Example: find light path difference p- δ=n11-n2 6=(r1e1+n1e1)-(r2e2+n2e2 c △6=n2L-n1L L=n1 r1- n2 r2 = (r1 -e1 +n1e1 )- (r2 -e2 +n2e2 ) s2 s1 r2 r1 n1 p n2 s2 S1p= r1 s1 p S2p= r2  = n2 L − n1 L Example: find light path difference