Monochromatic light refers to light that is of one colour. Since it only emits one colour, monochromatic light has a single wavelength. Wavelengths are seen as a rainbow of colours when light is passed through a spectrum. Therefore, monochromatic light would emit just one of these colours. For example, computer monitors emit light containing several wavelengths. However, a monitor that emits only monochromatic light displays just one colour on the computer screen. While the colour may appear in various shades, light to dark and everything in between, only one colour is visible. In most cases, the colour is green, red or white.
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Mono = one,  chroma = color. Monochromatic light is light of a single colour hence single frequency / wavelength. In the nature there is no monochromatic waves because emission of a light occurs at a defined fraction of time.

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