Question

Why electromagnetic waves have heating effect?

Answer 1

As frequency increases into the visible range, photons haveenough energy to change the bond structure of some individual molecules. ... For this reason, infrared, microwaves and radio waves are thought to damage molecules and biological tissue only by bulk heating, not excitation from single photons of the radiation.

Answer 2

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A portion of EM wave energy spends for charges motions. Besides, vibration of molecular dipoles and atoms due to the varying fields of EM wave causes energy absorption and hence the heating of materials.