Final answer:
A perfect absorber of light must also be a perfect emitter due to the principle of thermal equilibrium and conservation of energy. This balance ensures that the energy a blackbody absorbs is equal to the energy it emits, maintaining constant temperature.
Step-by-step explanation:
It is assumed that a perfect absorber of light, also known as a blackbody, must be a perfect emitter of light due to the requirement of thermal equilibrium. This stems from the principle of conservation of energy, which mandates that the rate of energy absorption must equal the rate of energy emission when a body is in thermodynamic equilibrium. A perfect blackbody is an idealized object that absorbs all incoming electromagnetic radiation without reflecting or scattering it. In astronomical terms, even stars are considered blackbodies because they absorb all incident radiation and re-emit energy across various wavelengths, thereby emitting light while also being perfect absorbers.
To illustrate, a silver object and a black object that are in thermal equilibrium will adjust their emissions to maintain equilibrium. The black object must emit as much radiation as it absorbs to keep its temperature constant, indicating it is as good at emitting as it is at absorbing. Conversely, a silver object that reflects more light will also absorb and emit less. Thus, emissivity is a property that controls both the absorption and emission of radiation.
Therefore, the correct answer to the student's question is that conservation of energy requires equal absorption and emission (option b).