Wien's displacement law connects the temperature of a glowing object to the color of light it emits most strongly. It explains why a heated iron bar glows dull red, then orange, then white-hot as it gets hotter, and why astronomers can estimate a star's surface temperature just by looking at its color.
How Wien's law works
Every object above absolute zero radiates electromagnetic energy across a range of wavelengths, with the intensity distribution described by Planck's law. Wien's displacement law is a simplified consequence of that distribution: it identifies the single wavelength, λ_max, where emission intensity peaks. The relationship is λ_max = b/T, where b = 2.898×10⁻³ m·K is an empirically measured constant and T is the object's absolute temperature in kelvin.
Because λ_max and T are inversely proportional, doubling an object's temperature halves its peak wavelength — shifting emission toward shorter, bluer, higher-energy light. This calculator applies the law in both directions: given a temperature, it finds the peak wavelength; given a peak wavelength or observed color, it solves for temperature.
Inputs and what they mean
The temperature input is the absolute temperature in kelvin (K) — not Celsius or Fahrenheit. Kelvin starts at absolute zero, so it must always be entered as a positive number. Typical values range from a few thousand kelvin for cool red stars and incandescent filaments up to tens of thousands of kelvin for hot blue-white stars.
The peak wavelength input is in nanometers (nm). Visible light spans roughly 380 nm (violet) to 750 nm (red); wavelengths shorter than 380 nm are ultraviolet, and longer than 750 nm is infrared. The Spectrum tab plots the computed peak against this visible band so you can see at a glance whether an object's peak emission is even in the range human eyes can perceive.
Limits and edge cases
Wien's law describes the peak of a smooth, idealized blackbody spectrum — real stars and heated objects are close approximations but not perfect blackbodies, so measured peak wavelengths can differ slightly from the prediction. The law also only identifies where emission is strongest, not the object's total brightness (that's governed by the separate Stefan-Boltzmann law, P = εσAT⁴).
At very high or very low temperatures, the peak wavelength moves outside the visible band entirely — into ultraviolet for extremely hot sources or infrared for cool ones — which is why an object's true peak color is often invisible to the naked eye even though the object still glows.