IA is incident at point A, forming an incident angle I between the air and the film surface. The reflected and refracted rays are AR1 and AD, respectively. D is the point where the partial reflection from the glass and film interface. CR2 and AR1 are inextricably linked.
μ(AD + CD) – AB
AD = AC = d/cos r
d tan r = AC/2
d = 1000 Ao