The light source is monochromatic (single colour) which means it gives out a narrow range of wavelengths.

The distance, D, is about 2m. On the screen we see a series of bright and dark lines called interference fringes.

For a constant interference pattern, we need two sources having a constant phase relation (two coherent sources).

The single slit ensures that the two slits in the double slit are coherent sources. In the following explanation, we will assume that waves leave the two slits in phase.

Point O is midway between the two slits so waves from the two slits will arrive at O in phase. At O we see a bright fringe due to constructive interference. This fringe is the central maximum of the interference pattern.

Point A is further from slit b than from slit a. The path difference is bA-aA. If this is equal to l/2 then destructive interference will occur and a dark fringe (a minimum) will be observed.

Point B The path difference is now bB-aB. If this is equal to l then constructive interference will occur and a bright fringe will be observed.

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