i would kndly like to know that waves originate from a source in form of wavelets (according to Huygen),i.e.,every particle on them must have same phase but when 2 waves pass through slits their occurs a phase difference that results in interference,why??

Dear kshitij

In this  case phase difference occure due to path difference.

In ydse whan wave passes through slit ,then for every point other than the point which lie on the perpendicular bisector of two slit, one wave must have to traveel greter distance than other . so due to difference in distance travell phase difference occure.

Please feel free to post as many doubts on our discussion forum as you can.
If you find any question Difficult to understand - post it here and we will get you
the answer and detailed  solution very  quickly.

 We are all IITians and here to help you in your IIT JEE preparation.

All the best.
 
Regards,
Askiitians Experts
Badiuddin