Explain why,
an optical pyrometer calibrated for an ideal black body radiation gives a very low 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

Explanation

The rate of cooling is faster with the cold surroundings. The rate of cooling does not arise because the temperature of the surrounding furnace is as high as the hot iron piece. Hence themmJ radiation will be consistent and the reading will be accurate.