LIGHT & OPTICS · LAB 22

Thin-film interference

Explore how reflections from a transparent film reinforce or cancel to create the colours of soap bubbles, oil films and optical coatings.

INTERFERENCECONSTRUCTIVE

The film is magnified; thickness values and optical calculations are shown to scale numerically

IncidentInternal pathEmerging reflections

LOCAL THICKNESS—

—

OPTICAL PATH—

—

PHASE REVERSALS—

π shifts at reflections

REFLECTED—

— transmitted

DOMINANT COLOUR—

—

WHAT TO NOTICE

Colour from phase

The two reflected waves travel different optical distances. Depending on their phase difference, they reinforce or cancel. Reflection from a higher-index boundary adds a half-wave phase shift, changing which wavelengths are bright.

OPD = 2nt cos θtΔφ = 2π·OPD/λ + φr

Educational thin-film model using ideal interfaces and approximate colour conversion.