We have to show (a) that a human body of area , emissivity , and temperature emits radiation at the rate of 910 W. (b) We have to answer why people do not glow in the dark.

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(a)

We will use the Stefan-Boltzmann law for answering the first part of the problem. It states that energy emitted by a body, which is at temperature T, per unit area per unit time of emissivity is given by

As the surface area of a human body is assumed to be and the temperature of the skin is assumed to be

,

and emissivity , the rate at which emission of radiation is taking place will be

(b)

The wavelength at which Planck’s spectral radiation has its maximum for will be

The radiation emitted by a human body is therefore in infrared part of the electromagnetic spectrum, and so the people do not ‘glow’.