A grey body is one whose absorptivity

A grey body is an idealized physical body used in thermal radiation studies.

It is defined as a body whose absorptivity and emissivity remain constant across all wavelengths and temperatures, although the value is less than 1 (unlike a black body which has a value of 1).

Option A: Varies with temperature
This is incorrect because a grey body’s absorptivity does not vary with temperature. It is assumed to remain constant for analysis purposes.

Option B: Varies with wavelength of the incident ray
This is incorrect. If the absorptivity varies with wavelength, the body is not grey. Grey body assumes constant absorptivity across all wavelengths.

Option C: Is equal to its emissivity
While Kirchhoff’s law states that emissivity equals absorptivity at thermal equilibrium, this is not the defining property of a grey body. This condition can apply to other bodies as well.

Option D: Does not vary with temperature and wavelength of the incident ray
This is correct. A grey body is defined as one whose absorptivity does not change with wavelength or temperature.

Correct Answer: Option D: Does not vary with temperature and wavelength of the incident ray