Completely black shadow cast through perfectly clear water.

In summary, the conversation discussed the observation of a vortex in a water tub creating a distinct black circle shadow surrounded by a bright white sunlight ring due to light refraction. The link provided by one of the speakers led to a video demonstrating a similar phenomenon.
  • #1
RonL
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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 ?

http://www.dailymail.co.uk/sciencet...rlpools-thought-work-way-space-phenomena.html

Thanks for any input :smile:
 
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  • #2
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.
 
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  • #3
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.

Thanks
 
  • #4
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.
 
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  • #5
Thanks Om, just the link I needed :smile:
 
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Q: What causes a completely black shadow to be cast through perfectly clear water?

A: This phenomenon is caused by the absorption of light by the water itself. As light passes through the water, it is absorbed and does not reach the bottom, creating a shadow.

Q: Can this phenomenon occur in any body of water?

A: Yes, this phenomenon can occur in any body of water as long as the water is clear and there is light shining from above.

Q: How does the depth of the water affect the darkness of the shadow?

A: The depth of the water does not affect the darkness of the shadow. The darkness is determined by the amount of light that is able to pass through the water, not the depth.

Q: Is there any way to prevent this shadow from occurring?

A: No, this shadow is a natural occurrence and cannot be prevented. However, certain factors such as water clarity and light intensity can affect the darkness of the shadow.

Q: Are there any animals or organisms that rely on this phenomenon for survival?

A: Yes, some animals such as deep sea creatures may rely on this shadow for camouflage and protection from predators. The shadow can also provide a cool and dark environment for certain species to thrive in.

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