Classical physics (the Rayleigh–Jeans law) predicted that the power radiated by a blackbody at short wavelengths would...
AApproach zero as wavelength decreases
BGrow without bound as wavelength decreases
CRemain constant regardless of wavelength
DDepend on the material the object is made of
The Rayleigh–Jeans law predicts intensity proportional to 1/λ⁴, which diverges as wavelength approaches zero — the 'ultraviolet catastrophe.' Planck's quantization suppresses this divergence by making high-frequency modes energetically costly to excite.
Question 2 True / False
A 'blackbody' is expected to be visually black in color in order to emit blackbody radiation.
TTrue
FFalse
Answer: False
A blackbody is defined as a perfect absorber and emitter of radiation at all wavelengths — not by its visible color. Stars (which appear white, yellow, or red) and the cosmic microwave background are excellent blackbody approximators. The word 'black' refers to the object absorbing all incident light, not to its appearance.
Question 3 Short Answer
What was Planck's key postulate that resolved the ultraviolet catastrophe, and what does it imply about the nature of electromagnetic energy?
Think about your answer, then reveal below.
Model answer: Planck postulated that electromagnetic energy is emitted in discrete quanta of size E = hf, where h is Planck's constant and f is frequency. This implies energy is not continuously divisible — it comes in indivisible packets proportional to frequency, suppressing the high-frequency modes that caused the classical divergence.
By requiring each oscillation mode to emit or absorb energy in chunks of hf, high-frequency modes become statistically unlikely to be excited (since each quantum is large). This naturally produces the observed dropoff in the Planck spectrum at short wavelengths, matching experiment perfectly where classical theory catastrophically failed.