A perfect emitter or absorber of radiant energy is a

Step 1: Analyze the question The question asks which object is a perfect emitter or absorber of radiant (thermal) energy.

Step 2: Explain the concept A black body is an idealised object that:

• Absorbs all the radiation that falls on it (none is reflected or transmitted).
• Emits the maximum possible radiation at any given temperature.

Good absorbers are also good emitters, so a perfect absorber is a perfect emitter. This is related to Kirchhoff's law of radiation. Dull, black surfaces are good absorbers and emitters. Shiny or white surfaces are poor absorbers and poor emitters because they reflect most of the radiation.

Step 3: Check each option

• A. Conductor: this describes how well a material conducts heat or electricity. It says nothing about radiation.
• B. Black body: absorbs and emits radiation perfectly. ✔
• C. White body: reflects most radiation, so it is a poor absorber and a poor emitter.
• D. Red body: it reflects red light, so it is not a perfect absorber.

Step 4: Answer The correct option is B. black body.

Exam tip: Remember "black = best" for radiation. Black surfaces are the best absorbers and best emitters, and also the poorest reflectors. This is why the bottoms of cooking pots are blackened and why solar heaters are painted black. Shiny surfaces such as thermos flask linings are used to reduce radiation.