Physics
Blackbody Radiation Spectrum
Quick fact
The Sun's surface temperature (~5,500°C) causes its blackbody spectrum to peak in the visible range, but a cooler star like Betelgeuse (3,500°C) appears red because its spectrum peaks at longer wavelengths.
Why this is interesting
Ever noticed how a metal glows red when heated, then orange, then white? That's the blackbody radiation spectrum in action. But why does the color change with temperature?
Read the full explanation
Understanding Blackbody Radiation Spectrum
Imagine a perfect object that absorbs all light falling on it and then re-emits energy purely due to its temperature. This idealized 'blackbody' gives off a continuous spectrum of radiation—a smooth curve of intensity across wavelengths. At low temperatures, most radiation is in the infrared (invisible heat). As the temperature rises, the peak of the curve shifts to shorter wavelengths: first red, then orange, yellow, white, and finally blue. This is why a heating coil starts to glow dull red and becomes bright white as it gets hotter. The spectrum's shape is universal: it depends only on temperature, not on the material. For example, a hot piece of iron and a hot piece of coal (if ideal) would emit the same spectrum at the same temperature.
A deeper explanation
The blackbody spectrum arises from the thermal motion of charged particles within the object. Classically, each possible vibration frequency would receive equal energy, leading to an 'ultraviolet catastrophe'—an infinite intensity at short wavelengths. This contradiction led Max Planck to propose in 1900 that energy is emitted in discrete packets (quanta) proportional to frequency: E = hf. Using this hypothesis, he derived Planck's law, which precisely gives the intensity at each wavelength for a given temperature. The law predicts two key features: the peak wavelength follows Wien's displacement law (λmax ∝ 1/T), and the total radiated power obeys the Stefan-Boltzmann law (P ∝ T⁴). These relationships allow astronomers to determine the temperature of stars from their spectra, and engineers to design efficient heaters, light bulbs, and infrared sensors. The blackbody radiation spectrum thus serves as a cornerstone of modern physics, revealing the quantized nature of light and matter.