r/Optics • u/Lonely-Competition-5 • 9h ago
Repeatable interference pattern using two flat glass plates and a custom liquid mixture – expected thin-film interference or something worth investigating?
Hello everyone,
I observed a repeatable interference pattern using a very simple setup, and I’m interested in understanding whether this is just a known thin-film interference effect or if the specific implementation is technically interesting.
Setup:
Green laser pointer
Two ordinary flat glass plates
A thin layer of a custom liquid mixture (3–4 components) between the plates
Projection on a distant screen
Observations:
Without the liquid, the characteristic ring pattern does not appear.
With this specific liquid mixture, a stable concentric ring pattern appears after about one minute and is repeatable.
Changing the laser wavelength (green vs. blue) changes the pattern significantly.
Changing the laser incidence angle can produce different beam profiles (for example, circular, elliptical, or even square-like patterns).
I tested several other liquids, but they did not produce the same stable and repeatable pattern. This specific mixture consistently does.
I have already discussed this with two photonics professors. Both suggested that the underlying physics is most likely classical thin-film interference/Newton’s rings, which I completely accept. I am not claiming a new physical phenomenon.
My question is different:
Could such a simple glass–liquid system be interesting from an optical engineering perspective (for example, passive beam shaping or sensing), assuming the behavior is genuinely repeatable?
I have attached photographs of the experimental setup and the resulting patterns.
I would appreciate your thoughts.
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u/Lonely-Competition-5 8h ago
When I replace the green laser with a blue laser, the interference pattern changes dramatically (elliptical instead of circular), while the setup and liquid remain the same. This seems consistent with wavelength-dependent interference, but I’m including it because the behavior is highly repeatable.
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u/TheMcSebi 7h ago
Did you rule out that your blue laser dot isn't circular?
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u/einstein1351 6h ago
Agreed. Blue diodes are traditional FP diodes that will have elliptical profiles. The green ones use one nonlinear crystal for 808->1064 conversion, and then another for SHG to 532. The phase matching condition usually limits the output to be more circular
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u/Projektz 8h ago
Look into liquid lenses for optics, and translated into optometry/opthalmology, variable focus intraocular lenses.
The big question is - how do you plan on “controlling” the liquid and its boundaries such that the behaviour is “genuinely repeatable”?
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u/Lonely-Competition-5 8h ago
At the moment, the repeatability comes from preparing the same liquid mixture with the same procedure, using the same glass plates and laser. Under these conditions, I consistently obtain the same pattern. I have not yet quantified the reproducibility statistically, but that would be the next step.
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u/lancerusso 6h ago
try with water or some other fluid- your liquid is staying liquid, right? It's not GRIN, or a liquid lens surface- presumably you think you have interesting dielectric properties relating purely to wave behaviour?
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u/frugal_cyclist 8h ago
Interesting!
I thought I had the answer, but after I started typing, I figured that it is more complicated than I thought.
My thought would be as follows:
The main large concentric rings are due to thin film interference.
With no liquid or non absorbing liquid (water or alcohols) then the ring structure should stay and just change the ring density slightly due to the change in index of refraction of the liquid.
In the picture you added, I assume that the liquid has some highly absorptive dye at the wavelength of your laser.
The main beam passes through. The back reflection, that ultimately creates the interference gets further absorbed due to the multipass, hence has little intensity, hence seeing the flat spot in the center. But this idea falls apart as the outer fringes still exist.
If you want help, please answer the following questions:
Do you see any deposits on the windows after you illuminate the sample for a long time?
If you shake the liquid ever so slightly, does this spot move around? Maybe you have some polymerization happening in the center.
Add a picture of the interference without any liquid. And one picture with only solvent (water or alcohol)
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u/Lonely-Competition-5 7h ago
A few observations from my side: I don’t observe any visible residue left on the glass after cleaning. The effect is highly repeatable when I use the same liquid mixture, the same glass plates and the same setup. I have tried other liquids, and they do not produce the same pattern. I also observed that changing the thickness of the glass changes the pattern, and using a blue laser instead of a green one also changes the pattern significantly.
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u/Lonely-Competition-5 7h ago
Based on these observations, do you think this is still fully explained by thin-film interference alone, or could the optical properties of the liquid itself be playing an important role?
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u/SomeClutchName 7h ago
Anything we can tell you is just gonna give you more ideas to pursue that may or may not be helpful. Frankly, I think you should write a paper.
1st, take pictures of your results. With/without plates, with/without liquid, different wavelengths, etc. Make sure your laser is always of normal incidence!
2nd, measure the fringes. You can probably find an equation that should confirm this.
3rd, look into the optical properties of your mixture (which you should not share here). Measure the absorption of the individual components and solutions to understand how this liquid plays a part.
If you have access to photonics people, they should have connections for any experiments you might need.
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u/Louisflakes 6h ago
not everything needs to be novel to be worth investigating - not everything novel needs to be investigated :)
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u/Dapper_Discount7869 8h ago
That it takes a minute to form is kind of interesting from a solution dynamics perspective, but that looks like the pattern of a Michelson interferometer (which is arguably what you built), which would change with wavelength and incident angle.
Essentially you made an etalon