Question

Out of the following, which statement is NOT true about black body radiation ?

• A. Intensity is more for shorter wavelengths (\(λ\)).
• B. Intensity is same for all wavelengths (\(λ\)).
• C. Intensity is less for longer wavelengths (\(λ\)).
• D. A black body emits all wavelengths (\(λ\)).

Hint:

The spectrum of a black body is a curve with a peak: intensity depends on wavelength and is not flat.

Answer 1

The Correct Option is B

Step 1: Understand the concept
A black body is an ideal absorber and emitter. Its emitted radiation is spread over all wavelengths, but the intensity varies with wavelength in a smooth curve described by Planck's law.

Step 2: Look at the curve
At a given temperature the intensity rises from zero at very short wavelengths, reaches a peak at \(\lambda_m\) (given by Wien's law, \(\lambda_m T = \text{constant}\)), and then falls for long wavelengths.

Step 3: Check statement (B)
The statement "intensity is the same for all wavelengths" would need a flat curve. The actual curve is not flat, because the intensity changes strongly with \(\lambda\). So (B) is NOT true.

Step 4: Other statements
Statements (A), (C) and (D) are the usual textbook ways of describing the black body curve: radiation is emitted at all wavelengths, and the intensity depends on wavelength, in contrast to the flat behaviour in (B). The statement that is not true is (B).

Final Answer:
Black body intensity is not the same for all wavelengths. This is option (B). \[ \boxed{\text{(B) }\text{Intensity is same for all wavelengths}} \]