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Pasta Films

Mario Freitas of Universidade Tecnológica Federal do Paraná likes pasta. 

I'm writing to you after dinner :)

I like to cook my pasta using a large water boiler as a pan. Today, before I washed it, I noticed at its inside bottom a rich colored pattern, probably due to a thin film interference.

The top image was taken from above the pan, looking downwards, with the camera flash on.   The second is a detail using macro zoom.  The third is the boiler on my gas stove. I am conscious that it's not exactly a beautiful image, but perhaps it's important to see the context.

Extensive further research in The OPOD Gastronomy Laboratories confirm these colours. They form when cooking pasta but not in the same pan when boiling vegetables.  Further work is needed on potatoes and rice.

The colours undoubtedly arise from optical thin film interference as Mario states.  

We might have some PhD students researching the starch chemistry relating to its role in forming optically thin films.  Are the pasta starches themselves producing thin transparent films or are they thermolysis/hydrolysis products?   An auxiliary PhD topic could be the effect of egg freshness and mass on dough kneadability/pasta cooking time and quality not to mention its thin film forming propensity.

All images ©Mario Freitas, shown with permission

The iridescent colours arise from interference between light waves reflected from the front and back film surfaces of the (starch?) film.

Take the red waves at right. Some of the incident waves are directly reflected from the front. Note that there is a 180° phase change when this happens. Some light enters the film and is internally reflected from its rear surface. The two waves combine. In the example the two waves are in phase and a red reflection would be seen in that direction.

Compare that with the shorter wavelength blue waves. The two outgoing waves are out of phase and cancel each other. No blue would be seen.

The particular colours produced depend on viewing angle, on the film thickness and its refractive index.