Thin-Film Interference Lab
Change a film's thickness and index and watch its reflected color shift, or find the exact quarter-wave thickness that cancels reflection for anti-reflective coatings.
Slide the film thickness and watch the reflected colour change (soap-film mode) or watch a single wavelength's reflection switch between bright and dark (anti-reflective coating mode).
About the Thin-Film Interference Lab
Free thin-film interference lab. Change a film's thickness and index and watch its reflected color shift, or find the exact quarter-wave thickness that cancels reflection for anti-reflective coatings. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for physics, the thin-film interference lab runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Change a film's thickness and index and watch its reflected color shift, or find the exact quarter-wave thickness that cancels reflection for anti-reflective coatings. Use it to explore physics ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Thin-Film Interference Lab
- Use the controls to change Case, Film thickness t, Film refractive index n, Wavelength λ. The simulation reacts instantly.
- Pick an option such as Anti-reflective coating (1 phase shift), Soap film in air (1 phase shift, symmetric) to switch modes or load an example.
- Press "Reset to defaults", "Lab Report" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Slide the thickness slowly and watch the reflected color swatch cycle.
- Design a quarter-wave anti-reflective coating for green light.
- Switch between the coating and soap-film cases and compare.
- Compute the optical path difference by hand and check it against the readout.
Key ideas you can learn
- Thin-film interference happens because light reflects off both the top and bottom surfaces of a thin film, and those two reflections can add constructively or destructively depending on the extra path length.
- The optical path difference is OPD = 2nt, where n is the film's refractive index and t is its thickness.
- Anti-reflective coatings use a quarter-wave thickness, t = λ/(4n), to cancel reflection of a target wavelength by destructive interference.
- A soap bubble's shifting colors come from its thickness changing as it drains, which shifts which wavelengths interfere constructively at each point.
Show my work
Challenges
Challenge 1 - design an anti-reflective coating
Use the coating case (one net phase shift, so destructive reflection needs 2nt = mλ, minimum thickness at m=1... but the standard quarter-wave AR design uses 2nt = λ/2, i.e. t = λ/(4n)). With n = 1.38 and λ = 550 nm, find the minimum non-zero coating thickness (in nm) that cancels the reflection of that wavelength.
Challenge 2 - find the optical path difference
Set the thickness slider to 200 nm and the index to 1.4. Compute the optical path difference OPD = 2nt in nanometers.
Where this is used in the real world
Camera lenses, eyeglasses and solar panels use engineered anti-reflective thin-film coatings, and the same physics explains the colors seen in soap bubbles and oil slicks on wet pavement.
Who is this simulation for?
Physics students in middle school, high school and first-year university, teachers who want a quick demonstration for the projector, and anyone revising for exams. It works well for flipped classrooms because students can explore before the lesson.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
Why do soap bubbles show rainbow colors instead of one plain color?
The film's thickness varies slightly across the bubble and changes over time as it drains, and different thicknesses satisfy the constructive-interference condition for different wavelengths, so different colors reflect strongly at different spots.
How does a quarter-wave coating stop reflections?
At thickness t = λ/(4n), the light reflecting off the bottom surface travels an extra half-wavelength compared to the top reflection, so the two reflected waves arrive out of phase and cancel by destructive interference.
Is the Thin-Film Interference Lab free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Thin-Film Interference Lab work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.