Table of Contents
Wien's Displacement Law
Wien's displacement law states that the peak wavelength of blackbody radiation is inversely proportional to temperature. Hotter objects emit at shorter wavelengths. Hot metal glows red, then orange, yellow, and blue-white as temperature increases. This law was discovered by Wilhelm Wien in 1893.
This relationship is fundamental to astrophysics for determining stellar surface temperatures, thermal imaging for designing IR cameras, and industrial pyrometry for non-contact temperature measurement of furnaces and molten metals.
The Formula
Where b = 2.8978 × 10&supmin;³ m·K and T is in Kelvin. Stefan-Boltzmann law gives total power: M = σT&sup4;, where σ = 5.67 × 10&supmin;&sup8; W/m²·K&sup4;.
Peak Wavelengths by Temperature
| Object | Temp (K) | Peak λ | Region |
|---|---|---|---|
| Human body | 310 | 9.35 μm | Infrared |
| Incandescent bulb | 2,800 | 1,035 nm | Near-IR |
| Sun | 5,778 | 502 nm | Green |
| Sirius A | 9,940 | 291 nm | UV |
Frequently Asked Questions
Why does the Sun appear yellow?
The Sun emits broadly peaking near 502 nm (green), but emits strongly across all visible wavelengths. Our brain sees this mix as white. Atmospheric Rayleigh scattering removes blue light, making it appear yellow from the surface.
Can this determine star temperatures?
Yes. Measuring a star's peak emission wavelength directly gives surface temperature. This is a primary method for stellar spectral classification (O, B, A, F, G, K, M types).
What is a blackbody?
An idealized object that absorbs all incident radiation and re-emits based solely on temperature. Stars, heated objects, and the cosmic microwave background (2.725 K) approximate blackbodies.