Know that thermal radiation is infrared radiation and that all objects emit this radiation.
Thermal Radiation Explained
Thermal radiation is a form of electromagnetic radiation emitted by objects due to their temperature. All objects with a temperature above absolute zero ($0 \ K$) emit thermal radiation. The hotter an object is, the more thermal radiation it emits, and the shorter the wavelengths of the emitted radiation.
Infrared Radiation
Thermal radiation falls within the infrared (IR) region of the electromagnetic spectrum. The electromagnetic spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet light, X-rays, and gamma rays. Infrared radiation is invisible to the human eye, but we can feel it as heat.
Properties of Thermal Radiation
All objects emit thermal radiation: This is a fundamental property of matter.
Temperature Dependence: The amount and wavelength distribution of thermal radiation depend on the object's temperature.
Wavelength Dependence: The peak wavelength of thermal radiation emitted by an object is inversely proportional to its temperature (Wien's Displacement Law).
Emission Spectrum: The spectrum of thermal radiation emitted by an object is continuous, meaning it emits radiation across a range of wavelengths.
Wien's Displacement Law
Wien's Displacement Law describes the relationship between the peak wavelength ($\lambda_{max}$) of thermal radiation emitted by an object and its absolute temperature ($T$):
$$ \lambda_{max} = \frac{b}{T} $$
Where:
$\lambda_{max}$ is the peak wavelength (in meters)
$b$ is Wien's displacement constant ($b \approx 2.89 \times 10^{-3} \ m \cdot K$)
$T$ is the absolute temperature (in Kelvin)
This law tells us that hotter objects emit radiation with shorter wavelengths (e.g., hotter objects emit more in the blue/violet end of the spectrum). Cooler objects emit radiation with longer wavelengths (e.g., cooler objects emit more in the red/infrared end of the spectrum).
Example Table: Wavelength and Temperature
Temperature (K)
Peak Wavelength (m)
Color (approximate)
577 K
0.000000795 m (795 nm)
Violet
373 K (Room Temperature)
0.00000128 m (1280 nm)
Red
1000 K
0.00000034 m (340 nm)
Blue-White
Applications
The emission and absorption of thermal radiation have many applications, including:
Infrared Thermography: Detecting temperature variations in objects by detecting their infrared radiation. Used in medical diagnosis, building inspection, and security systems.
Thermal Imaging Cameras: Cameras that detect infrared radiation and create images based on temperature differences.
Heating Systems: Radiators and heaters emit infrared radiation to warm objects and people.
Remote Controls: Many remote controls transmit infrared signals to electronic devices.
Suggested diagram: A simple diagram showing an object emitting infrared radiation, with arrows indicating the direction of the radiation. Include a label indicating that the radiation is invisible.