The thermopile is a device for detecting

Analyze the question: It asks what a thermopile is used to detect.

Explain the concept: A thermopile is made of several thermocouples connected in series. Each thermocouple joins two different metals (for example, bismuth and antimony). When one junction is hotter than the other, a small e.m.f. is produced (the Seebeck effect). Connecting many in series adds up their e.m.f.s, so the device becomes much more sensitive.

Step-by-step solution:

1. The "hot" junctions are blackened so they absorb heat radiation well.
2. When radiant heat (infrared or thermal radiation) falls on them, those junctions become warmer than the "cold" junctions.
3. The temperature difference produces an e.m.f., and a sensitive galvanometer connected to the thermopile registers a deflection.
4. The size of the deflection shows the amount of radiant heat energy received.

So a thermopile detects and measures radiant (heat) energy.

Checking the options:

• A. Radioactive radiations are detected by a Geiger-Müller tube or a scintillation counter, not a thermopile.
• B. Radiant energy. Correct.
• C. X-rays are detected by photographic film or an ionization chamber.
• D. The presence of electrons would be detected with a cathode ray tube or a fluorescent screen.

Exam tip: Remember "thermo" means heat. Thermopile gives heat radiation detection, and thermocouple gives temperature measurement. Whenever you see "thermopile", think "radiant heat detector".