Question 1

What is thin-film interference?

Accepted Answer

Thin-film interference happens when light reflects off both the top and bottom surfaces of a transparent film a few hundred nanometres thick. The two reflected waves overlap; depending on the extra path travelled by the lower wave, they reinforce or cancel for each wavelength, producing shifting rainbow colours.

Question 2

Why do soap bubbles show rainbow colours?

Accepted Answer

A soap film is a thin layer of water bounded by air. Light reflecting from the front and back of the film interferes. Because the film thickness varies and drains over time, different regions reinforce different wavelengths, so you see swirling bands of colour that change as the film thins.

Question 3

What is the path difference equation?

Accepted Answer

The optical path difference between the two reflected rays is 2·n·d·cos(θ), where n is the film's refractive index, d its thickness and θ the angle of refraction inside the film. A half-wavelength phase shift is added when light reflects off a denser medium.

Question 4

When is the reflection constructive?

Accepted Answer

For a film with a single hard reflection (e.g. soap in air), constructive interference in reflection occurs when 2·n·d·cos(θ) = (m + 1/2)·λ, and destructive when 2·n·d·cos(θ) = m·λ, for integer m. The extra half wavelength comes from the π phase shift on the air-to-film reflection.

Question 5

Why does the colour change with viewing angle?

Accepted Answer

As the viewing angle increases, the angle of refraction θ inside the film grows and cos(θ) shrinks, reducing the effective path difference 2·n·d·cos(θ). This shifts the reinforced wavelength toward the blue end, so the same patch of film looks a different colour when tilted.

Question 6

How thick is a typical iridescent film?

Accepted Answer

Visible interference colours appear for films roughly 100 to 1000 nanometres thick — comparable to the wavelength of visible light. Thicker films wash out into white because many wavelengths reinforce at once and the colours overlap.

Question 7

Why does an oil slick look different from a soap bubble?

Accepted Answer

An oil film floats on water, so light reflects at air-to-oil (low to high index) and then oil-to-water. If both reflections flip phase the net half-wave shift cancels, changing which condition gives constructive interference. The refractive indices of oil and water also differ from a soap film, shifting the colour palette.

Question 8

What is a Newton's ring and how does it relate?

Accepted Answer

Newton's rings are thin-film interference fringes formed in the air gap between a curved lens and a flat plate. The gap thickness changes smoothly with radius, producing concentric bright and dark rings — exactly the same physics as the colour bands on a soap film.

Question 9

Does refractive index affect the colours?

Accepted Answer

Yes. The path difference scales directly with n, so a higher refractive index reinforces longer wavelengths at the same thickness. Anti-reflection coatings exploit this by choosing n and d so that destructive interference cancels reflection at a target wavelength.

Question 10

How does this simulation compute the colour?

Accepted Answer

For each point on the film the simulation finds the local thickness and refraction angle, evaluates the reflected intensity of red, green and blue wavelengths from the interference condition, and combines them into an RGB colour. Sweeping thickness and angle reproduces the full iridescent spectrum.

🌈 Thin-Film Interference — Soap Bubbles & Oil Slicks

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