What emits infrared radiation?

• A. Everything
• B. Only hot objects
• C. Only metals
• D. Only living things

Answer: (A)

Anything with a nonzero temperature emits infrared radiation as part of its thermal radiation. Planck’s law shows that all objects above absolute zero radiate energy across a spectrum that includes the infrared region, and the amount and peak wavelength of that emission depend on temperature. At typical temperatures, most of the emission falls in the infrared, so you can detect heat from rocks, furniture, or even a person with an IR camera. Only at absolute zero would there be no emission.

That’s why the correct idea is that everything emits infrared radiation. It’s not limited to hot objects, metals, or living beings—any object above 0 K emits IR to some extent, with hotter objects emitting more and shifting more of their energy into shorter wavelengths.