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Thermodynamics

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Non Reflecting Films

One of the practical applications of the interference phenomenon is the making of non-reflecting films. The reflection from a lens or a prism can be decreased to a minimum by coating a thin film of proper thickness.

The material is properly selected such that its refractive index is intermediate between air and glass. Equal quantities of light are reflected its outer surface A and also from the inner surface A and also from the inner surface B between it and the glass. Here light is reflected from the surfaces A and B, while passing from a rarer to a denser medium. The rays are reflected under similar conditions and the same phase change π occurs in each case. If the thickness of the film is λ/4, the two interfering rays, reflected from the upper and the lower surfaces, differ in path by λ/2 and complete destructive interference occurs.

In this case, the thickness λ/4 of the film will be true for one particular wavelength. For this purpose, the wavelength is selected from yellow-green portion of the spectrum where the eye is most sensitive. Therefore, if the incident light is white, some light is reflected at shorter and longer wavelengths and the reflected light has a purple hue. But this a very useful method to decrease the reflection to a minimum value. Such thin coatings of a liquid of proper thickness are known as non-reflecting films.