The electromagnetic spectrum is made up of all electromagnetic radiation frequencies that spread energy and travel through space in the form of waves.
People use the electromagnetic spectrum every day! Did you know the microwave oven you use to heat your food and the cell phone you use every day rely on the electromagnetic spectrum? Even the light we see with our eyes is part of the spectrum.
Longer wavelengths with lower frequencies are a part of the radio spectrum. Shorter wavelengths with higher frequencies are a part of the optical spectrum. Waves can be very big like a building or very small like an atom.
