Which type of electromagnetic radiation has the longest wavelength?

The electromagnetic spectrum runs from radio waves at the long-wavelength end through microwaves, infrared, visible light, ultraviolet and X-rays to gamma rays at the short end. All of it is the same phenomenon travelling at the same speed in a vacuum; only wavelength and frequency differ. Because wavelength and frequency multiply to a fixed value, long wavelength means low frequency, and photon energy tracks frequency, so radio photons carry the least energy of all. That is precisely why radio is safe to bathe in and gamma rays are not: high-energy photons can knock electrons off atoms and damage DNA, while radio photons simply lack the energy to do so. Long wavelengths also diffract around obstacles and follow the curve of terrain, which is why radio signals reach behind hills while visible light does not. Radio waves can be metres or even kilometres long, whereas gamma rays are shorter than the width of an atom.

Microwaves heat food because their frequency happens to make water molecules rotate and rub against each other.