DOUBLE SLIT INTERFERENCE
The diagram shows that first and second bright fringes that result from the interference of two coherent light sources. Considering the top bright fringe, the top slit wave path W and the lower slit path X. By pythagoras W² is equal to (y - a / ₂)² - D² and X² is equal to (y + a / ₂ )² - D². Recognising W² - X² as the difference of two squares, W - X = ( ( y + a / ₂)² - ( y - a / ₂)² ) / ( W + X ), or more simply W - X = ( 2 a y ) / ( W + X ). If the distance D is large then the angle of the first fringes will be small and W + X can be approximated to D, i.e.

• This above experiment (young's double slits) determines that light is wave like in nature as it can cancel itself out.

COHERENCE
In order to produce a good interference pattern the light sources must be coherent, that is, they must have a constant phase difference and the same frequency. Above by using slits the light source is the same.