More rainbows this weekend - and snow, hail, sleet, rain, thunder, even a little sunshine - a real mixed bag of weather.Standing in the stable yard looking up at the rainbow that had just appeared against the dark grey sky (more rain, snow, sleet, or was it going to be hail?) I glanced down and saw another rainbow on the concrete beneath my Wellington booted feet. Was I standing on, as my Uncle Charlie once told me, the spot where the rainbow ends? What a wonderful idea, but no this was diesel from the tractor probably. Still that’s what old Charlie used to tell me, the rainbows in the puddles were the spots where the rainbow ended or sometimes sprung up from. We used to stand, man and boy, staring into the iridescent puddles trying to work out which – ‘start or end?’
Stood remembering I wondered, not for the first time, what causes this miracle to happen?
Here’s the scientific answer - the reason that petrol appears colourful when it contacts water has to do with the relative density of the materials, both optical density and mass density. Petrol, as a liquid chemical, has a significantly lower mass per volume, or density, than the water it sits on top of. This causes a very thin film of petrol to spread out onto the waters surface. This film is so thin that the distance between it's top and bottom is something approaching the wavelength of the visible light spectrum. So, a film of petrol on a body of water will appear colourful, because each particular area of the film is not a uniform thickness.
On balance I think I prefer Uncle Charlie’s explanation. Yes, I’ll stick with that – the place where rainbows start from or where the rainbows end.
How do you tell the difference between the start and end?
ReplyDeleteI never did work that one out - perhaps old Charlie could tell us.
ReplyDeleteI don't want an explanation - I like Uncle Charlie's version. Beautiful photos. I just stopped and stared down at one in B&Q's car park - see what you are doing to me!
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