Interference



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              Interference effects are commonly observed in thin films, such as thin layers of oil on water or the thin surface of a soap bubble. The varied colors observed when white light is incident on such films result from the interference of waves reflected from the two surfaces of the film. Consider a film of uniform thickness t and index of refraction n, as shown in figure. Let us assume that the light rays traveling in air are nearly normal to the two surfaces of the film. To determine whether the reflected rays interfere constructively or destructively, we first note the following facts:

• A wave traveling from a medium of index of refraction n1 toward a medium of index of refraction n2 undergoes a 180° phase change upon reflection when and undergoes no phase change if
• The wavelength of light λn in a medium whose refraction index is n; 

                                        λn = λ / n

    where λ is the wavelength of the light in free space.