Abstract: Certain areas of the pattern show zero contrast in the Young＇s double slit experiment with a nonmonochromatic point source. Two theoretical explanations are proposed here, with one applying the contrast defined by optical path difference and another indicating it is due to wave trains cannot overlap when the length of them is less than the optical path difference.

Key words: interference, Young＇s double slit experiment, nonmonochromatic light, contrast, wave train