2.3.3 Radiation

Objective: Know that thermal energy transfer by thermal radiation does not require a medium.

What is Thermal Radiation?

Thermal radiation is a form of electromagnetic radiation that is emitted by all objects with a temperature above absolute zero (0 Kelvin or -273.15 Celsius). This radiation carries energy and is how heat can be transferred without the need for a medium.

Key Concepts

• Electromagnetic Radiation: Energy travels in the form of waves. Electromagnetic radiation includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.
• Thermal Equilibrium: When an object absorbs and emits energy at the same rate, it is in thermal equilibrium.
• Blackbody Radiation: An ideal blackbody is a perfect absorber and emitter of radiation. Real objects are not perfect blackbodies, but they approximate this behavior.

Radiation Does Not Require a Medium

Unlike conduction and convection, thermal radiation can travel through a vacuum. This is because electromagnetic radiation is self-propagating; it doesn't need a material to carry the energy.

Example: The Sun transfers energy to Earth through thermal radiation. Space is a vacuum, yet the Sun's energy reaches us.

Properties of Thermal Radiation

Property	Description
Wavelength	The distance between successive crests or troughs of a wave. The wavelength of thermal radiation depends on the temperature of the object.
Frequency	The number of waves that pass a given point per unit time. Frequency is related to wavelength by the equation: $$f = \frac{c}{\lambda}$$, where f is frequency, c is the speed of light (approximately $3.0 \times 10^8$ m/s), and λ is wavelength.
Energy	The amount of energy carried by a single photon of radiation. The energy of a photon is given by: $$E = hf$$ or $$E = \frac{hc}{\lambda}$$, where E is energy, h is Planck's constant ($6.626 \times 10^{-34} \, J \cdot s$), f is frequency, c is the speed of light, and λ is wavelength.
Temperature Dependence	The wavelength of maximum emitted radiation shifts to shorter wavelengths as the temperature increases. This is described by Wien's displacement law: $$\lambda_{max} = \frac{b}{T}$$, where $\lambda_{max}$ is the wavelength of maximum emission, b is Wien's displacement constant ($2.89 \times 10^{-3} \, m \cdot K$), and T is the absolute temperature in Kelvin.

Examples of Thermal Radiation

• A hot stove radiating heat.
• The warmth felt from the Sun.
• An infrared lamp emitting infrared radiation.
• A glowing filament in an incandescent light bulb.