Everything from X-Rays to visible light to radio waves are forms of electromagnetic radiation. You can think of it like ripples on a pond: they move through space and they carry energy, but they have no mass. (These waves don't move through water, of course, they move through the electric field.) If you look at a wave as a curve that goes up and down, you can see waves three key attributes: frequency (number of up-and-down cycles per unit of time), amplitude (how high up and down the wave goes), and wavelength (the distance between consecutive cycles).
[Picture of a wave]
Wavelength (distance per cycle) times frequency (cycles per time) gives us speed (distance per time). All electromagnet radiation moves at the speed of light. This means that everytime you increase the frequency of a wave, the wavelength decreases (and vice versa). This leaves us with frequency and amplitude as the two features we can adjust.
Different types of waves are distinguished by their frequency. (Frequency is measured in cycles per second, called Hertz (Hz).)
Radio waves, where different frequencies identify different stations, are between 3000 to 300000000 Hz. When you tune your radio to 91.5 FM, you're listening for waves with 91500000 cycles per second.
Light waves, where different frequencies identify different colors, have a frequency around 430000000000000 Hz (red) to 760000000000000 Hz (violet). When something appears red to you, it's because your eyes are being hit with waves with 430000000000000 cycles per second.
Collectively, the space of all different frequencies is called the spectrum.
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