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Why is the blue cave (of Kastellorizo) blue?

August 2, 2013

Six short words, bar the brackets: a simple question, but not simple to answer. One of the children asked me yesterday whilst we were in the cave itself, at Meis, the Greek isle (Kastellorizo in Greek, Meis is the Turkish name for it) yesterday.

Whilst the short answer 'it is reflection/refraction' satisfied Beardsworth minor's question, it didn't satisfy me, I wanted to know more, and it gave me something to think about on the sunbed a Saint George's the islet we stopped off at on the way back to the main town.

I got thinking of rainbows.

Well not, in this way, but isn't it amazing what results googling produces?


I was pretty sure that, once I got home, and checked, that I would fine out that all rainbows are red on top, blue in the centre: and so it was. Blue light refracts more than the other colours, and this is part of why the blue cave is blue.
There are two more pieces of the jigsaw. The water would be black were it not for a light source, and here the source is the small crack– you have to fold yourself down, low into the boat, in order for the boat to get through the narrow cave.

The white light comes in, in the main, horizontally. When the sun is highest in the sky, at noonish, I think the effect will be greatest: there will be less light coming in at an angle: whether the position of the sun has any practical effect I don't know, or whether the effect is minor, is something I will try to find out on another year.

The last element is the wall and ceiling– which, alas, I didn't look at, or focus on, for the few minutes we were in the cave. The shape of the wall will affect the effect. At the extreme, if the back wall was plain and vertical, then the effect would be lessened. Here, the cave's wall must be sloped, giving the light several occasions to reflect and refract, and the greater the number of refractions, the water will appear bluer.

Finally,I am intrigued by the highlighting – the sparkling nature-of some of the water. Repeating my first picture:

To the extreme left, there is some highlighting. I cannot remember where the boat was when I took the pictures- the captain kept moving the boat around, quite quickly, but I know that we weren't near the entrance: I am guessing, but I suspect the highlighting is because there will be other holes, not discernible to the tourist's eye, perhaps just cracks, perhaps just below the water line, which cause this underwater lighting effect. On the boat trip back, I stared a lot at the rocks, and think they are virtually all limestone: so probably porous, possibly cracked, and this helped me in my thinking about blueness.

Finally, googling has shown up surprisingly little about this: what I have found via google tends to be refraction/reflection/blue higher energy: all true, but I suspect the narrowness of the opening, the shape of the wall, and the rock being limestone all help cause the effect.

Five hundred words to explain six.


From → Science, Travel

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