Young's double slit experiment produces regular intervals of bright fringes and dark fringes. The bright fringes happen when the 2 sources of wave are in phase and superimpose while the dark fringes are produced when the 2 waves are in antiphase and vectorially cancel each other out. However, I've a little confused. It is said that for the experiment to work, the sources of light must be coherent. Meaning that they have constant phase difference; always in phase/ antiphase, having a fixed phase difference. The fact that there are dark and bright fringes indicate that the phase difference is not constant meaning that they are not always in phase/ antiphase. Am I missing something here or am I interpreting it wrongly?