What is the electromagnetic spectrum?

The most basic answer is that the electromagnetic spectrum is the range of all the wavelengths (frequencies) of electromagnetic radiations. It basically describes the range of light in our universe.

Visible light is the only part of the EM spectrum we can see, but there are many kinds of invisible light waves. There are seven kinds of wavelengths in the EM spectrum: radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma. Radio waves have the lowest frequency, and therefore have the largest wavelengths, which means that gamma rays have the highest frequencies and smallest wavelengths in the electromagnetic spectrum. 

The range of wavelengths or frequencies over which electromagnetic radiation extends I think

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