The instrument used for detecting heat rays is:
(A) Weight thermometer
(B) Thermo heater
(C) Expansion Meter
(D) Pyrometer

Hint To answer such questions we must know the meaning and applications of the terms mentioned, and after that we just have to compare them and their properties as what suits the best to select as an answer.

Complete step by step answer

Let’s compare all of the given terms by defining them,
1. Weight thermometer: It is a glass vessel which determines the liquid’s thermal expansion coefficient by measuring its mass which is needed to fill the vessel twice at different temperatures.
Thermo heater: It is used in heating fluids mainly.
2. Expansion Meter: It is a device with some numbers and needles on it used to determine expansion.
3. Pyrometer: It is a device used to measure the temperature of the surface of an object which depends upon the radiations emitted by that object.

So, after comparing their definitions the best option to choose is Pyrometer as it is used to measure only high temperature.

Therefore, the correct option is D.

Note We must know that there are two kinds of Pyrometers, i.e. optical pyrometers and Infrared pyrometers. They are used in different applications such as in metallurgy industries, to measure the temperature of furnaces, steam boilers, hot air balloons, etc.

Also, the Pyrometers are closely similar to Bolometers as both are used to detect high temperatures. So, it gets easy for the students to get tricked if they don’t know the difference between them.