Thin-film interference
(O3.3.1) O3.3 · Thin Films →The condition for constructive interference between light reflected from the top and bottom surfaces of a film of thickness t and refractive index n.
| refractive index of the film | — | |
| film thickness | m | |
| fringe order | — | |
| wavelength | m |
A film thin enough, and light coherent enough, for the two reflected waves to interfere; illumination close to normal incidence.
Requires the film to be thin enough and light coherent enough for the two reflected waves to interfere, near normal incidence — a thick or non-coherent setup does not produce clean interference.
The path difference is twice the film's thickness, since light travels down and back up through it; the constructive/destructive condition also depends on whether either reflection introduces an extra half-wavelength phase shift.
Whether a reflection introduces an extra half-wavelength phase shift depends on whether that reflection is off a HIGHER or LOWER index medium — the single most common source of error in thin-film problems.