• A rainbow is created by sunlight passing
through drops of water, which act as prisms.
This distribution of colors is called a spectrum; separating light into a spectrum is called spectral dispersion.
The reason that the human eye can see the spectrum is because those specific wavelengths stimulate the retina in the human eye. The spectrum is arranged in the order red, orange, yellow, green, blue, indigo, and violet according to the different wavelengths*1 of light; the light in the region with the longest wavelengths is seen as red, and the light in the region with the shortest wavelengths is seen as violet. The light region which the human eye can see is called the visible light region.
If we move beyond the visible light region toward longer wavelengths, we enter the infrared region; if we move toward shorter wavelengths, we enter the ultraviolet region. Both of these regions cannot be seen by the human eye.
Light is just one portion of the various electromagnetic waves flying through space. The electromagnetic spectrum covers an extremely broad range, from electrical and radio waves with wavelengths of several thousand kilometers to gamma (³) rays with wavelengths of 10-13m and shorter. The visible light region is only a very small portion of this: from approximately 380 to 780nm*2. The light reflected from an object and which we recognize as color is (with the exception of man-made monochromatic light) a mixture of light at various wavelengths within the visible region.
*1 Wavelength: Light has wave characteristics; wavelength is the peak-to-peak distance of two adjacent waves.
*2 nm (nanometer): A unit of measure often used when discussing wavelengths of light; mm (micrometer) is also sometimes used.