Physics
Thermal Emission
Quick fact
The Sun's surface temperature of about 5500°C makes its thermal emission peak in visible light, while a human body at 37°C emits mostly infrared radiation, invisible to our eyes.
Why this is interesting
You've seen a hot stove glow red and felt warmth from a campfire, but did you know that even cold objects like ice emit radiation? Why does this happen, and what determines the color and amount of the emitted energy?
Read the full explanation
Understanding Thermal Emission
All objects with temperature above absolute zero (-273°C) emit electromagnetic radiation due to the thermal motion of their atoms and molecules. This thermal emission ranges from low-frequency radio waves to high-frequency gamma rays, depending on temperature. At low temperatures, most emission is in the infrared range; as temperature rises, the radiation becomes visible (red, then white-hot) and eventually ultraviolet. The total power emitted increases dramatically with temperature—this is why a hot stove feels much hotter than a warm hand. The emission spectrum also shifts to shorter wavelengths at higher temperatures, explaining why a heated metal glows red first, then yellow, then white.
A deeper explanation
Thermal emission originates from the accelerated motion of charged particles (mainly electrons) in matter due to thermal energy. According to classical electrodynamics, accelerating charges emit electromagnetic waves. Quantum mechanics refines this: the energy levels of oscillating charges are quantized, leading to the blackbody radiation spectrum described by Planck's law. The ideal blackbody absorbs all incident radiation and emits a characteristic spectrum determined solely by temperature. Real objects have emissivity (0 to 1) that modifies the ideal blackbody emission. The Stefan-Boltzmann law states that the total radiated power per unit area is proportional to the fourth power of absolute temperature (P = σT^4). Wien's displacement law gives the wavelength of peak emission: λmax = b/T. These principles explain why the Sun emits mostly visible light, why Earth emits infrared, and how thermal imaging detects temperature differences. Understanding thermal emission is crucial for climate science (greenhouse effect), astronomy (stellar spectra), and engineering (heat management).