The Electromagnetic Spectrum
The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. Electromagnetic energy travels through space in waves at 299,792.458 km per second, the speed of light. Wavelength is the distance from wavecrest to wavecrest. Frequency is the number of wavecrests passing a given point per second.
The spectrum is divided into regions based on wavelength ranging from short gamma rays, which have wavelengths of 10-6 µm or less, to long radio waves which have wavelengths of many kilometers. Because the range of electromagnetic wavelengths is so vast, the wavelengths are often shown graphically on a logarithmic (powers of 10) scale.
Visible light is composed of wavelengths ranging from 0.4 (blue) to 0.7 (red) µm. This narrow portion of the spectrum is the entire range of the electromagnetic energy to which the human eye is sensitive. When viewed through a prism, this range of the spectrum gives produces a rainbow.
Just beyond the red end of the visible bands are three regions of infrared energy waves. The near-infrared, middle-infrared, and the thermal-infrared regions, along with the visible bands, are commonly used in remote sensing.