Thin Films

Thin films -- due to reflection and interference -- provide the beautiful colors we seen in soap bubbles and the non-reflective coatings for camera lenses and for glass in picture frames.

Light reflected from both surfaces of a thin film interfer to provide constructive or destructive interference.

There may also be an unexpected phase change at one of the surfaces due to the nature of the media on both sides of the surface.

Let us begin by looking at a thin layer of oil floating on top of water.

Light is reflected from the first surface of a thin film (at point A in this diagram). And some of the light is refracted and passes through the film and continues on to the second surface where it is again reflected (at point B in this diagram). This light passes through the film again and some of it is refracted (at point C in this diagram) and continues on with and interferes with the light originally reflected at the first surface.

The angle if incidence does make a difference.

However, we will restrict our present attention to the situation shown here, where the light is nearly perpendicular to the thin film.

With air on both sides of this thin soap film, the characters of the two reflections are different. One is from a material of low index of refraction to a material of high index of refraction (n1 < n2) while the other is from a material of high index of refraction to a material of low index of refraction (n2 > n3). These are like the reflections of a pulse in a rope with a fixed end or with a free end. This means there is an extra "phase shift" of one-half a wavelength. This changes our conditions for maxima and minima.

For such a thin film, like a soap bubble, with lower indices of refraction on both sides, . . .

Optical Resolution


Return to Ch 20, Wave Optics: Interference and Diffraction
(c) Doug Davis, 2002; all rights reserved


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