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Old 09-10-2015, 03:08 PM #1
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Default Strange Shadow Effect in Shallow Water?

So I was playing around with my lasers late at night, and I discovered an interesting effect when shining my lasers into the kiddie pool at my apartments. What's interesting is that it only happens in the kiddie pool, which is 1 or 1.5 ft deep. If I shine it in the normal pool, which goes down to about 4 ft deep, the effect disappears. I am quite sure it is due to the coherence of the beam, but I can't for the life of me come up with why it would only happen in the shallow water.

Also interesting to me is the fact that the shadow appears to be perfectly circular around the beam regardless how steep the angle is between the beam and the surface of the water. I imagine this is due to the water diffracting the beam and making the beam more perpendicular to the plane of the surface once it is actually in the water.

Some further testing I would like to do is determining at what depth the effect begins to appear, if it disappears again with more shallow water than the kiddie pool, and if focusing/defocusing the beam affects the shadow. What do you guys think?











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Old 09-10-2015, 03:12 PM #2
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Default Re: Strange Shadow Effect in Shallow Water?

Not a clue as to what it could be, but it looks pretty damn cool. It's as if there's a whole bunch of fog flowing over the ground.
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Old 09-10-2015, 03:35 PM #3
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Default Re: Strange Shadow Effect in Shallow Water?

Hmm, interesting.

I guess some more information would help us to figure it out. Was the pool calm or were there ripples, and what happens to the pattern with ripples.

How large is the circular "shadow"?

Maybe you could shine the laser close to the edge of the pool and observe what happens to the "shadow" on the wall of the pool.
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Old 09-10-2015, 04:05 PM #4
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Default Re: Strange Shadow Effect in Shallow Water?

Best guess: This is caused by total internal reflection. The light from the dot escapes the pool at steep angles, but is totally internally reflected back into the pool at shallow angles, illuminating the bottom. It would still happen in a deep pool, but the reflected light would be much dimmer because of the distance it diverges over, and the area it needs to illuminate. Here are a few google images to help explain.



Imagine you were a fish below a swimmer (or dive a few feed down and look up, next time you're in a pool - I have and it's impressive to witness first hand). This is what you would see. There is a clear "barrier" between light that escapes/enters the pool, and light that reflects off the surface of the water. This "barrier" occurs at the total internal reflection angle.

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Old 09-10-2015, 04:28 PM #5
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Default Re: Strange Shadow Effect in Shallow Water?

Quote:
Originally Posted by Cyparagon View Post
Best guess: This is caused by total internal reflection. The light from the dot escapes the pool at steep angles, but is totally internally reflected back into the pool at shallow angles, illuminating the bottom. It would still happen in a deep pool, but the reflected light would be much dimmer because of the distance it diverges over, and the area it needs to illuminate. Here are a few google images to help explain.


Imagine you were a fish below a swimmer (or dive a few feed down and look up, next time you're in a pool - I have and it's impressive to witness first hand). This is what you would see. There is a clear "barrier" between light that escapes/enters the pool, and light that reflects off the surface of the water. This "barrier" occurs at the total internal reflection angle.
Good call Cyp! That's gotta be it. I just tested it, the "shadow" outlines the shape of the laser and increases with the water's depth.
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Old 09-10-2015, 08:33 PM #6
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Default Re: Strange Shadow Effect in Shallow Water?

I'm with you in that appears to be related to total internal reflection. What I'm having trouble grasping is exactly which light you're saying is being totally internally reflected. Are you saying the reflection off the bottom of the pool is being totally reflected, except for the portion where we see the shadow, which is mostly escaping the water?


Quote:
Originally Posted by Cyparagon View Post
Best guess: This is caused by total internal reflection. The light from the dot escapes the pool at steep angles, but is totally internally reflected back into the pool at shallow angles, illuminating the bottom. It would still happen in a deep pool, but the reflected light would be much dimmer because of the distance it diverges over, and the area it needs to illuminate. Here are a few google images to help explain.



Imagine you were a fish below a swimmer (or dive a few feed down and look up, next time you're in a pool - I have and it's impressive to witness first hand). This is what you would see. There is a clear "barrier" between light that escapes/enters the pool, and light that reflects off the surface of the water. This "barrier" occurs at the total internal reflection angle.

