The original cropped image shows a colour sequence that is a fit to thin film interference simulations.
One by Prof Zawischa (www.itp.uni-hannover.de/fileadmin/it... ) – as a function of optical pathlength difference in nm – is shown bottom left.

Explanation. It’s a dryer drum – stainless steel. It is reported that when stainless steel is heated more chromium diffuses towards the surface and more oxygen diffuses in from the air. The thin film of chromium oxide, which passivates the surface, increases in thickness.

ANU Student Capter of Optica and SPIE IDL2025 event - student presenters and chapter office bearers @optica.org @idlofficial.bsky.social @ausphysics.bsky.social

Happy International #lightday2025 . Here’s an optics-in-the-home puzzle to get those with optics knowledge thinking. What’s the explanation for the attached image? @idlofficial.bsky.social #lightday2025 @ausphysics.bsky.social
@david-optica.bsky.social @optica.org

Happy International Day of Light, May 16th 2025. There was rain but no rainbows in Sydney this morning. So here’s some wet leaves photographed under a rainbow umbrella instead. @idlofficial.bsky.social #lightday2025 @ausphysics.bsky.social
@david-optica.bsky.social @optica.org