# Strange Optical Phenomenon (mirror in shower)

Discussion in 'Physics & Math' started by Neddy Bate, Oct 19, 2021.

1. ### Neddy BateValued Senior Member

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I'm not seeking any google hits or anything like that. If you can make a coherent explanation of the effect, whether it entails an optical illusion or not, I will be more than happy to hear it. Clearly we can agree that it seems to be a bit complicated to explain. That is why I posted the thread.

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3. ### Neddy BateValued Senior Member

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Here is a sketch of what I think might be going on here:

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The light ray comes from the upper right, and hits the shower stream. The light bounces off the water droplet and travels back toward the eye, which normally would cause the eye to see an image of the water droplet. But the light ray also travels through the water droplet and back toward the mirror, so in this sense, the shower stream acts sort of like a beam splitter. After that ray is reflected off the mirror, it travels back toward the eye along the exact same line of sight as the "image" of the water droplet. I don't think any of this paragraph is really in question, it is all pretty normal stuff.

However, somehow those two rays which are on the same line of sight are out of phase from one another. This could be caused by the light having come out the back side of the water droplet, or something to do with the glass of the mirror, I'm not sure of the details there. But the end result is that the eye receives two rays along the same line of sight, and due to destructive interference of the waves being out of phase, the eye sees a black circle instead of the image of the water droplet that it normally would see. This happens at the location of the pupil in the mirror reflection because that is where the light rays are perfectly orthogonal.

The reason I never made such an explanation or diagram before is because it still begs the questions of how two light rays can possibly be out of phase like that. But at least I think I have the basic mechanics of it explained here.

Last edited: Jul 19, 2022
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5. ### DaveC426913Valued Senior Member

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That was a joke. That's a dangerous collection of words to Google without appropriate hazmat equipment.

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7. ### Neddy BateValued Senior Member

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Oh, I get it now! LOL

8. ### Neddy BateValued Senior Member

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After studying this effect more closely in the mirror, I noticed something that I had not noticed before. The water droplets that I am seeing in front of my face, (but mysteriously not at the pupil), are actually the water droplets of the reflected image of the back of the shower stream, not the shower stream itself.

I was able to prove this to myself by holding my hands out as if to measure the width of the shower stream from left to right. The actual hands that I see in front of me are much wider than the reflected hands that I see in the mirror, due to the fact that the reflected image appears farther away. I can make out that all of the water droplets that I see in the mirror are located between the narrower space of my reflected hands. The actual shower stream is much wider, but that is not what is readily visible in the reflected image, as it is much closer and therefore somewhat out of focus when I am looking at my distant face in the mirror. Plus I can reach around to the back of the shower stream and basically watch my hand touch those water droplets in the mirror, whereas touching the droplets in front of the stream do not appear to be the ones in the mirror.

So, my above diagram is not accurate, because it assumes that I am seeing the actual shower stream, but in reality I am actually seeing the reflected image of the back of the shower stream. Below is an updated more accurate diagram:

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Another reason the rear of the shower stream is so clear in the reflection is because it is probably being illuminated by light reflecting off the mirror, as shown in this updated diagram. I still don't know what is causing the black circles at the pupils, but at least the diagram is more accurate now.

Last edited: Aug 9, 2022
9. ### wegsMatter and Pixie DustValued Senior Member

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When I conducted the experiment, I saw the ''black circle'' too, but remember...the lighting in my bathroom is different; no direct lighting in the ceiling above the shower. It seems like an optical illusion if we're both seeing the same thing, given that the lighting is different, no?

10. ### Neddy BateValued Senior Member

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I think of optical illusions as things which can be very clear to see, but they are not persistent. You might be able to see something that is not really there, but if you look directly at it, you can tell that it is not actually there because it goes away. For example, this black dot illusion in between the number keys:

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You can see the dots when you are looking at other places, but they are never located where you are actually looking. If you look only at one place between specific keys, such as the central space located in the middle of keys 4, 5, 7, 8, you can prove to yourself that the dot is not really there.

The pupil effect is 100% persistent in my case where the lighting happens to be ideal. I can look directly at one pupil of one of my eyes in the mirror, and the black dot never goes away. So, in that sense, it is difficult for me to say it is an optical illusion.

However, I do understand that the black dot is not really there. There is actually a flowing water stream in front of the pupil, but for some unknown reason the water stream does not show up visibly in that one place, and there is a black dot instead. So, yes, it is some kind of illusion, at least in that the black dot is not really something that is there. But the same could be said about a rainbow in the sky. It is not a real thing that is there, it is an optical phenomenon, but it is hard for me to call it an optical illusion, because it is persistent.

