature
- Peak wavelength: The wavelength at which a black body emits most intensely
- Infrared radiation: EM radiation emitted by cool/room-temperature objects — not visible to the naked eye
- Greenhouse effect: The trapping of heat by gases that absorb and re-emit outgoing infrared radiation
- Radiation balance: The state where an object's rate of absorption equals its rate of emission
Must-Know Facts
- ALL objects emit EM radiation — the wavelength depends on temperature
- Hotter objects emit MORE total radiation and at SHORTER peak wavelengths
- Higher temperature → peak shifts from infrared → red → orange → yellow → white → blue
- A perfect black body absorbs ALL radiation that hits it (reflects none)
- Earth is stable when: rate of absorption from Sun = rate of emission to space
- Greenhouse gases absorb outgoing infrared from Earth, then re-emit in all directions
- More greenhouse gases → Earth must warm to a higher temperature to restore balance
- The Sun's surface (~6000°C) emits peak visible light; Earth (~15°C) emits peak infrared
Temperature vs Colour
- Cool object → infrared (invisible — thermal cameras detect this)
- ~1000°C → dull red glow
- ~3000°C → bright orange/yellow
- ~6000°C (Sun) → white (all visible wavelengths)
- ~10,000°C+ → blue-white
Exam Tips
- Greenhouse gases absorb Earth's outgoing infrared — NOT incoming sunlight
- Energy is not lost — it is absorbed and RE-EMITTED by greenhouse gases
- "Peak wavelength decreases" means MORE energy per photon, NOT less total emission
- Always link temperature change to re-emission: more CO₂ → less escaping to space → Earth warms → emission increases → new equilibrium
Key Equations
- Higher temperature → shorter peak wavelength (hotter = bluer)
- Higher temperature → greater intensity emitted at all wavelengths
- Emitted power ∝ T⁴ (qualitative — doubling T gives 16× the power)
- Wien's law (qualitative): peak wavelength × temperature = constant
Common Mistakes
- Greenhouse gases absorb incoming sunlight: They absorb Earth's outgoing infrared radiation — not the incoming shortwave solar radiation
- Saying hotter stars are red: Red stars are cooler; blue/white stars are hotter — peak wavelength decreases as temperature increases
- Thinking emission stops at equilibrium: At equilibrium, Earth still emits — it just emits as much energy as it absorbs
- Confusing peak wavelength with total emission: Shorter peak wavelength means higher energy per photon AND more total emission — both increase with temperature
- Forgetting re-emission: Greenhouse gases don't just absorb — they re-emit in all directions, including back towards Earth
This topic summary covers Knowledge Organiser: Black Body Radiation within Black Body Radiation for GCSE Physics. Revise Black Body Radiation in Energy for GCSE Physics with 16 exam-style questions and 5 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 7 of 7 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Black Body Radiation
What is a perfect black body?
Explain how the radiation emitted by an object changes as its temperature increases.
Quick recall flashcards
Define:
A black body is a theoretical perfect absorber and perfect emitter of radiation.
Perfect emitter
Emits the maximum amount of radiation at every wavelength