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Sometimes, I think to myself...
Refraction is rather neat when attempting to detect disturbances in water..

In the case of still water, the uniform refraction of light gives a roughly even coverin to the bottom of a container (for example, a sink). If everything is working evenly, the bottom of the sink will have as much light on it as it would in air.

However, small disturbances in the water change the refractive angle, which gives rise to lighter and darker areas on the bottom of the sink, where there are either greater or fewer points on the surface at such an angle and position as to direct light to that point on the bottom.

But the main reason I'm finding this interesting today is because I found a miniature whirlpool phenomenon in the sink at work.

I managed to spot it because the curved edges of said whirlpool bent light away from the centre. That is to say, the southernmost edge of the whirlpool angled light towards the south - the end result being that the area directly beneath the whirlpool (at least in the direction of the refracted light) was quite dark, because all the light was being scattered away. In fact, that dark circle was surrounded by a bright ring, where light that would normally fall there was joined by the light that had been diverted away from the centre.

So yes, these ring structures kept on appearing in the sink. Oh, and the reason I was messing about with the sink runs as follows..

When I got into the restroom, I found the sink half full of white water. Evidently somebody last thing on Friday thought it would be amusing to close the plug hole, and dissolve a bar of soap in the sink or something. Ha ha. Obviously I don't want to reach into water whose contents is unknown to me, so I had to find another way to empty the sink. With that in mind, I started adding water, using the overflow mechanism to remove any excess, which gradually diluted the sink contents down to regular water, which I was then able to put my hand into to open the plug hole.

But here's the thing.. these whirlpools were being created in line with the overflow hole and the area where the water was being poured in. The vortex formed near where the water was coming in, moved away, stabilised (characterised by the bright ring around the dark shadow on the bottom of the sink), and then destabilised, moving around a lot before disappearing. A close examination of the surface of the water showed a hint of the whirlpool, but only as a slightly distorted speck..

Actually, looking at it, the point of stability appears to be similar to a Lagrange point. If you imagine the sink as a clock, with the overflow hole at 9, then the water enters between the centre and the number 9 - the whirlpools tend to form between the centre and the number 3. There seems to be a fairly narrow band in which the vortex is stable - when it moves out of that, the whole thing tends to collapse, and a new one forms. I think I know why it forms, but I can't put it into words.. :o)

But hey, here's a picture of it anyway.. The water from the tap enters at the top left of the photo, just out of sight (but you can see the disturbance from it).. The vortex is to the right and down a little from the plug, you can see it as a sort of pinch in the light reflecting off the surface.. More importantly, you can see the shadow (and ring) that it casts on the bottom of the sink (one for each of two light sources in the room).

(picture in next update, from mobile)