Refraction
Wave 2 has N2 = L/?2 = (L/?)n2 wavelengths in block
Wave 1 has N1 = L/?1 = (L/?)n1 wavelengths in block
hence N2 -N1 = (L/?)(n2 -n1)
phase change of wave 1 is k1x-?t (2?/?1)L - ?t
phase change of wave 2 is k2x-?t (2?/?2)L - ?t
phase difference =(2?L/?)(n2 -n1) = 2?(N2 -N1 )