The visible light spectrum is the section of the electromagnetic radiation spectrum that is visible to the human eye. It ranges in wavelength from approximately 400 nm (4 x 10-7 m) to 700 nm (7 x 10-7 m). It is also known as the optical spectrum of light.
The wavelength (which is related to frequency and energy) of the light determines the perceived color. The ranges of these different colors are listed in the table below.
Some sources vary these ranges pretty drastically, and the boundaries of them are somewhat approximate as they blend into each other. The edges of the visible light spectrum blend into the ultraviolet and infrared levels of radiation.
The Visible Light Spectrum Color Wavelength (nm) Red 625 - 740 Orange 590 - 625 Yellow 565 - 590 Green 520 - 565 Cyan 500 - 520 Blue 435 - 500 Violet 380 - 435
Most light that we interact with is in the form of white light, which contains many or all of these wavelength ranges within them. Shining white light through a prism causes the wavelengths to bend at slightly different angles due to optical refraction. The resulting light is, therefore, split across the visible color spectrum.
This is what causes a rainbow, with airborn water particles acting as the refractive medium. The order of wavelengths (as shown to the right) is in order of wavelength, which can be remembered by the mneumonic "Roy G. Biv" for Red, Orange, Yellow, Green, Blue, Indigo (the blue/violet border), and Violet. You'll notice that in the image and table Cyan is also appears fairly distinctly, between green & blue.
By using special sources, refractors, and filters, you can get a narrow band of about 10 nm in wavelength that is considered monochromatic light.
Lasers are special because they are the most consistent source of narrowly monochromatic light that we can achieve.