Light from a monochromatic line source passes through a lens and is focused on to a single slit S. It then falls on a double slit (S1 and S2) and this produces two wave trains that interfere with each other. The interference pattern at any distance from the double slit may be observed with a micrometer eyepiece or by using a screen.
Where m = 0,1,2,3 etc. and so the mth bright fringe for m = 3 is 3lD/d from the centre of the pattern. The distance between adjacent bright fringes is called the fringe width (x).