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What are light wavelengths?

The wavelengths of electromagnetic radiation span from the very short wavelengths of cosmic and gamma rays, to the very long waves that are used for radio broadcasting. The wavelengths plants use for photosynthesis fall within a very small portion of the total electromagnetic spectrum called the visible and near visible light spectrum. 

The wavelengths of the visible and near visible light spectrum is measured in nanometers, 100 nm – 1000 nm, which refers to the length of the wave. At the low end of the spectrum is short wave ultraviolet light, 100 nm. At the high end of the spectrum is long wave infrared, 1000 nm. In the middle of the spectrum are the wavelengths that plants use for photosynthesis, 400 nm – 700 nm.  

This range of 400 nm – 700 nm is referred to as PAR. PAR is an abbreviation of photosynthetically active radiation.

Across the range of PAR, there are different wavelengths of light. We see these wavelengths of PAR as colours like this:

400 nm – 500 nm /  blue – green

500 nm – 600 nm / green – orange

600 nm – 700 nm / orange – red