Emission and absorption of infrared — black bodies
A perfect black body is a theoretical object that absorbs all electromagnetic radiation that falls on it (no reflection, no transmission). It also emits radiation perfectly across all wavelengths.
Real "black bodies"
Stars are good approximations. So are the inside of an oven (cavity radiation), and very dark, dull surfaces.
Black body emission
- All bodies above 0 K emit thermal radiation.
- The hotter the object, the more total energy emitted.
- The hotter the object, the higher the peak frequency emitted.
✦Worked example— Examples of peak frequency
- Room temperature (300 K): peak in IR.
- Hot iron (1000 K): peak still in IR, glows dull red.
- Light bulb filament (3000 K): peak in IR, but visible light extends into all colours.
- Sun (5800 K): peak in visible, hence we evolved to see this band.
Energy transferred per second
For thermal equilibrium: an object absorbs as much radiation as it emits. For a heating object: emission > absorption (it radiates away to cool); for a cooling, absorption > emission (until equilibrium reached).
The Earth's energy balance
The Earth's temperature is set by the balance between:
- Absorbing sunlight (visible/UV).
- Emitting thermal IR back to space.
If absorption increases (more incoming sunlight or less reflected) or emission decreases (greenhouse gases trap IR), the Earth warms.
Greenhouse effect
- Sun's visible light passes through the atmosphere and warms the surface.
- Surface emits longwave IR.
- Greenhouse gases (CO₂, CH₄, water vapour) absorb some IR and re-emit it in all directions, including back down.
- Net effect: surface stays warmer than it would otherwise be.
Without the natural greenhouse effect, Earth would be ~33 °C colder. Anthropogenic CO₂ has enhanced this effect, raising global average temperature by ~1 °C since 1900.
⚠Common mistakes
- Calling a "black-body" something that's just a black surface — strict black body emits across the spectrum at all temperatures.
- Saying the greenhouse effect is "bad" — without it, Earth would be uninhabitably cold. The issue is the enhanced greenhouse effect from extra CO₂.
- Confusing absorption and emission — a good emitter is also a good absorber.
- Forgetting that all warm objects emit IR, not just hot ones.
AI-generated · claude-opus-4-7 · v3-deep-physics