Which of the following thermometers measures temperature from the thermal radiation emitted by objects?

Step 1: Analyze the question The question asks which thermometer measures temperature from the thermal radiation emitted by an object. The key idea is that it works without touching the object.

Step 2: Explain the concept Every hot object emits thermal (infrared and visible) radiation. The hotter the object, the more radiation it emits, and the shorter the wavelengths. This is described by the Stefan-Boltzmann law (total energy radiated is proportional to T⁴) and by Wien's law. A thermometer that uses this radiation is called a radiation pyrometer, or an optical pyrometer.

Step 3: Check each option

• A. Pyrometer thermometer: It measures the intensity or colour of radiation from a hot body, such as a furnace, molten metal or a glowing filament. It needs no contact with the object, so it suits very high temperatures. This matches the question.
• B. Platinum resistance thermometer: It works by the change in electrical resistance of platinum wire with temperature. It must be in contact with the object and does not use radiation.
• C. Thermocouple thermometer: It uses the e.m.f. (Seebeck effect) produced at the junction of two different metals held at different temperatures. It is not radiation based.
• D. Constant pressure gas thermometer: It uses the expansion of a gas at constant pressure. It is a contact thermometer and does not use radiation.

Step 4: Answer The pyrometer is the only thermometer that measures temperature from the thermal radiation emitted by the object.

Exam tip: Link each thermometer to its property:

• Pyrometer → radiation (no contact)
• Platinum resistance → resistance
• Thermocouple → e.m.f.
• Gas thermometer → pressure or volume

If you see "radiation" or "very hot objects like furnaces or stars", think pyrometer.