Question

A cavity with a small hole will always behave as a:

• A. White body
• B. Transparent body
• C. Black body
• D. Opaque body

Hint:

A small hole in a cavity simulates a perfect black body because radiation entering the hole is almost entirely absorbed through multiple reflections.

Answer 1

The Correct Option is C

A cavity with a small hole is one of the classic idealizations used in thermal radiation studies to model a perfect black body.
Here's how it works:
- Any radiation entering the small hole in the cavity undergoes multiple reflections inside the cavity.
- Because of these repeated reflections and internal absorption, almost no radiation escapes the cavity once it enters.
- As a result, the hole appears perfectly black from the outside since nearly all incident radiation is absorbed.
This setup mimics the behavior of an ideal black body, which absorbs all incident radiation regardless of wavelength or angle.
Thus, a cavity with a small hole behaves as a near-perfect absorber and emitter, i.e., a black body.
Let’s briefly consider why other options are incorrect:
- A white body reflects all incident radiation — the opposite of a black body.
- A transparent body allows radiation to pass through without significant absorption — not the case with a cavity.
- An opaque body blocks transmission, but that doesn’t mean it absorbs all radiation like a black body does.
Hence, the correct answer is option (3): black body.