Thin film interference
Examples of thin film interference.
Soap bubbles
Oil film on puddle
Explaining thin film interference
Why do we see the colours reflecting from the thin film?
The thickness t of the film determines the path difference between ray 1 and ray 2
As an approximation, the path difference = 2t
Where path difference = nλ for a wavelength (colour) we get constructive interference
between reflecting rays and we see that colour as a bright colour.
Where path difference = (n+1/2) λ for a wavelength (colour) we get destructive interference
between reflecting rays and that colour will not be seen
Using thickness t to calculate path difference is an approximation and, as such, has
limitations.
Phase changes at boundaries
Thin film interference occurs between the light reflected from the top and bottom surfaces
of a film. In addition to the path length difference, there can be a phase change.
When light reflects from a medium having a refractive index greater than that of the
medium in which it is traveling, a 180º phase change (or a λ/2 shift) occurs.
This could mean that one or both of the surfaces has a phase change on reflection.
Example Soap bubble:
Top boundary: airoil = 180 phase
change (λ/2 shift)
Bottom boundary: oilair = no
phase change (no shift in λ).
Uses of thin film interference
Camera lenses are coated with a thin film to reduce reflections and increase transmission.
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