Thermal radiation law: The wavelength corresponding to maximum monochromatic emissive power varies inversely with absolute temperature according to which law?

Difficulty: Easy

Correct Answer: Wien’s displacement law

Explanation:


Introduction / Context:
Blackbody radiation describes how ideal emitters radiate energy as a function of wavelength and temperature. Several fundamental laws describe different aspects of this emission.



Given Data / Assumptions:

  • We consider the wavelength λ_max at which the spectral emissive power is maximum for a given absolute temperature T.
  • Emphasis is on the inverse proportionality λ_max * T ≈ constant.


Concept / Approach:
Wien’s displacement law states λ_max * T = b, where b is Wien’s constant. This gives the simple inverse relationship and explains why hotter bodies radiate at shorter peak wavelengths (e.g., the Sun peaks in the visible, a hot stove in the infrared).



Step-by-Step Solution:

Identify the target: relation between peak wavelength and temperature.Recall the formula: λ_max * T = b (b ≈ 2.898 × 10^−3 m·K).Conclude the governing law: Wien’s displacement law.Contrast with other laws: Stefan–Boltzmann relates total emissive power to T^4; Planck gives full spectral distribution; Kirchhoff relates emissivity and absorptivity at thermal equilibrium.


Verification / Alternative check:
Apply to two temperatures to see the shift: doubling T halves λ_max, confirming the inverse proportionality.



Why Other Options Are Wrong:

  • Kirchhoff’s law: not about peak wavelength.
  • Stefan–Boltzmann: total power, proportional to T^4, not spectral peak.
  • Planck’s law: full spectrum formula; the inverse result emerges from differentiating Planck’s law, but the named simple relation is Wien’s.


Common Pitfalls:
Confusing total emissive power (Stefan–Boltzmann) with the wavelength of maximum emission (Wien’s).



Final Answer:
Wien’s displacement law

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