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

$\sigma$ = 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, $E=\sigma T^4$