Thin-Film Interference

When light strikes a thin film, it reflects from both the top and bottom surfaces. These two reflected rays can interfere constructively (producing bright colors) or destructively (producing darkness) depending on their relative phase shift.

Experiment Simulation

Use the Thin-Film Interference module to verify the calculation and explore other conditions.

1. Set Film refractive index n to 1.5, Substrate to Air, Film thickness d to 100, and Vacuum wavelength lambda to 600. Read the round-trip optical length 300.0 nm, relative phase 1.0 cycles, reflection shift 0.5 cycles, and Normalized Intensity 1.00000000000.
2. Change Film thickness d to 200. The round-trip optical length becomes 600.0 nm. The propagation shift is now 600 / 600 = 1.0 cycle. Added to the 0.5 reflection shift, the total phase is 1.5 cycles, giving an intensity of 0 (a dark reflection).
3. Change Substrate to Glass and Film refractive index n to 1.25. Now the bottom reflection goes from n=1.25 to Glass (n=2.5), causing a second phase inversion. The relative reflection shift is 0. Set Vacuum wavelength lambda to 500 and Film thickness d to 100. Read relative phase 0.50 cycles and intensity 0.
4. Set Film thickness d to 0 with Substrate Air. The propagation path is 0, leaving only the 0.5 reflection shift. The relative phase is 0.5 cycles (dark).
5. Set Film thickness d to 0 with Substrate Glass. Both surfaces cause phase inversions, so the relative shift is 0. The relative phase is 0 cycles (bright).