A black body is one that
Correct answer: B. absorbs all radiation falling on it and is also the best possible emitter
- A. reflects all radiation falling on it
- B. absorbs all radiation falling on it and is also the best possible emitter
- C. emits no radiation at all
- D. is black in colour at all temperatures
Explanation
A perfect absorber must also be a perfect emitter at the same temperature, otherwise it could not stay in thermal equilibrium with its surroundings. The spectrum of the radiation it emits depends only on temperature, which is why the concept is so useful for stars. A small hole in a hollow cavity is the standard practical approximation.
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About Quantum Theory and Radiation
Quantum theory treats electromagnetic radiation as packets of energy called photons, with energy proportional to frequency through E equals hf. The topic covers Planck's hypothesis, photon momentum, the photoelectric effect, threshold frequency, work function and wave particle duality, distinguishing quantised energy exchange from classical continuous radiation.
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