6.11 Thin Film Interference

Two Reflections, One Path Difference

Thin film interference occurs when light reflects off both the top and bottom surfaces of a thin film (like a soap bubble or oil slick). The two reflected waves recombine and interfere. The result depends on the extra path length (about 2t, where t is film thickness) and any phase shifts from reflection.

Phase Shifts on Reflection

When light reflects off a boundary going from low to high index of refraction, it undergoes a 180° (half-wavelength) phase shift. Reflecting off a boundary going from high to low index causes no phase shift. Counting shifts at both surfaces tells you whether to use constructive or destructive conditions.

Constructive vs. Destructive Conditions

Using wavelength inside the film (λ_film = λ/n): if the two reflections have the same phase shift (both or neither shifted), constructive interference needs 2t = mλ_film; if the reflections have different phase shifts, constructive needs 2t = (m + ½)λ_film. Swap conditions for destructive interference.

Key terms

Thin film interference

Interference between light reflected from the top and bottom surfaces of a thin film, depending on path difference and phase shifts.

Phase shift on reflection

180° shift when reflecting low-to-high index; no shift when reflecting high-to-low index.

λ_film formula

λ_film = λ/n, the wavelength of light inside a medium with refractive index n.

Constructive interference (same phase shifts)

2t = mλ_film, where m is an integer (0, 1, 2, ...).

Constructive interference (different phase shifts)

2t = (m + ½)λ_film, due to the extra half-wavelength shift between the two reflections.

Try an AP-style question

Why do soap bubbles show colors?

1. The air inside the bubble glows faintly
2. The bubble absorbs only some colors of light
3. The soap is dyed many different colors
4. Light reflecting from the film's two surfaces interferes