It explains the difference between a blackbody and a gray body following Kirchhoff's law of thermal radiation.

1. Blackbody Vs. Gray body
Blackbody Gray body
It absorbs incident light of all
wavelengths falling on its surface
at a constant temperature.
Its surface absorptivity is
independent of incident light
wavelength and temperature
conditions. And it absorbs some
fraction of incident light that falls
on its surface.
The absorptivity of a blackbody is
one. And its reflecting and
transmitting powers are zero.
Its absorptivity varies between 0
and 1. And it has definite reflective
and transmitting powers depending
upon the nature of the substance.
The energy density of radiations
emitted by a blackbody is
independent of its nature, size,
and shape. Instead, it depends
only on temperature conditions.
The energy density of emitted
radiations by a gray body depends
upon its nature, size, shape, and
composition.
The blackbody's emissive power is
a universal constant at each
wavelength in the thermodynamic
equilibrium state.
At a particular wavelength and in
thermal equilibrium conditions, the
emissive power of a gray body is the
product of its absorbing power and
the blackbody's emissive power at
that wavelength.
The blackbody radiation is
isotropic in nature.
The emissivity of a blackbody is
one.
The gray body radiations are
anisotropic in nature.
The emissivity of the gray body
varies between 0 and 1.
Rayleigh-Jeans formula
of blackbodies
Planck's formula of
blackbody
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