An optional pyrometer is used for measuring high temperature, if it is calibrated for an ideal black body radiation. When the temperature of a red-hot piece of iron is measured in the open, the pyrometer gives too low value but when the same piece is in the furnace, the pyrometer gives the correct values of temperature for the iron piece . Explain.

An optional pyrometer is used for measuring high temperature, if it is

1.	An optional pyrometer is used for measuring high temperature, if it is calibrated for an ideal black body radiation. When the temperature of a red-hot piece of iron is measured in the open, the pyrometer gives too low value but when the same piece is in the furnace, the pyrometer gives the correct values of temperature for the iron piece . Explain.
Answer» Solution :Optical PYROMETER MEASURES the amount of heat that is RADIATED from an object. If T is the temperature of red HOT ironpiece in the furnace, then heat radiated per second per unit area is ` E = sigma T^4` When the red-hot IRON piece is placed in the open with a temperature `T_0` , heat radiated per second per unit area is ` E. = sigma (T^4 - T_0^4)` So, E. < E and hence, optical pyrometer gives a low value for the temperature measured in the open.