Question

Explain why an optical pyrometer (for measuring high temperatures) calibrated for an ideal black body radiation gives too low a value for the temperature of a red hot iron piece in the open but gives a correct value for the temperature when the same piece is in the Furnace.

Answer 1

An optical pyrometer calibrated for an ideal black body radiation gives too low a value for temperature of a red-hot iron piece kept in the open. Black body radiation equation is given by $E=\sigma(T^4-T_0^4)$
this equation is also known as Stefan's law of radiation .

Where,
E = Energy radiation
T = Temperature of optical pyrometer
$T_0$ = Temperature of open space
σ = Stefan's constant

from above formula, it is clearly shown that, an increase in the temperature of open space reduces the radiation energy. When the same piece of iron is placed in a furnace, the radiation energy, E = σT⁴ .