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Old 09-11-2015, 12:33 AM #7
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Default Re: Strange Shadow Effect in Shallow Water?

It's an interesting theory, but i'm not sure it holds true in this situation. I would actually expect the opposite: a bright circle around the entry point caused by the dot projected onto the bottom and a dimmer area around that due to internal reflection.

I bet that is exactly what you would get shining the laser -straight down- into a pool. In these pictures the laser is at a shallow angle, making me think it might actually have something to do with the (vertical) walls of the pool to some degree. The 3rd photo with the beam near a corner of the pool having a much more pronounced effect would suggest the walls have something to do with it.

Shining a laser straight down into a pool is obviously tricky unless it has a high diving board or a bridge that gets you right over the water.
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Old 09-11-2015, 02:46 AM #8
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Default Re: Strange Shadow Effect in Shallow Water?

This is something very easy to test yourself. That's what I did before my last post, I just took a bucket of water and filled it with about one inch of water and saw the effect clearly with a normal pointer with the lights off.

EDIT: It looks like the with the index of refraction of air being 1 and water 1.33 then the critical angle for light leaving the water is 48.8 degrees. I'll see if I can do the calculations tomorrow, but I think that would mean that the circle's radius would be very roughly twice the water's depth.

Here's a supper awful drawing. The thick red line coming in from the right would be the laser. The blue the water. The smaller red lines would be the diffused light from the laser hitting the bottom, and the black dot (on the bottom right) would be where the circle or light would start.)
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532nm: 500mw CNI PGL-III-C
515nm: 36mW laserBtB
503nm SB1573N91 Assembly in progress...
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475nm: 50mW-80mW Assembly in progress...
462nm: 2W M462 Kryton Groove
450nm: 94mW laserBtB
445nm: 2.5W NDB7875 Kryton Groove - 40mW Pen laser using NDB7242E diode
405nm: 100mW PHR-805T

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Old 09-11-2015, 07:44 AM #9
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Default Re: Strange Shadow Effect in Shallow Water?

Might be temp related too.
I'm wondering if the shadow effect is conical rather than flat.

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Old 09-11-2015, 10:42 AM #10
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Default Re: Strange Shadow Effect in Shallow Water?

Quote:
Originally Posted by TaterMay View Post
Are you saying the reflection off the bottom of the pool is being totally reflected, except for the portion where we see the shadow, which is mostly escaping the water?
Yes.

Quote:
Originally Posted by Benm View Post
a bright circle around the entry point caused by the dot projected onto the bottom and a dimmer area around that due to internal reflection.
The beam's termination (dot) is the main source of illumination here. Not the water entry point.

Quote:
Originally Posted by VisibleGreen View Post
Might be temp related too.
How?
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Old 09-11-2015, 01:14 PM #11
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Default Re: Strange Shadow Effect in Shallow Water?

Quote:
Originally Posted by Benm View Post
It's an interesting theory, but i'm not sure it holds true in this situation. I would actually expect the opposite: a bright circle around the entry point caused by the dot projected onto the bottom and a dimmer area around that due to internal reflection.

I bet that is exactly what you would get shining the laser -straight down- into a pool. In these pictures the laser is at a shallow angle, making me think it might actually have something to do with the (vertical) walls of the pool to some degree. The 3rd photo with the beam near a corner of the pool having a much more pronounced effect would suggest the walls have something to do with it.

Shining a laser straight down into a pool is obviously tricky unless it has a high diving board or a bridge that gets you right over the water.
I believe Mattronium's illustration is accurate. If you look at where the light reflects from the surface back down to the bottom, everything inside that point, up to the actual beam, should be a much darker shadow. It just really makes sense with what I've observed.

Quote:
Originally Posted by Mattronium View Post
This is something very easy to test yourself. That's what I did before my last post, I just took a bucket of water and filled it with about one inch of water and saw the effect clearly with a normal pointer with the lights off.

EDIT: It looks like the with the index of refraction of air being 1 and water 1.33 then the critical angle for light leaving the water is 48.8 degrees. I'll see if I can do the calculations tomorrow, but I think that would mean that the circle's radius would be very roughly twice the water's depth.

