# What is causing the diffraction pattern on my ceiling?

When I wake up in the morning and look at my curtains, I see a pattern on the ceiling made by the light going through the gap between my curtains. I have added a picture of it below. I remember from high school that when a laser was being shot through a very thin slit, that you would get a similar pattern. However, I see two important differences here:

1. This is regular divergent sun light instead of parallel single wavelength light from a laser.
2. The gap in my curtains is orders of magnitude thicker than the gap in a slit experiment.

So if this is not a diffraction pattern caused by the wave particle duality of light, then what is causing this pattern to appear on my ceiling?

Edit: Okay, so I looked again at the light pattern and the building across and now think that the vertical white beams are causing the pattern. The light pattern is just the horizontally mirrored reflection of those white beams. The part of the building in the front right has 6 beams, which correspond to the 6 bright beams of light. The back left has a lot more beams which are further away and seem thinner because of it, so those we see on the right in the picture.

Now I am wondering why the horizontal white beams are not showing up on my ceiling.

• interesting (+1), you are right that it doesn't seem to be a usual diffraction pattern. A few questions. What's behind the curtain, is it a usual transparent pane of glass? Double or single glazing? Is the sun low in the sky, is the bright patch at the bottom of the picture the sun viewed straight on? Perhaps it's an image of the curtains, but it's hard to see how... Dec 28, 2021 at 11:15
• What happens if you slightly increase the gap? Dec 28, 2021 at 19:32
• If you show us a picture of what the view outside the window is like, it would help a lot. Of particular interest are things within 10-20 feet of the window (on your presumed balcony) and regularly spaced...maybe vertical railing supports or the like. Dec 28, 2021 at 20:21
• If you search the web for similar images with Google Lens or Yandex image search, several similar images turn up. That makes me think it's not an outside object but something inherent in the curtain structure, maybe different shadows through the ondulation of the curtains reflected by the shiny metal on top of the curtains. Dec 29, 2021 at 9:07
• It is dark outside now, so I will try to remember to take a picture tomorrow to help you guys out. Dec 30, 2021 at 18:35

The rays are white, so it does not look like diffraction. Maybe it is an effect similar to that in camera obscura, so it is a (transformed) image of some outside object.

EDIT (Jan 1, 2022): So the OP provided a photo of the outside objects and asked "why the horizontal white beams are not showing up on my ceiling". I am not sure about the following, but my guess is the gap between your curtains is a "line", not a round hole, as in camera obscura, so it works as camera obscura only in one dimension. Maybe the faint wide transverse white bands on your ceiling are the images of the horizontal beams, but again, I am not sure. One could calculate the image of the array of white beams in your "optical instrument" (gap between the curtains), but I am not sure I am going to do that:-)

• Yeah this must be it, see my edit. Jan 1, 2022 at 14:18

Since the beams are going up to the ceiling, they must be reflecting from something low outside. Take a look.

• It could be the low thing (or things) is a wide surface and the reflected light comes in through the gap at different angles. Dec 28, 2021 at 22:11
• If the light comes from below, then it could be caused by my French balcony, but I am pretty sure that I ruled that one out. I remember trying to cover it with objects to make the lines go away, but they remained on my ceiling. Dec 30, 2021 at 19:11

It looks as though it may be a reflection off the curtain rod.

It is a crepuscular rays phenomenon.

crepuscular rays, shafts of light which are seen just after the sun has set and which extend over the western sky radiating from the position of the sun below the horizon. They form only when the sun has set behind an irregularly shaped cloud or mountain which lets the rays of the sun pass through a cloud in bands. The radiating appearance of the bands is caused by perspective, as demonstrated by the fact that when on rare occasions the rays extend across the entire sky, they appear to converge again on the eastern horizon.

Something in you curtain setup is macroscopically scattering sun light early morning when the sun is low at the horizon and sunlight comes horizontally inside your room creating the same phenomenon.

From the data you are given if it is not something else in your general curtain setup my best guess is that the gap is responsible and just the correct amount for scattering sun light inside the room and creating the crepuscular rays.

You never said what happens if you open more your curtains. I guess the rays would vanish.

• I doubt that this is the same phenomenon. The apparent "spreading" of crepuscular rays is due to the long distances involved, and the somewhat forced perspective of very distant objects (the same reason all celestial objects look like they're the same distance away). In reality, the rays are nearly parallel. The apartment doesn't seem large enough to get this kind of forced perspective, so more likely we're looking at several evenly spaced and fairly close (probably reflected) light sources.. Dec 30, 2021 at 20:29