optical path difference of air wedge

1. The problem statement, all variables and given/known data



i cant understand why the optical path diffrence is 2nt+0.5λ. why is it so? the 0.5λ is due to the path difference change of 180 degree of when it is reflected lower ray when it is reflected from the lower glass surface... why there's also phase change of 2nt. since both ray pass thru the glass surface twice , so it should cancel each other , leaving only phase change of 0.5λ . Am i right?



here's the link :

http://ift.tt/1w1ykcL

2. Relevant equations







3. The attempt at a solution