The GATE Grind

GATE 2026 CH – Question 13

Heat Transfer · Radiation · 1 mark · Multiple choice

A body is placed inside a perfectly black enclosure and is allowed to reach thermal equilibrium. According to the Kirchhoff's identity, which one of the following is equal to the emissivity of the body at a given wavelength?

  1. absorptivity of the body at the same wavelength
  2. reflectivity of the body at the same wavelength
  3. Stefan-Boltzmann constant
  4. zero

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Show answer and explanation

Correct answer: (A) absorptivity of the body at the same wavelength

Explanation

Kirchhoff's law says that at thermal equilibrium the spectral emissivity of a body equals its spectral absorptivity at the same wavelength (and temperature). It does not relate emissivity to reflectivity directly, and the Stefan-Boltzmann constant is a universal constant.