On many mornings I get this cool light pattern on my ceiling:
It's light coming in through the blinds, but there is this rippling/wavy/moving effect. Its intensity varies (as can be seen a little in the GIF). I am basically just curious as to what it is and why it occurs. My partner has seen this in other houses but I never have.
The light is coming in after bouncing off an AC unit This ended up not being really correct; see edit below:
So I was wondering if that has something to do with it (e.g. some sort of mirage?). Otherwise, I was having trouble finding out other information online. Has anyone seen this before? Any potential explanations for what is happening?
First time poster here; please help me improve the post if needed!
More information (and answer):
I have now verified that this is actually light bouncing off water (caustic reflections); I will accept the answerer who suggested/explained this possibility. Here are the fancy lights happening again this morning after the AC had been off for a few minutes:
Sure enough, it rained yesterday, and there is a puddle outside which is redirecting direct sunlight directly into the window:
So, I apologize for my original post being misleading. I said this was light "bouncing off the AC unit" – this is I guess true (sort of trivially), but not as significant as the direct sunlight reflecting off the puddle. I realize this incorrect/incomplete information introduced a bias. That being said, kudos to the answerer who proposed the correct explanation! It is kind of a particular set of prerequisites that have to occur for this light effect to take place (i.e. it rains enough to create the puddle, but sunny for the ~hour when the angle is correct).
Just as an aside, there is this funny sort of confounding variable thing going on. The asphalt shown in the puddle photo was recently all torn out; I think the puddle only started appearing since then, concurrent with about when we started running the AC more.
One final note is that the pattern is much less striking when the blinds are more open:
There is some movement, but not nearly the amount seen when the blinds are closed (and the boundaries of the light don't grow and shrink as much).
Anyway cheers to all who have given input on this post!
Best Answer
These are probably Reflection Caustics.
The video Taming light reflection to create images discusses engineered caustics (refractive rather than reflective in this case). See also this post from the EPFL Geometric Computing Laboratory:
I don't think the explanations of refraction from density variations in air actually work in this case. Schlieren photography is a real thing, but the variation of index of refraction of air at different temperatures is very small, of order $10^{-6}$ per degree C, so you need special circumstances, highly collimated light and/or long distances to see such an effect. See this work for example.
You might need one of these (from Window Air Conditioner and Rain):
I saw a similar effect years ago and looked out the window to find that sunlight was reflecting off of a puddle of water on top of a slightly concave top panel of the AC unit. Either somebody set something heavy on it to try to stop noisy vibrations, or sat on it, or it perhaps deformed over time for other reasons.
These normally have a matte finish, AC window units are not normally shiny polished metal that can reflect so nicely.
So I suspect there is a puddle of water on top of a slightly concave metal panel, and we are seeing caustic effects from Fresnel reflections from ripples on the water induced by wind blowing or AC compressor vibrations.
From James Gurney's blogpost Caustic Reflections
From forums.sketchup.com's Water Reflection on Surfaces:
Concave AC unit top with matte finish: These don't reflect by themselves, there's probably water on top.
Source: Shutterstock