Plane-parallel thin film

 

Plane-parallel thin film

A monochromatic light ray $S$ is incident obliquely on the upper surface of the film at $P_1$ with an angle of incidence $i$. At $P_1$, the incident ray is divided into a reflected ray $R_1$ and a refracted ray that enters the film at an angle $r$. The refracted ray travels through the film and reaches the lower surface at $C_1$, where it is again divided into a transmitted ray $T_1$ and a reflected ray. This reflected ray travels upward to the upper surface at $P_2$, where a part emerges as the second reflected ray $R_2$, while the remaining part is reflected again towards the lower surface. In the same manner, repeated reflections and transmissions take place between the two parallel surfaces of the film, producing successive reflected rays $R_1, R_2, R_3,\ldots$ and transmitted rays $T_1,T_2,\ldots$. Since all these emergent rays are parallel and originate from the same incident wave, they are coherent and can interfere with one another.