Describe the behavior of light that interacts with a thin film.

• When light travels from one medium to another, some of the light is transmitted, some is reflected, and some is absorbed.
• The phase change of a reflected ray depends on the relative indices of refraction of the materials with which the ray interacts.
• A phase change of 180 degrees occurs when a light ray is reflected from a medium with a greater index of refraction than the medium through which the ray is traveling.
• No phase change occurs when a light ray is reflected from a medium with a lower index of refraction than the medium through which the ray is traveling.
• The phase of a wave does not change when it is refracted as it passes from one medium into another.
• Thin-film interference occurs when light interacts with a medium whose thickness is comparable to the light’s wavelength.
• The interactions between the initial reflected light and the light exiting the thin film after being reflected from the second interface exhibit wave interference behavior, resulting in a single wave that is the sum of the two interacting waves.
• The amount of constructive or destructive interference between the two reflected waves depends on the relationship between the thickness of the film, the wavelength of light, any phase shifts, and the angle at which the incident light strikes the film.
• Practical examples of thin-film interference include the color variations seen in soap bubbles and oil films, as well as antireflection coatings.
• The spectrum of colors observed in oil films and soap bubbles arises from differences in the thickness of the film.
• Antireflection coatings eliminate reflected light by applying the relationships between indices of refraction, phase shift, and wave interference to create destructive interference of the light reflected from the two surfaces of the coating.
• The simplest antireflection coating has a thickness equal to one-quarter of the wavelength of the light in the coating, and the index of refraction of the coating is greater than that of air and less than that of the surface upon which the coating is applied. This assumes incident light is normal to the surface.

Quantitative analysis of thin-film interference is limited to waves that are normal to the incident surface.

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• Thin Film Interference