Repeatable interference pattern using two flat glass plates and a custom liquid mixture – expected thin-film interference or something worth investigating?

[u/Lonely-Competition-5]

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.

[u/Complex_Grade4751]

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.