On some sunny mornings, I’ve noticed a circle of bright light on a wall in my bedroom. The light is obviously coming from a gap around the molding of my poorly installed air conditioner. The light comes in through the gap and shows as a perfect circle on the wall about 5 meters across from the air conditioner window.
For a while, I figured that it might coming in through a hole in the a/c unit itself, since I couldn’t identify any gap in the molding that would be a perfect circle.
Finally one recent morning, my curiosity forced me to investigate further, and I was pretty shocked at what was happening. I first blocked the circle with my hand and then moved my hand to the exact spot that it was coming in from (since there are quite a few very small gaps). I’m going to blur the next part if anyone wants to guess the shape of the gap:
The gap is actually a sliver the shape of an upside down Idaho, but thinner. When putting my hand directly in front of the gap, it shows on my hand that same shape. As I get further and further away from the gap, the shape continually changes until it’s an almost perfect circle at the wall..
I am going to speculate it has more to do with the shape of the sun than the shape of the hole. For example, a partial solar eclipse will make pac-man shapes in the same way you experienced. I suspect the distance between filter (the Idaho) and the wall plays a factor as well. Someone better at physics should be along here to explain it in more detail.
(My impression from the OP was that we were talking about a hole much larger than something that would act as a pinhole. But on rereading I think I’m mistaken, so I have deleted my reply. See Chronos below.)
Regardless of the shape of the actual hole, if it’s small enough, it’ll act like a pinhole. “Small enough”, here, means that it’s much smaller than the size of the light spot. And as a pinhole, it’s producing an image of the light source, in this case the Sun. During an eclipse, you would indeed see the eclipse in the spot.
Another application of this same phenomenon: Walk under a leafy tree on a sunny day, and you’ll see a bunch of little splotches of light underneath the tree, from where the Sun shines through gaps in the leaves. Those splotches are also each images of the Sun, and would also change shape during a partial eclipse. Which can be a very eerie effect if you don’t know what’s going on, because you can see at a glance that something’s not normal, but it’s not immediately obvious what it is.
A lot of sources invoke diffraction for this and similar phenomena, but it’s not actually relevant. It’s purely a ray optics phenomenon. If it were based on diffraction, then you’d see significant color-splitting at the edges, since the red light would be affected nearly twice as much as the violet light.
Yes, it’s length is about an inch, and the base (the top of the upside down Idaho shape) is about a 1/4 inch. If my room was half as wide, it would still look more like Idaho than a perfect circle. Not sure what would happen if the room was wider, but I assume a larger circle (since the whole shape gets larger as I move further from the gap and as it gradually becomes the circle).
I enjoy tracking down these sun images and have done it a few times. Mark the spot on the wall and see how it moves every day. It won’t last very long as the angle of the sun changes.
At certain times of the year the sun as it rises will shine through the side/ bottom glass panes of the garden window in my galley kitchen. And the reflection gets bounced to the back wall. it’s perfectly round unless a tree branch or clouds are blocking it.
So actually one thing about the hole was that it didnt have reflections happening in it, it was a clean hole, eg in thin material or non reflective material.
If it has reflections occurring in the hole, then there are multiple images painted on the wall at the same spot.
The sun (and the moon) are 30 minutes of arc across - so about “1 in 120” spread (Actually a radian is about 57°). With a pinhole, that means the image of the sun will be, say, 1.2 inches across at 10 feet. Make the aperture bigger than a pinhole, and the image will be bigger by the size of the hole. When the circle that would be created is significantly bigger than the opening, the shape of the opening is not obvious.
Let’s say, we have a rectangular hole 1/4"x 1/2"; and a wall 12 feet away. With a pinhole, you’d expect an image of the sun 1.44" diameter. With the rectangular opening, you will have a oval-shaped image about (1.44+.25)" x (1.44+0.5)". A quarter inch off circular in almost 2 inches, with fuzzy boundaries, is not that obvious.
You can see the effect on paper - draw an opening, and have parallel rays coming from the sun at 1/2 degree apart. They go through the hole, then where do they go?
If the hole is bigger, then light will create a less focused image until it is just a big circle. The focus “circle of confusion” is the size of the aperture.
