Blackbody Radiation
Introduced| English | Meaning |
|---|---|
| blackbody radiation/ˈblækbɒdi ˌreɪdɪˈeɪʃn/ | The glow of every hot object. |
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:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
A hotter body peaks at a shorter wavelength — the glow shifts toward blue.
A hotter blackbody emits less intensity at short wavelengths than a cooler one.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
False. A hotter blackbody's curve is taller at every wavelength, and its peak shifts to shorter 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?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The peak wavelength (colour) shifts with temperature, so colour reveals it.
Select all true statements about blackbody radiation.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
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 ____.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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.
Write from a peak at 400 nm
- One hot object's spectrum peaks at 400 nm.
- Another peaks at 800 nm. Say which object is hotter.
Two blackbodies are compared. One has its radiation peak at 400 nm. The other has its peak at 800 nm. Which object is hotter, and how can you tell?
The shorter peak wavelength belongs to the hotter body.
Which blackbody is hotter?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
A shorter peak wavelength means a higher temperature.