Frying Pan Optics

Samantha Lawler (Sundog Vegetables) caught this fleeting iridescence.

"I was frying an egg from one of my ducks for breakfast, and noticed a lovely little rainbow, almost like the iridescence you see in a soap bubble or oil slick.  I only managed one decent picture before the bottom layer of the egg cooked and the white turned opaque, which happened within seconds.  I would guess that the iridescence is either from the interface between the white and the melted butter in the pan, or possibly from protein layers within the white itself?  We joked that our organic free-range ducks are so happy that they now include rainbows in their eggs.

©Samantha Lawler, shown with permission

Red light waves reflected from the front and back of a thin film that is denser than its surroundings. Here the waves just happen to be in phase and the film looks red

Shorter wavelength waves
are out of phase

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Swirling iridescent colours of oil films, soap bubbles, cracks in ice and even frying eggs (but not clouds or contrails or bird wings) sign that there are thin films about of different optical properties (refractive index) to their immediate neighbours.

Light reaching a boundary between two different refractive indices is partially refracted into the next medium and also partially reflected. An opportunity for interference. Rays can be reflected towards the eye from (1) the nearest film surface and (2) the far surface. When the film is thin enough the waves of the two reflections interfere. When in phase we see brightness, otherwise degrees of darkness. The in-phase condition depends on viewing angle and film thickness and, importantly, wavelength (colour).