What is a monochromatic light?

Ans:

Breaking the word down mono denotes one and chromatic means of colour so a fairly literal translation for the adjective monochromatic would be having one colour. The colour of light is resolute by its wavelength so it makes sense that monochromatic light is light of a single wavelength.

For the reason that the speed of light in vacuum is constant and that speed is related to wavelength and frequency by c = lf specifying the frequency of light is essentially equivalent to specifying its wavelength. Consequently monochromatic light can also be defined to be light of a single frequency.