Wien's displacement law
/ VEEN /
Why does a cooling ember glow red while a hotter flame burns blue-white? Wien's displacement law is the simple rule behind this everyday observation: the hotter an object is, the shorter (bluer) the wavelength at which it shines most brightly. As temperature rises, the peak of the glow 'displaces,' or slides, toward the blue end of the spectrum — hence the name.
The law is wonderfully exact. The peak wavelength times the temperature equals a fixed constant, so peak wavelength is simply that constant divided by temperature. In practical numbers, a body at about 6,000 degrees (like the Sun's surface) peaks at roughly 500 nanometers, smack in the visible range; double the temperature to 12,000 degrees and the peak halves to 250 nanometers, in the ultraviolet; cool it to 300 degrees (room temperature) and the peak slides out to about 10 micrometers, deep in the infrared, which is why people glow in the infrared but not in visible light.
Wien's law is one of the astronomer's most powerful shortcuts. Find the wavelength at which an object is brightest and you immediately know its temperature — no need to visit it. This is how we know the surface temperatures of stars from their colors, why hot young stars look blue and cool old ones look red, and why the cosmic microwave background, peaking in the microwaves, corresponds to a chilly 2.7 degrees. It is one of the two pillars (with the Stefan-Boltzmann law) that turn the Planck curve into measurements.
A red giant with a surface near 3,000 degrees peaks at about 1 micrometer (near-infrared), so most of its light is actually invisible to us even though it looks bright orange-red. Wien's law tells us, from that peak alone, how hot its surface is.
Peak wavelength times temperature is a constant — so the color of the peak is a direct thermometer.
Wien's law gives the wavelength of peak brightness, not the only wavelength emitted. A 6,000-degree star still radiates plenty of red and ultraviolet light — it just peaks in the green-yellow.