Physics

Sir in your lecture #54 part 2  , you say that when light strikes with soap bubble , the condition for constructive interference  is reversed and for destructive interference,  the condition will be 2t =m (lamda) . Sir i can't understand , in destructive  interference one Ray can't match with 2nd Ray or they cancel out each other , so how could  you say destructive  interference equals to 2t = m (lamda). Plz sir explain it or if anyone know the answer then solve my confusion ??