A perfectly black body absorbs all incident radiation. If a black body is at a temperature T , what can be said about its emissivity (e) and absorptivity (a)?

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According to Kirchhoff's law of thermal radiation, for any given temperature and wavelength, the emissivity of a body is equal to its absorptivity ( $e = a$ ).

A perfectly black body is defined as a body that absorbs all incident radiation, meaning its absorptivity $a = 1$ . Therefore, its emissivity must also be $e = 1$ .

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JEE Main 2028 (Class 11)