關於黑體輻射的問題

Q1: 為何太陽可以被當作絕對黑體?

A black body is one that emits all possible frequencies when heated to a temperature.

The sun is in plasma state. Any such object at high temperature have their atomic ions with drastic collisions which results in all posssible vibrational energies. These energies when emitted are in the form of electromagnetic radiation, thus covering all possible frequencies of the electromagentic spectrum.

Q2: 其他恆星/行星又是否可以被當作絕對黑體?

Stars are those heavenly bodies that emits light by themselves, whereas planets cannot emit light. Hence, visible stars can be regarded as black bodies but not planets.
Q3: 用灰體造成的空腔的小孔, 其溫度會否比灰體表面的溫度高?

Be aware that the "hole" is a blackbody, because any electromagentic waves that fall onto it are absorbed. The temperature of such "black body" is the temparture of the inner surface of the cavity.

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