Electromagnetic wave(radio wave)
Water waves are formed by vibration in liquid. These waves transfer energy from one to the other. All waves can be thought as a vibration that transfer energy. In this sense, electromagnetic wave is the energy which is derived form electric wave and magnetic wave. We call it electromagnetic wave because electromagnetic waves are formed by relationship between electric fields and magnetic fields. Every wave has a frequency that is a number of wavelength. The frequency bands using for geodetic VLBI observations are 2GHz and 8GHz.
QUASAR are the inner core of active galaxies in billions of the light-years away from the earth. It is about trillions times brighter than the Sun, located the end of the space, and the source of electromagnetic waves.
Depending on the wavelength region, telescopes are separated by optical telescopes and radio telescopes.