Radio and radar waves, microwaves, infrared rays, light rays, ultraviolet rays, X-rays and gamma rays all form what is called the electromagnetic spectrum. They are all various kinds of radiant energy and they differ only in their wavelength of frequency. Those with long wavelengths are generally called waves and the shorter ones rays.
Before 1800, only light was known, as all the other rays and waves are invisible. Infrared and ultraviolet rays were discovered were discovered in 1800 and 1801, but no one was aware of any other waves and rays. Then in the 1860s, the British physicist, James Clerk Maxwell, worked out the nature of light rays. He showed that they consist of electric and magnetic fields, and so he called light an electromagnetic radiation.
Maxwell then went on to predict that other kinds of electromagnetic radiation must exist. He was proved correct when, inspired by his teachings, the German scientist, Heinrich Hertz, first generated radio waves in 1881. Other waves and rays were discovered later.