Energy from sunlight is called the electromagnetic spectrum. Energy in the form of light waves travels from the sun to Earth. Light waves travel similarly to waves traveling across a lake. The distance from the peak of one wave to the peak of the next wave is the wavelength. Wavelengths are measured in micrometres (µm) or nanometers (nm). One µm is one-millionth of a meter or 1,000 nm. A nanometer is one-billionth of a meter. The visible region (light seen be human eyes) of the electromagnetic spectrum is from 400 nm to 700 nm.
The wavelengths used in most agricultural remote sensing applications cover only a small region of the electromagnetic spectrum. The green color associated with plant vigor has a wavelength of approximately 500 nm.