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Wien's displacement law
Intermediate
Peak wavelength of blackbody radiation shifts with temperature.
λ_max T = 2.9×10⁻³ m·KThe peak wavelength of blackbody radiation is λ_max = b/T, with b = 2.898×10⁻³ m·K. Hotter objects glow bluer.
Sun: 500 nm; human body (310 K): 9.3 μm infrared.
Gives temperature of stars.
In more detail
Wien's displacement law states that the peak wavelength of blackbody radiation is inversely proportional to temperature, λmax T = 2.9 × 10⁻³ m K. Hotter objects emit at shorter wavelengths.
Real-world uses
- Estimating the temperature of stars from their colour.
- Infrared thermometers use peak wavelength to measure temperature.
- A red-hot poker glows red while a white-hot star appears blue-white, because the peak wavelength shifts with temperature.
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Frequently asked questions
Why is the Sun yellow?
Its surface temperature of about 5800 K gives a peak in visible light.
Is a red star cooler than a blue star?
Yes. Red stars have lower surface temperatures and peak at longer wavelengths.
Why do hotter objects look bluer?
Their peak emission moves to shorter wavelengths, so the colour shifts from red toward blue as the temperature rises.
Related topics in Heat & Thermodynamics
Radiation · Stefan–Boltzmann law · Newton's law of cooling · Blackbody radiation · Ideal gas laws · Boyle's law