What is this wavy light coming through my blinds?

Question

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!

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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!

Answer 1

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):

The light is coming in after bouncing off an AC unit

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:

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Concave AC unit top with matte finish: These don't reflect by themselves, there's probably water on top.

Source: Shutterstock

Answer 2

Without knowing all the details, it seems very likely to me that these are refraction shadows from hot and cold air mixing. The fact alone that it happens near an AC unit which gets cold on the inside and hot on the outside supports this.

Here's what you can do to test whether this is true: Get a fan and direct it onto the outside part of the AC unit. If the pattern disappears or moves a lot quicker, that would be further support for the assumption of refraction.

Answer 3

It's caused by the same effects as cause a mirage: uneven atmospheric densities. In your case, the A/C housing is warm, either because it's running or because the sunlight has heated it. As a result, there is a column of heated air flowing upwards, mixing with cooler air, and generally going (mildly) turbulent.
The different densities of the air lead to different angles of refraction, which redirects the sunlight into the patterns you see.

A mirage is a bit different, in that it usually is formed when there's a low-density layer of air near the ground (and higher density above). Due to total-internal-reflection, which happens for low angles of incidence when light from above hits the layer boundary, the image of a distant object is reflected on its way to your eyeball. In some cases, depending on the relative densities of different layers, the image may be upside down!

Answer 4

Welcome to the physics SE! This is a really cool question to which I hope to add a really cool answer.

I like Noah's answer because it includes an experiment. Here is a simpler experiment: Note the pattern with the AC running, then turn off the AC unit entirely, wait a few minutes for it to quiet down, and then see if the pattern disappears.

If it does not disappear, then you have a different sort of thing going on, as follows.

There is something called a pinhole camera effect which projects light through a small hole into a dark space and creates a blurry backwards image of the outside world on the walls of the dark space. This forms the basis of something called the camera obscura.

In that image, something like a bright shiny car moving past the hole left-to-right outside will be projected as an upside-down car moving right-to-left inside the dark space.

This effect works for narrow slits as well as small holes but with a slit, the resulting image is sharper in one dimension than in the other because the effective "diameter" of the "hole" isn't the same in two dimensions.

This will then project waving lines of light into the darkened room, and if there's a tree with shiny leaves outside the slitted window, then as they glisten and move in the breeze you'll get moving, random lines of light being projected into the room.

Note that this "slit camera obscura" effect would work with variable density air as cited by Carl and Noah as well, in which case it is that slit focusing effect that magnifies the line images and makes them easier to see!

Answer 5

It might be the schlieren effect, which would concur with the effect being lessened when your blinds are more open. Basically when the light is partially blocked, in your case by your blinds, you see the movement of the air streams, the hot and the gold air moving around causing turbulence.

Wikipedia says

Schlieren flow visualization is based on the deflection of light by a refractive index gradient[4] The index gradient is directly related to flow density gradient. The deflected light is compared to undeflected light at a viewing screen. The undisturbed light is partially blocked by a knife edge. The light that is deflected toward or away from the knife edge produces a shadow pattern depending upon whether it was previously blocked or unblocked. This shadow pattern is a light-intensity representation of the expansions (low density regions) and compressions (high density regions) which characterize the flow.