I wonder if you were able to see the dot persistently, or if you only caught a fleeting glimpse of it due to your lighting not being ideal? If so, then I can understand why you might call it an optical illusion, because it might seem to be just like the dot illusion in the image above. But if you could see the dot persistently, would you still call it an illusion?

Last edited: Aug 9, 2022
11. ### wegsMatter and Pixie DustValued Senior Member

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This a great point - then why do you think our brain is “only” seeing our pupil “over” the shower stream? Looking back to when I did this, I remember the shower stream but because I was so focused on my eyes, the stream itself seemed faded away, if that makes sense. I may try this experiment again only this time, focusing on the water stream and see if the same effect happens.

12. ### DaveC426913Valued Senior Member

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Proof you were in the shower. Pix or it didn't happen.

13. ### wegsMatter and Pixie DustValued Senior Member

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ha, you're silly!

Wait, why is this thread posted under Physics/Math? Just noticed that. I’m considering it to be an optical illusion so maybe that's why I'm not seeing the connection.

14. ### Neddy BateValued Senior Member

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I think the reason for that is because the pupil is the only location where the incident light rays (approaching the mirror) and the reflected light rays (departing from the mirror) are exactly orthogonal to each other. For all other places, there is an angle between the incident and reflected light rays.

Yes, it is always a good idea to repeat the experiment. In my case, the experiment is repeated every weekend when I travel, because that is where the shower is that has the perfect lighting for it. It is always the same, very clear, and persistent.

I have tried looking directly at the shower stream in front of my pupils (in the mirror), and that is exactly what appears to be missing! The black dot is there instead. I can see the shower stream over the rest of my eye very clearly, but not at the pupil. But please do the experiment yourself and report back here. I am very glad you were able to see it, so hopefully your input can help me figure it out.

I was hoping for a physics/optics explanation. I was thinking it was something to do with the light waves being out of phase, and cancelling each other out by destructive interference. I still think it is something like that, and I don't really consider it to be just a simple optical illusion, for that reason.

Last edited: Aug 10, 2022
15. ### DaveC426913Valued Senior Member

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Let's change the descriptor to "optical phenomenon". I think the answer will still be solved optically, so it's still physics.

16. ### Neddy BateValued Senior Member

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Thanks. Just out of curiosity, would you think a rainbow would be described more accurately as an optical illusion, or an optical phenomenon?

17. ### Michael 345New year. PRESENT is 72 years oldlValued Senior Member

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I would go for optical phenomenon

Illusion gives me the impression you are seeing something not actually present

The colours of the rainbow are physically present and detectable, think camera

Phenomenon is the physics of different wavelengths of the light spectrum bending at different wavelengths passing through a prism (raindrop)

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18. ### DaveC426913Valued Senior Member

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Yes. It was I who introduced the word illusion - and deliberately. But I'm not married to it.

19. ### Neddy BateValued Senior Member

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So, which category would you say a rainbow falls into? Does your "yes" mean you agree with Michael 345 that it would be more accurately described as an optical phenomenon, rather than optical illusion?

20. ### DaveC426913Valued Senior Member

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No. I think we're in danger of having the cart before the horse here. The explanation should inform the category, I don't think the category should inform the explanation.

IOW the semantics of it isn't a hill worth dying on.

21. ### Neddy BateValued Senior Member

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I specifically asked you about a rainbow. I assume you have seen a rainbow in your life, correct? So what category would you put a rainbow into?

Last edited: Aug 11, 2022
22. ### DaveC426913Valued Senior Member

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I saw that. I kind of hoped you'd see the question as moot by pointing out that it's merely semantical. (Maybe a bigger box would change my mind?)

All that really matters is the actual explanation. Category follows explanation. Predefining it is likely to bias the search for an explanation (you toward objective and I toward subjective, it seems). And neither of us know - because neither of us know whether it has an objective or subjective cause.

Bearing that in mind, does it still matter what I think of rainbows?

Are you working your way toward a sylogism as argument?
Premise 1: A rainbow is a real phenom.
Premise 2: Neddy's Black Pupil is the same as a rainbow.
Conclusion: Neddy's Black Pupil is a real phenom.

Last edited: Aug 11, 2022
23. ### Neddy BateValued Senior Member

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Okay, so you refuse to tell me which way you would categorize a rainbow, just in case, later, I might say that the pupil effect is "the same" as a rainbow.