The heat produced by the Sun travels from the Sun to the Earth via waves known as electromagnetic waves. These waves can vary greatly in their wavelength. Have you ever thrown a rock into a lake? What happens when you throw a small pebble into the water? The resulting ripples are small, and each wave is close to the next wave; they have a short wavelength. Now, consider what would happen if you threw a large boulder into the water. Instead of small ripples, you get large waves. These larger waves are spaced further apart from one another. Thus, they have a large wavelength.
Because the electromagnetic waves traveling to the Earth from the Sun come in a variety of lengths, scientists consider them to be a spectrum. Thus, we refer collectively to all these waves as the electromagnetic spectrum. The electromagnetic spectrum is a big word that simply refers to all the different sized waves of energy traveling outward from the Sun, as well as from many other objects in the Universe.
To better understand the electromagnetic spectrum, scientists break it into three separate categories or divisions. The shortest waves are called ultraviolet waves. The medium sized waves are called visible light waves, and the longest waves are called infrared waves. Even longer waves also are categorized into radio waves, microwaves, and so forth.