Reflection from a single thin film

The appearance of the film will depend on the extent of this interference. In the case of an observer looking down on a film in a perpendicular direction, some of the light incident on the surface and seen by 

If the path difference, [p], is equal to a half-integral number of wavelengths the film will then appear bright, because constructive interference will occur  

At other path differences, the film will appear to have an intermediate tone, depending on the exact phase difference between the rays. 

Should the light beam fall on the surface at an angle of incidence Q, producing an angle of refraction, the path difference, [p], between rays 1 and 2 is  

The reflectivity of a thin film in air will be different from that for a thick plate, given in Section 14.5.1, as interference effects from the bottom surface also need to be considered. 

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