my doubt is that everyone knows that velocity of light is constant everywhere in universe.then why does velocity of light decreases when light passes through any denser medium like glass or water with respect to air.

the velocity of light is constant not in whole universe it depends from medium to medium there may be a constant speed of light in one medium but it may be some other in other medium.

Many scientist have tried to explain this phenomena Eienstein has said that when light travels in the form of packets i.e. photons . when this light enters from one medium to another then these photons strike the molecules present in medium and thus their speed slows down just like hurdles kept in ones way.

the velocity of light in a medium is constant everywhere. what confused you must have been an ambiguous statement that didnt say anything about the medium. for eg. the velocity of light in vacuum is constant everywhere in the universe i.e. approximated to 3 * 10^8 m.s^-1

velocity of light in medium is related to refractive index of that medium as

         c/v =u (refractive index) 

 c is speed of light vaccume

 so,           v is inversly proportional to refractive index

      therefore on passing through dencer media speed gets reduced...