2.光程差两束光光程之差 S p 1p=r1 p- δ=n1r1n2r δ=(r1-e1+n1e1)-(r2-e2+n2e2) 图20-1 图20-28 2.光程差—两束光光程之差 = = s2 s1 r2 r1 n1 p n2 图20-1 s2 S1p= r1 s1 p S2p= r2 图20-2 n1r1- n2r2 (r1 -e1 +n1e1 ) - (r2 -e2 +n2e2 )