A Reddit user has captured an unusual optical phenomenon in their apartment, gaining widespread attention on the online forum.
The viral post, shared by user Mob_Slime, features an image of what appears to be an inverted projection of the street outside, visible through a keyhole. The intriguing post’s caption says, “Projection from a keyhole in my apartment showing the other side of the street.” The post has garnered over 60,000 upvotes since it was shared on September 22.
The image displays a beam of light passing through the keyhole and projecting a faint but clear picture of a window and surrounding buildings onto the apartment door. Many Reddit users expressed surprise and fascination at the rare sight, with some speculating about how the projection came to be.
Photographer Emma Carter, based in Pittsburgh, explained the phenomenon, attributing it to a natural optical occurrence known as the camera obscura effect.
“What you’re seeing is a camera obscura effect,” Carter told Newsweek. “Imagine you’re in a dark room and there’s a tiny hole in one of the walls. Outside, it’s bright and sunny. When the light from outside goes through that little hole, it forms a picture of the outside world on the opposite wall inside the room, but the picture is upside down.”
The term “camera obscura” can refer to “any dark place in which a projected image is visible,” says an April 2020 study in the Encyclopedia of Early Modern Philosophy and the Sciences. “In its simplest form, the camera obscura is merely a dark room with a single aperture.” (That aperture is the opening in a lens through which light passes to enter a camera.)
The study went on: “The camera obscura was a simple astronomical tool that evolved into an important optical instrument in the seventeenth century. During its development, the camera obscura generated various philosophical discussions about perception and image formation.
“This phenomenon can take place naturally, and descriptions of its accidental occurrence go back to antiquity. The camera obscura became intentionally used as an instrument by Renaissance astronomers. Leonardo da Vinci, for example, described it and the first known illustration of the camera was published by Gemma Frisius [a Dutch mathematician],” the study said.
‘It’s Like a Magic Trick’
Carter further explained that light always travels in straight lines, which plays a crucial role in creating the inverted image. “The light travels in a straight line through the hole and flips as it comes inside,” she said. “It’s like a magic trick that shows you the outside world on the wall but flipped.”
In the case of the Reddit post, the keyhole acted as the small opening through which the light entered, projecting the outside scene onto the door. The process, Carter said, is simple in theory but remarkable to witness in real life.
Sam Savat, a photographer based in Phoenix, told Newsweek: “What’s happening here is basically the same concept they use to make a pinhole camera.”
He went on: “When light passes through a small opening, like this keyhole, it projects an upside-down image on the wall or surface inside. It’s pretty cool, actually. The keyhole in the apartment works like a tiny lens that focuses the light and projects whatever’s outside.”
Savat compared the keyhole projection to how the human eye functions, noting the similarities in how light is processed.
“The images we see pass through our pupils—those small openings—and get reflected on the back of our eyeballs where the optic nerves are,” he said. “The images we receive on the back of our eyeballs are actually inverted, and our optic nerves send that information to our brain for it to process, and we see the images right-side up in the real world.”
Savat added that the phenomenon, while rare in such a setting, is a natural result of light traveling in straight lines.
“The outside street image you see through the keyhole is inverted because light travels in a straight line,” he said. “So in order for you to see things that are higher in your field of view, like the top of a building or birds, the light from above has to go through the small keyhole. Because light travels in a straight line, it hits the bottom part of the surface inside of your eyeball, making the image appear upside down.”
‘So Cool’
While the camera obscura effect has been known and documented for centuries, seeing it happen with an apartment keyhole has introduced many Reddit users to the phenomenon for the first time, leaving them both intrigued and impressed by the science behind it.
Royalchariot wrote, “That is so cool.”
Thereminz said, “There is something ‘otherworldly’ about a camera obscura projection. maybe it’s in the way the diffraction changes the image a little bit, sort of like tilt shift photography…”
Muggaraffin wrote, “I’m just surprised I’ve never seen this in person. Seems like something that would happen more often. Unless I’ve just got lazy keyholes that let any old light in.”
User summervogel said: “I’ve never seen a camera obscura occur naturally like this but I hope I do one day.”
TwistedxBoi wrote: “Local Redditor discovers camera obscura. It’s funny how many of these get created on accident in the world.”
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