Optics
Thin-film interference
Reflections off the two surfaces of a thin film interfere; 2nt = (m+½)λ picks the brightly reflected colour (soap-bubble colours).
Thin-film interference — interactive Optics simulation. Reflections off the two surfaces of a thin film interfere; 2nt = (m+½)λ picks the brightly reflected colour (soap-bubble colours).
Thin-film interference
Reflections off the two surfaces of a thin film interfere; 2nt = (m+½)λ picks the brightly reflected colour (soap-bubble colours).
Reflections from top and bottom film surfaces interfere. A π phase shift may occur on reflection from a denser medium. Thickness and wavelength pick which colors constructively reflect—the iridescence of soap bubbles and oil slicks.
- 2nt = (m+½)λ
- Soap-film colours
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
Light reflecting off both surfaces of a thin transparent film (like a soap bubble) interferes, producing bright or dark reflection depending on the film's thickness. Does a thicker film always shift which interference 'order' produces bright reflection?
Predictions to weigh
- Yes — the interference order for bright reflection scales directly with the film's thickness (in units of the wavelength inside the film).
- No — the film's thickness doesn't affect which order is bright.
- Thickness only affects the color, not a numerical 'order.'
Variable roles
What you set:
- Film thickness t (nm)
What you measure:
- Bright-reflection order m
How the investigation runs
- Open the thin-film preset and press Reset. The film has index n = 1.33 (like a soap film); light wavelength is 550 nm.
- Enable the interference-order readout.
- Set the film's thickness for each trial and record the bright-reflection interference order.
Governing equation
Thin-Film Bright-Reflection Order — 2nt = (m+½)λ
Light reflecting off both surfaces of a thin film interferes constructively (bright reflection) when the round-trip optical path length matches a half-integer multiple of the wavelength: m = 2nt/λ − 0.5, accounting for the front-surface phase shift.
What the printable worksheet asks students to work out
- For one trial, compute m = 2nt/λ − 0.5 using n = 1.33, λ = 550 nm. Compare to the table.
- Explain why the −0.5 term appears: light reflecting off the FRONT of the film undergoes a half-wavelength phase shift (reflecting off a higher-index medium), while light reflecting off the BACK doesn't — this asymmetry shifts the usual integer-order condition by half a step.
Where this shows up beyond the lab
- A soap bubble's colors constantly shift and swirl because the film's thickness varies across its surface and changes over time as it drains and evaporates — different thicknesses satisfy the bright-reflection condition for different wavelengths (colors) at any given moment. Explain why a perfectly uniform-thickness film would instead show a single uniform color (or none) across its whole surface.
- Camera lenses use thin anti-reflective coatings, engineered so the front and back reflections DESTRUCTIVELY interfere for a chosen wavelength (usually green, the eye's most sensitive color), minimizing unwanted reflection. Explain why such coatings often still show a faint purple tint — the wavelengths at either end of the visible spectrum aren't as fully cancelled.
- AP Physics 2 — Unit 14: Waves, Sound, and Physical Optics
- IB Physics — C.3 Wave phenomena
- General High School Physics — Light & optics
- NGSS High School Physics — Wave properties
- Welcome to Thin Film
- Select the thin film
- Press Play
- Colour from film thickness
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.