To describe the differences between types of light like visible and x-ray, scientists often talk about the length of the light's waves or "wavelength." The various types of light that make up the electromagnetic spectrum differ in the length of their waves. However, this doesn't mean the entire length of the wave train. The length of a light's wave or the wavelength is the distance between two nearest peaks (the highest places) or two nearest troughs (the place where it dips the lowest).
Test yourself on this diagram
The length of this wave is...
One common unit used to measure the length of light waves is not feet or inches but rather something called an angstrom. You'll find out more about angstroms later on.

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