Thermal Radiation Properties
Imagine holding your hand near a hot iron — you feel warmth without touching it. That warmth travels through empty space, not through air or metal. This is thermal radiation: energy emitted by any object purely because it has a temperature.
Unlike conduction (touching) or convection (fluid flow), radiation doesn't need a medium. It's electromagnetic waves — mostly infrared, but at high temperatures, visible light too. A glowing red-hot steel rod is radiating; so is your own body, though you can't see it.
The Core Idea: Every Object Radiates
Every object above absolute zero (0 K) emits radiation. The amount and type depend on two things:
- Temperature — hotter objects radiate more energy, and at shorter wavelengths.
- Surface properties — some surfaces are good emitters, others are poor.
This leads to three key properties that describe how a real surface behaves compared to an ideal "perfect" radiator.
1. Emissivity (ε)
Intuition: A black matte surface feels hotter in sunlight than a shiny white one. Why? The black surface emits radiation more efficiently.
Definition: Emissivity is the ratio of radiation emitted by a real surface to the radiation emitted by an ideal blackbody at the same temperature.
ε=EblackbodyEreal
- ε=1 for a perfect blackbody (ideal emitter).
- 0<ε<1 for all real surfaces.
- ε depends on material, surface finish, and wavelength.
A good emitter is also a good absorber. This is Kirchhoff's law: at thermal equilibrium, ε=α (absorptivity) for the same wavelength and direction.
2. Absorptivity (α)
Intuition: A black car roof gets hotter in summer than a white one. It absorbs more sunlight.
Definition: Absorptivity is the fraction of incident radiation that a surface absorbs.
α=incident radiationabsorbed radiation
- α=1 for a perfect blackbody (absorbs everything).
- α=0 for a perfect reflector.
- For opaque surfaces: α+ρ=1, where ρ is reflectivity.
Don't confuse absorptivity with emissivity. A shiny metal has low α (reflects most light) and low ε (emits poorly). A black surface has high α and high ε.
3. Reflectivity (ρ) and Transmissivity (τ)
Intuition: A mirror reflects; a glass window transmits; a brick wall does neither.
Definition: For any surface, incident radiation is either absorbed, reflected, or transmitted:
α+ρ+τ=1
- Reflectivity ρ: fraction reflected.
- Transmissivity τ: fraction transmitted through.
- For opaque solids: τ=0, so α+ρ=1.
The Blackbody: The Ideal Reference
A blackbody is a theoretical surface that:
- Absorbs all incident radiation (α=1).
- Emits the maximum possible radiation at any temperature.
- Follows Planck's law, Stefan-Boltzmann law, and Wien's displacement law.
Real surfaces are compared to this ideal. The blackbody is a standard — like comparing a real engine to a Carnot engine.
Stefan-Boltzmann law for a blackbody:
Eb=σT4
where σ=5.67×10−8 W/m2K4.
For a real surface: E=εσT4.
Putting It Together: A Real Surface
A hot metal plate at 500 K with ε=0.3 emits:
E=0.3×5.67×10−8×(500)4≈1063 W/m2 …