Work out thin film interference instantly with clear inputs, formula shown and shareable results.
Light reflected from the two surfaces of a thin film interferes. Reflection at the front surface flips the phase by half a wavelength, so constructive reflection needs 2nt = (m + 1/2)lambda while cancellation needs 2nt = m.lambda. A 400 nm soap film at n = 1.33 reinforces around 709 nm, giving it a reddish sheen.
Constructive reflection
2 n t = (m + 1/2) lambda
Destructive reflection
2 n t = m lambda
Reflection off a medium of higher index inverts the wave. Only the front surface does this here, so the two reflections start half a cycle apart.
Yes. A quarter-wave layer of the right index makes the two reflections cancel, which is how camera lens coatings suppress glare.