Know that thermal energy transfer by thermal radiation does not require a medium.
What is Thermal Radiation?
Thermal radiation is the transfer of energy in the form of electromagnetic waves emitted by all bodies that have a temperature above absolute zero. The energy is carried by photons, which travel at the speed of light and can move through a vacuum.
Key Features
Occurs from any surface that has a temperature \$T > 0\ \text{K}\$.
Does not need a material medium – it can pass through empty space.
The amount of radiation emitted depends on temperature, surface area, and the nature of the surface.
Electromagnetic Spectrum and Radiation
All electromagnetic waves travel at the speed of light \$c = 3.0 \times 10^8\ \text{m s}^{-1}\$ in vacuum. Thermal radiation lies mainly in the infrared region, but hotter objects emit visible light and even ultraviolet radiation.
Black‑body Radiation
A perfect black body absorbs all incident radiation and re‑emits the maximum possible amount for its temperature. Real objects are approximated by a black body with an emissivity \$e\$ (0 < \$e\$ ≤ 1).
Stefan‑Boltzmann Law
The total power \$P\$ radiated by a surface of area \$A\$ is given by
\$P = \sigma \, e \, A \, T^{4}\$
where
\$\sigma = 5.67 \times 10^{-8}\ \text{W m}^{-2}\text{K}^{-4}\$ is the Stefan‑Boltzmann constant,
\$e\$ is the emissivity of the surface,
\$T\$ is the absolute temperature in kelvin.
Factors Affecting the Amount of Radiation
Temperature: Radiation increases with the fourth power of temperature.
Surface Area: Larger area radiates more energy.
Emissivity: Dark, matte surfaces have high \$e\$ (≈1), while shiny, reflective surfaces have low \$e\$.
Distance: The intensity of radiation falls off with the square of the distance from the source (inverse‑square law).
Comparison with Conduction and Convection
Aspect
Conduction
Convection
Radiation
Medium required
Yes – solid or liquid
Yes – fluid (liquid or gas)
No – can occur in vacuum
Mechanism
Transfer of kinetic energy between adjacent particles
Bulk movement of fluid carrying energy
Emission of electromagnetic waves (photons)
Dependence on temperature
Proportional to temperature gradient
Proportional to temperature difference and fluid motion
Proportional to \$T^{4}\$ (Stefan‑Boltzmann law)
Everyday Examples
The Sun’s energy reaches Earth through the vacuum of space.
A hot iron rod glows red and can heat nearby objects without touching them.
Heat loss from a house’s walls at night, even when the air is still.
Common Misconceptions
“Radiation needs air.” Incorrect – radiation travels through empty space.
“All radiation is harmful.” Incorrect – thermal radiation is a normal part of everyday heat transfer.
Quick Check Questions
Explain why the Moon, which has no atmosphere, still gets warm during the day.
Two objects have the same temperature but different colours (black vs. shiny metal). Which emits more radiation and why?
Calculate the power radiated by a black‑body sphere of radius 0.05 m at \$300\ \text{K}\$. (Use \$e = 1\$.)
Suggested diagram: Sketch showing a hot object emitting infrared rays that travel through a vacuum to a cooler object.
Summary
Thermal radiation is a mode of heat transfer that does not require any material medium. It is governed by the Stefan‑Boltzmann law, depends strongly on temperature, surface area, and emissivity, and is the only way heat can be transferred across the vacuum of space.