What you are looking at
A "blackbody" is an ideal object that absorbs all light falling on it and, when warm, re-radiates a glow that
depends
only on its temperature. The curve shows how much energy it gives off at each
wavelength; the swatch on the left is the colour your eye would actually see. Drag the temperature and watch
the whole curve transform.
The Planck curve
Classical physics predicted this glow should blow up to infinity at short wavelengths — the "ultraviolet
catastrophe". In 1900 Max Planck fixed it by assuming energy comes in discrete
quanta, and
out fell the exact law:
B(λ,T) = (2hc²/λ⁵) · 1/(e^(hc/λkT) − 1)
That single bold assumption — energy in lumps — was the birth of quantum theory. The curve rises, peaks, and
falls, with no catastrophe.
Two rules you can watch
Wien's law — the peak wavelength moves inversely with temperature, so hotter means bluer:
λ_peak = b / T (b = 2.90 × 10⁻³ m·K)
Cool objects peak in the infrared (a warm body, an ember), the Sun peaks in the visible, and very hot stars
peak in the ultraviolet.
The Stefan–Boltzmann law — the total energy radiated (the whole
area under the curve) climbs as the
fourth power of temperature:
P_total ∝ T⁴
Double the temperature and the object radiates
sixteen times as much. That is why the power
readout rockets up as you heat it.
Why things glow the colours they do
As you raise the temperature the glow marches through the spectrum: dull red → orange → yellow-white →
blue-white. "Red hot" and "white hot" are literally different points on this curve, and astronomers read a
star's temperature straight from its colour.
Things to try
Step through the presets — ember, bulb, Sun, blue star — and watch the peak sweep left and the area explode.
Notice the Sun's peak sits right in the middle of the visible band. Push to the maximum and see the peak
cross into the ultraviolet while the visible glow turns icy blue.