On many mornings I get this cool light pattern on my ceiling:

enter image description here

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:

enter image description here

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:

enter image description here

Sure enough, it rained yesterday, and there is a puddle outside which is redirecting direct sunlight directly into the window:

enter image description here

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:

enter image description here

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!

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    $\begingroup$ Is the AC running? $\endgroup$ Commented May 19, 2021 at 14:36
  • 13
    $\begingroup$ Since you're a new poster here I'll mention that this is an excellent question. $\endgroup$
    – noah
    Commented May 19, 2021 at 17:05
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    $\begingroup$ @Not_Einstein the AC is running. My impression was that the effect is stronger when it is running, but I will need to double check this (as others have suggested). $\endgroup$
    – Tom
    Commented May 19, 2021 at 17:22
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    $\begingroup$ Nobody seems to have addressed why this is much more prominent with the blinds closed than open: you have created a 1-dimensional "pinhole camera". When the blinds are open, you get light of various locations and directions, but with the blinds closed, you select only the beams which travel through a narrow space. That means you're getting an "upside-down" picture of the outside, but only along the dimension perpendicular to the blinds (which is why the pattern shows little variation along the length of the blinds). I used to get the same effect from the reflection of towels on a clothes line. $\endgroup$
    – Dave
    Commented May 20, 2021 at 16:00
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    $\begingroup$ @Tom congrats on completion of your experiment! You can ask a new question about the relationship between the widths of the slits in the blinds and visibility of the caustic effect. Theoretically at least, it's related to the depth of the puddle and the strength of Earth's gravity, just the sort of answer folks will love to write! $\endgroup$
    – uhoh
    Commented May 20, 2021 at 21:20

5 Answers 5


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:

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):

from Window Air Conditioner and Rain https://www.hvachowto.com/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 James Gurney's blogpost Caustic Reflections http://gurneyjourney.blogspot.com/2010/07/caustic-reflections.html From James Gurney's blogpost Caustic Reflections http://gurneyjourney.blogspot.com/2010/07/caustic-reflections.html

From forums.sketchup.com's Water Reflection on Surfaces:

from forums.sketchup.com's Water Reflection on Surfaces https://forums.sketchup.com/t/water-reflection-on-surfaces/79291

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


Source: Shutterstock

  • $\begingroup$ still unanswered in Math SE: Constraints on conical coffee cup constructions of cardioids & catacaustics $\endgroup$
    – uhoh
    Commented May 20, 2021 at 1:09
  • $\begingroup$ so you need special circumstances, highly collimated light - Sunlight is highly collimated. $\endgroup$
    – J...
    Commented May 20, 2021 at 21:16
  • $\begingroup$ @J... no, not by a long shot! FWHM is almost a half degree. Now we can argue if that's a little or a lot, but in order to perform Schlieren photography in one's bedroom I think it's insufficient. If you can convince me otherwise using math or providing an example that would we great, I love to be proven wrongl $\endgroup$
    – uhoh
    Commented May 20, 2021 at 21:22
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    $\begingroup$ It's not enough for true Schlieren, but it's enough to create what would be technically called a "shadowgraph" similar to what OP was demonstrating. We know that's not the case from their follow up, of course, but it's not an unrealistic possibility. If you want an example you can go watch Yuri Gagarin blast off on his first flight - watch the shadows on the ground. Or wait for a sunny day and go find a working BBQ, then look at its shadow on the ground. $\endgroup$
    – J...
    Commented May 20, 2021 at 21:44
  • $\begingroup$ @J... You are right! Now that you mention these surely I've seen light on the ground passing through a big campfire or something else hot at some point in my life. Though I can't remember exactly when this certainly sounds very familiar. So that bit in my answer needs to be adjusted. If you can think of a good edit feel free, otherwise it will take me another 30 minutes for my coffee to fully kick in, it's early here. Shadowgraph $\endgroup$
    – uhoh
    Commented May 20, 2021 at 21:49

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.


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!


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!

  • $\begingroup$ I was getting ready to add an answer mentioning the effect of slits, but I think you've already covered it nicely. Until I read the part about the reflecting puddle, I was expecting it was "heat" (lower-density air) rising from heated pavement or something similar. $\endgroup$
    – jeffB
    Commented May 22, 2021 at 1:20

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.

Showing schlieren effect


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