Here's a supper awful drawing. The thick red line coming in from the right would be the laser. The blue the water. The smaller red lines would be the diffused light from the laser hitting the bottom, and the black dot (on the bottom right) would be where the circle or light would start.)
At some point yesterday, it finally clicked for me, and I believe your drawing illustrates it well. I feel that this is the most likely explanation. I believe the light is mostly escaping where we see the shadow. Then, where the light begins again is where the critical angle has been reached, and the light is totally internally reflected from the surface back down to the bottom of the pool.


Quote:
Originally Posted by VisibleGreen View Post
Might be temp related too.
I'm wondering if the shadow effect is conical rather than flat.
What makes you think temp has anything to do with this? I can't see such small relative temperature changes having any significant effect on the light distribution.
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Old 09-12-2015, 12:59 AM #12
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Default Re: Strange Shadow Effect in Shallow Water?

As per your diagram there shold be bright inner circle where the beams escape from the water, and a dim area around that, eactly as you have drawn. But the effect seems to be the exact opposite of that.

There could however be an explanation for this: The beams that make it out of the water do just that, but don't go in your direction so it does not appear bright.

Outside that area there is "total" reflection, but this is actually not total and some light scatters from the water-air interface making it look brighter from any angle. This would be consistent with the photos.

Obviously this effect should not be large on a perfectly flat body of water, but if there are some ripples in the pool i think it could be a realistic mechanism for what we see.
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Old 09-12-2015, 03:41 AM #13
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Default Re: Strange Shadow Effect in Shallow Water?

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Originally Posted by Benm View Post
As per your diagram there shold be bright inner circle where the beams escape from the water, and a dim area around that, eactly as you have drawn. But the effect seems to be the exact opposite of that.

There could however be an explanation for this: The beams that make it out of the water do just that, but don't go in your direction so it does not appear bright.

Outside that area there is "total" reflection, but this is actually not total and some light scatters from the water-air interface making it look brighter from any angle. This would be consistent with the photos.

Obviously this effect should not be large on a perfectly flat body of water, but if there are some ripples in the pool i think it could be a realistic mechanism for what we see.
The circle of light that we are seeing is on the bottom of the pool. When the laser hits the bottom it becomes the main light source. The light from the "new" light source that hits the water at greater then a 48 degree angle (the light going mostly "up") is transmitted and leaves the scene. The light that hits the water at less then 48 degrees is reflected back down again. This light that is reflected is the cause of the circle of light at the bottom of the pool. And naturally the circle is brightest at the closest point that has total internal reflection and becomes weaker farther away because of the inverse square law.
Does that answer it?

Like I said before too, it's something that can easily be experimentally tested with only a container of water and the always elusive lasers (none of us has any of those now, do we ).
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Old 09-12-2015, 05:16 AM #14
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Default Re: Strange Shadow Effect in Shallow Water?

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Originally Posted by TaterMay View Post
What makes you think temp has anything to do with this? I can't see such small relative temperature changes having any significant effect on the light distribution.
You're probably right.
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Old 09-13-2015, 12:28 AM #15
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Default Re: Strange Shadow Effect in Shallow Water?

Quote:
Originally Posted by Mattronium View Post
The circle of light that we are seeing is on the bottom of the pool. When the laser hits the bottom it becomes the main light source. The light from the "new" light source that hits the water at greater then a 48 degree angle (the light going mostly "up") is transmitted and leaves the scene. The light that hits the water at less then 48 degrees is reflected back down again. This light that is reflected is the cause of the circle of light at the bottom of the pool. And naturally the circle is brightest at the closest point that has total internal reflection and becomes weaker farther away because of the inverse square law.
Does that answer it?

Like I said before too, it's something that can easily be experimentally tested with only a container of water and the always elusive lasers (none of us has any of those now, do we ).
You could be correct there indeed - so it is caused by light that went up from the original spot where the laser hit the bottom and is then reflected back to the bottom again from the water surface. I'd reckon in that case the depth of the pool would make a serious difference.

I'm not sure how well it would translate to a bit of water in a bucket, but it should be fairly easy to 'prove' using an underwater camera and an actual pool, displaying what the pattern on the bottom is regardless of what it looks like from above the surface.
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Old 09-13-2015, 04:12 AM #16
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Default Re: Strange Shadow Effect in Shallow Water?

Just make sure you guys wear protective wear when testing it out. Who knows what kind of random reflections can occur
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