Knowledge Organiser: Black Body Radiation
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 → the peak moves to shorter wavelengths, so the glow changes colour: dull red → orange and yellow → white → blue-white
- 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 (~5500°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
- ~5500°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 the radiation shifts towards shorter wavelengths, NOT less total emission: a hotter object emits more at every wavelength
- Always link temperature change to re-emission: more CO₂ → less escaping to space → Earth warms → emission increases → new equilibrium
Key Relationships
- Higher temperature → shorter peak wavelength (hotter = bluer)
- Higher temperature → greater intensity emitted at all wavelengths
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: A hotter object has a shorter peak wavelength AND emits more radiation in total. Both happen together as the temperature rises.
- Forgetting re-emission: Greenhouse gases don't just absorb — they re-emit in all directions, including back towards Earth
Memory Aid
- MORE radiation emitted per second
- SHORTER peak wavelength
- Black body absorbs everything
- Emits maximum radiation
- All objects radiate thermally
- Darker surfaces beat shiny ones