Electromagnetic radiation: 6. Microwaves
Microwaves in radio telecommunications are the very shortest wavelengths usable for communication. They have wavelength 1mm to 30cm, or frequency 1-300 GHz.
An important frequency is 1420 MHz (wavelength 21.1cm). Interstellar hydrogen radiates at this frequency, making it a very useful frequency for radio astronomers.
Microwaves have only fairly recently been regarded as distinct from very short wavelength radio waves. Their recognition is probably due to the popularity of microwave ovens. In fact, these ovens typically use frequencies of 300MHz to 1GHz, or wavelengths 30cm to 100cm, frequencies in the lower half of the UHF band hence slightly below the radio telecommunications definition.