Question

A perfect black body:

• A. Reflects all incident radiation
• B. Absorbs all incident radiation
• C. Transmits all radiation
• D. Scatters all light

Hint:

A perfect black body is a theoretical standard against which the radiative properties of real surfaces are compared. Its absorptivity and emissivity are both exactly equal to 1.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the defining radiative property of an idealized physical body known as a "perfect black body".

Step 2: Key Formula or Approach:
For any material surface, the sum of its absorptivity (\( \alpha \)), reflectivity (\( \rho \)), and transmissivity (\( \tau \)) must equal unity by conservation of energy:
\[ \alpha + \rho + \tau = 1 \]

Step 3: Detailed Explanation:

• Definition of a Black Body: A perfect black body is an idealized physical object that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence.
Therefore, its absorptivity is maximum:
\[ \alpha = 1 \]

• Reflection and Transmission: Because it absorbs everything, it reflects nothing (\( \rho = 0 \)) and transmits nothing (\( \tau = 0 \)).
- This rules out Option A (reflects all) and Option C (transmits all).
- Scattering (Option D) is a form of reflection, which is zero for a black body.

• Emission Characteristics: According to Kirchhoff's Law of thermal radiation, at thermal equilibrium, the emissivity (\( \epsilon \)) of any body is equal to its absorptivity (\( \alpha \)).
Thus, a perfect black body is also a perfect emitter of thermal radiation (\( \epsilon = 1 \)).


Step 4: Final Answer:
Thus, a perfect black body absorbs all incident radiation, matching Option (B).