Every silver lining
Yesterday I got back home from Turkey. I'd been running a workshop on IA for gp4 teachers. Taking a flight home at 4am doesn't seem too drastic but leaving the hotel at 1:30 was. When I got back to Flekke at 19:30 my wife had left the car at the bus stop so I could drive the last 3km home. It was in the car park that I spotted the rings in the photo. The colours are a classic example of thin film interference. Light reflecting off the top and the bottom of the oil film interferes cancelling out the light unless it happens to be of the correct wavelength for constructive interference. This depends on the thickness of the oil film, if its very thin the only constructive interference happens for violet light (shortest wavelength), if a bit thicker then blue etc. in this way the colours create "contours of thickness" . The rings are formed because the oils is draining of a small lump in the tarmac. Its thinnest in the centre so this is violet followed by the other colours. I have to be purposely vague about the colours as I am colour blind and can't really tell them apart. Towards the outside the colours repeat as the optical path difference becomes 2λ. I'm sure it's very pretty for those of you who can see the colours. When I got home I saw that there were more oil rings outside my garage. Its seems that the oil is leaking from my car. Hence the saying "every silver lining is in the middle of an ugly storm cloud".