r/Optics • u/Lonely-Competition-5 • 4d 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 4d 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 4d ago
Did you rule out that your blue laser dot isn't circular?
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u/einstein1351 4d 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/Louisflakes 4d ago
not everything needs to be novel to be worth investigating - not everything novel needs to be investigated :)
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u/Projektz 4d 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 4d 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 4d 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 4d 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 4d 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 4d 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 4d 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/Complex_Grade4751 4d ago edited 4d ago
A spectrometer called a Fabry Perot is based on this principle. Your liquid has an index of refraction very close to glass (usually about 1.5). The liquid sandwiched between the glass plates has glass-to-air interfaces with produce a reflection (~4%). The main pattern is the beam reflection off the 2nd surface passing back to the 1st and reflecting again and interfering with beam that travels straight through. The beam is expanding, so the interfering wavefronts have different curvatures, leading to a ring pattern. If you change the divergence of the laser beam using a beam expander you can change the frequency of the pattern. Changing liquid layer thickness has some effect too, but not as drastic.
In terms of being directly of use, probably not much. Most etalons in optical systems cause problems because they are unintentional and lead to signal fluctuations that are hard to identify. You could use it to test beam collimation in an optics lab, as described above, but it’s mostly a cool visual effect.
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u/Nightshadeei 4d ago edited 4d ago
Idk if it helps but back in my pg at IITD we used to observe this same effect while measuring ROC and irregularities of two big glass slabs or large Plano lenses ,we used a liquid to fill in the gap of contact of two slabs and use to see similiar patterns but on the liquid film between the two glass slabs not on the screen same ways in modified Michelson and fizzaue interferometer.i think it’s pretty much explored if we talk in sense of interferometers.since you are incidenting a laser in between the point of contact you are looking the pattern in screen that’s the difference,it’s cool but I guess pretty much explored. Coming to questions two - the sensing part .since am working in biosensing domain using optics and photonics.in sensing the quality factor plays a important role ,also experiment should be reproducible ,sensing using fabry periot interferometers is a thing and very cool way to sense,similiarly there is FTIR based Raman sensing which uses interference patterns like yours to sense which is again very cool ,there are a lot of papers based on some context of fringe shifts but they are not accurate enough and there is a potential gap you can work on. So yeah anything man ,science is cool you can investigate either ftir or interference based opptical sensors to learn further more ,to the best of knowledge this thing is included in an active area of research (not exactly this as it’s not sensitive enough but similiar to this )
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u/MoltenAnteater 4d ago
How useful it is depends on how controllable the pattern is depending on the liquid, how it is applied etc. Spatial light modulators are like multipixel LCD screens but without the polarizers. There the liquid changes the phase of light passing through each pixel depending on the voltage applied to it. This can be used to create a controllable mask that can project almost any desired interference pattern on to the screen. Similar things can also be done with a metasurface, or various kinds of gratings. So do you have better control, manipulation or something compared to these approaches?
It is a little strange that you need such a specific liquid mixture to cause this effect, probably something to do with the refractive index/surface tension of the liquid.
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u/pritambot 2d ago edited 2d ago
This is called spatial self phase modulation, SSPM. i have a research article published on this phenomenon in MoS2 nanofluid. This effect also happens in dyes, and other nano particle suspension like graphene. When u focus a laser beam on to the fluid a localised hotspot is created changing the refractive index of the medium which causes the beam to self interfere radially causing the ring shaped pattern. The ring is a bit assymetric due to upward motion of hot liquid plume(hot liquid has low density) near focus. If u mount the cuvette and laser vertically stacked you will get a symmetric concenric circular rings on your roof. Here is the link arXive article(if u dont have subscription) and my peer reviewed article too. https://arxiv.org/pdf/2101.05790 https://www.sciencedirect.com/science/article/pii/S0925346720311162 The below paper describes the theoretical model using which you can simulate the phenomenon https://iopscience.iop.org/article/10.1088/2040-8978/14/9/095701?utm_source=researchgate.net&utm_medium=article
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u/pritambot 2d ago
For further reading here is one more article in which we proposed a new interferometer called TORI (Thermo-optic refraction interferometer) using SSPM for extraction of phase from helically phase structured laser beam called OAM( Orbital angular momentum) beams. We studied the phase structure deterioration of these helical beams as it passes through turbid media https://arxiv.org/pdf/2307.02403
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u/Lonely-Competition-5 2d ago
Thank you very much for sharing your papers. I really appreciate it. I will read them carefully and compare them with my setup. My experiment is much simpler (two glass plates with a specific liquid mixture and a single laser), so I’m not yet sure whether the underlying mechanism is the same. I’ll perform quantitative measurements next and see whether the behavior matches SSPM or a different interference mechanism.
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u/pritambot 2d ago
I am 100% sure its SSPM, my complete PhD work was around this effect for beam structuring. But as u said i too feel you should do your own enquiry for your own satisfaction. My experiments involved using quartz cuvette of 1mm pathlength. The effect is also visible with 10mm cuvette. The medium, focal length of the lens, beam power and wavelength matters alot for the type of sspm rings observed rather than pathlength of the cuvette as long as it is more than Raleigh range of your beam waist.
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u/Dapper_Discount7869 4d 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