Infrared waves are detected by which of the following?

The correct answer is Option (2) → Thermopile

Each band of the electromagnetic spectrum has its own detectors, chosen by how the waves interact with matter.

Option (1) Geiger tube — Incorrect. A Geiger tube is a detector for the shortest waves of the spectrum, X-rays and gamma rays.

Option (2) Thermopile — Correct. Infrared waves are heat waves. When absorbed, they increase the thermal motion of the molecules and heat the absorbing surface. A thermopile responds to this heating, so it is used to detect infrared radiation.

Option (3) Point contact diode — Incorrect. Point contact diodes are the detectors for microwaves, of wavelength 0.1 m to 1 mm.

Option (4) Receiver’s aerial — Incorrect. A receiver’s aerial detects radio waves (wavelength above 0.1 m), whose fields drive currents in the aerial wire.

NCERT (2026-27):
Electromagnetic Spectrum