14.6.A.1 When light travels from one medium to another, some of the light is transmitted, some is reflected, and some is absorbed.
14.6.A.2 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° 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.
14.6.A.3 The phase of a wave does not change when refracted as it passes from one medium into another.
14.6.A.4 Thin-film interference occurs when the light interacts with a medium whose thickness is comparable to the wavelength of light.
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 and the wavelength of light and the angle at which the incident light strikes the film.
14.6.A.5 Practical examples of thin-film interference include the color variations seen in soap bubbles and oil films, as well as anti-reflection coatings.
The spectrum of colors observed in oil films and soap bubbles arises from differences in the thickness of the film resulting in varying wavelengths of the waves constructively interfering due to thin-film interference and the varying angle at which the light is incident on the surface.
Anti-reflection coatings eliminate reflected light by applying the relationships between the indices of refraction and phase shift and wave interference to create destructive interference of the light reflected from the two surfaces of the coating.
The simplest anti-reflection coating has a thickness equal to one-quarter wavelength of the incident light 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.