Blackbody Radiation
Introduced| English |
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| blackbody radiation/ˈblækbɒdi ˌreɪdɪˈeɪʃn/ |
Heat metal and it glows red, then orange, then white
- A cool poker is dark; heat it and it glows a dull red, then orange, then dazzling white.
- Every warm object glows — the colour tells you its temperature.
- This glow is blackbody radiation 黑体辐射, given off by all hot bodies.
- Explaining its shape needed a brand-new idea — the birth of quantum theory.
The radiation curve
- A hot body emits a smooth spread of wavelengths, with a clear peak.
- Plot intensity against wavelength and you get a curve that rises to a peak, then falls.
- A hotter body glows brighter (a taller curve) at every wavelength.
- Its peak also shifts to a shorter wavelength — from red toward blue.
As a blackbody gets hotter, its peak wavelength:
A hotter body peaks at a shorter wavelength — the glow shifts toward blue.
A hotter blackbody emits more intensity at every wavelength than a cooler one.
The hotter curve is taller at all wavelengths.
Colour tells temperature
- Because the peak shifts with temperature, the colour of the glow reveals how hot something is.
- A red star is cooler; a blue-white star is hotter (the opposite of the "hot = red" everyday feel).
- Astronomers judge a star's temperature from the colour of its light.
- Even your own body glows — in the infrared, which thermal cameras see.
A red star and a blue-white star: which is hotter?
Blue-white light peaks at a shorter wavelength, so that star is hotter.
The colour of a glowing object can be used to estimate its temperature.
The peak wavelength (colour) shifts with temperature, so colour reveals it.
Select all true statements about blackbody radiation.
Hotter = brighter, peak bluer, colour = temperature. Classical physics failed; Planck's quanta were needed.
Why it launched quantum theory
- Classical physics predicted a hot body should blast out infinite ultraviolet energy — clearly wrong.
- Max Planck fixed it by assuming energy comes in quanta (packets), $E = hf$.
- That single quantum assumption gave the correct curve — and started quantum physics.
- Blackbody radiation is where the quantum world first announced itself.
Hotter or cooler blackbody?
A blackbody's spectrum shifts with temperature. Sort each fact.
Planck explained the blackbody curve by assuming energy comes in packets called ____.
Energy quanta ($E = hf$) gave the correct curve and started quantum theory.
For a hotter body, the peak shifts to a shorter wavelength (toward blue), not a longer one. This is the reverse of everyday intuition where "hot = red". A blue-white star is much hotter than a red one.
Two stars glow, one red and one blue-white. Which is hotter?
- The blue-white star is hotter — its blackbody peak is at a shorter wavelength.
- The red star is cooler, with its peak toward longer wavelengths.
Blackbody radiation is the glow of every hot object. A hotter body is brighter and its peak shifts to a shorter wavelength (bluer) — so colour reveals temperature. Explaining the curve required Planck's idea that energy comes in quanta ($E = hf$), launching quantum theory.