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B Completely black shadow cast through perfectly clear water.

  1. Aug 7, 2016 #1


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    Today while filling a mortar tub with water for all the wildlife in my backyard and surrounding area, I noticed that vortex swirls produced a completely black circle shadow, surrounded by a white sunlight ring, the ring was very bright and the reflection on the bottom of the tub (black) was very distinct, it would move around until the water vortex would disappear into water with less motion.
    I'm not good with search queries, the link below is the best I could find. Only a few times would the water in my tub form a funnel, but almost always the shadow would form a near perfect circle.
    I wonder if anyone has studied this type of light refraction ?


    Thanks for any input :smile:
  2. jcsd
  3. Aug 7, 2016 #2


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    The vortex bends the surface of the water above it into a convex lens. The refraction of light through this lens bends the light away from the middle of the vortex and towards a point off from the center. Because this "lens" is more strongly cured towards the center of the vortex, light passing near the center is more strongly refracted than the light passing nearer to the edge and the end result is a ring of light on the bottom of the tub surrounding a darker patch at the center.
  4. Aug 7, 2016 #3


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    I'll try to take a picture next time I fill the tub, what I found so interesting was the shadow was about 25mm in diameter and the light ring about 2mm, a very distinct and sharp meeting of the two and the black disc of shadow has no variation in it, even when there was a vortex funnel, there was no obvious center.
    It just seems so strange to me.
    The water depth at full is about 150mm, but depth did not seem to make any difference as the tub was being filled.

  5. Aug 8, 2016 #4


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    A bit after the 3 minute mark in the following video, Physics Girl draws some pictures that display the lens effects that Drak described.

    I remembered the shadows from this video from a couple of years ago, as being similar to your description. And "pool vortex shadows" brought it up as the number one hit on google.
  6. Aug 8, 2016 #5


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    Thanks Om, just the link I needed :smile:
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