If a monochromatic light source – light of a single wavelength, frequency or colour - is set behind a screen with a single slit a which allows light to pass, and then allowed to fall on a screen with two other slits as shown below, then the two slits b and c will act as point sources of light.
Such an arrangement is Young's double slit experiment. Light from these two sources will be coherent and monochromatic and an interference pattern will be produced as shown, with alternating bright fringes, where constructive interference occurs, with the path difference for light from the two slits being a whole number of wavelengths, and dark fringes, where destructive interference occurs, with the path difference for light from the two slits being a whole number of wavelengths plus half a wavelength. Lines of constructive and destructive interference will spread out from the midpoint of bc as shown below.
