Thin films

The index of refraction (h) for a medium is defined as the speed of light in vacuum (c) divided by the speed of light in the medium(v).

cv

Thin filmsReflected light will experience a 180 degree phase change when it reflects from a medium of higher index of refraction and no phase change when it reflects from a medium of smaller index.

Thin films

This is analogous to a wave on a rope reflecting from a stiff wall, or from soft air.

Figure 14-7A Reflected Wave Pulse: Fixed End

Figure 14-8A Reflected Wave Pulse: Free End

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Air gap

Soap filmThe interference colors from a soap film can be related to the thickness of the film by using the interference condition and noting that there is a 180 degree phase change upon reflection from the film surface, but no phase change for the reflection from the back surface. The color seen depends also upon the angle of view.

Soap film

Soap film

Oil filmThe interference colors from an oil film on water can be related to the thickness of the film by using the interference condition and noting that there is a 180 degree phase change upon reflection from the film surface, but no phase change for the reflection from the back surface. This presumes that the index of refraction of the oil is greater than that of the water. The color seen depends also upon the angle of view.

Oil film

Oil film

Antireflective coatingsThe idea behind anti-reflection coatings is that the creation of a double interface by means of a thin film gives you two reflected waves. If these waves are out of phase, they partially or totally cancel. If the coating is a quarter wavelength thickness and the coating has an index of refraction less that the glass it is coating then the two reflections are 180 degrees out of phase.

Antireflective coatings

Example 25-3Roses are Red, Violets are Violet

Figure 25-8The Electromagnetic Spectrum

Electromagnetic spectrum

All electromagnetic waves travel at the

same speed through vacuum

Electromagnetic spectrum

This speed is 3x108m/s

Electromagnetic spectrum

The relationship between the speed of EMR and its frequency and wavelength is

c f