February 15, 2026
The finger wars begin
Interference Pattern Formed in a Finger Gap Is Not Single Slit Diffraction
Finger physics sparks a nerd fight: magic, trick, or bad idea to stare at the sun
TLDR: A physicist claims the finger-gap light trick is “edge diffraction,” not the classic single-slit demo. Commenters split between safety warnings and sharp skepticism, while others turn it into DIY eye science and lamp-squinting memes—because home physics is fun, but your retinas would like a word.
A physics blogger says the “finger-gap light show” isn’t your high-school single-slit diffraction trick but a fancier “edge diffraction” effect—basically light bending around the sides of your fingers. Reddit and Hacker News promptly turned it into a soap opera of optics, eyeballs, and memes. Fans loved the DIY angle (“two fingers, instant science!”), but skeptics came in hot. lefra demanded proof: if it’s just edge stuff, why does the pattern vanish with one finger? And why are we using formulas for a wall when the “screen” is your retina? Meanwhile, kelseyfrog escalated the home-brew lab vibes with a freaky-cool tip: move a tiny hole and you can literally “see your own retinal vasculature.” Eye horror meets physics flex.
Then dark-star threw the safety flag: “Please don’t tell people to look into sunlight.” Cue a chorus of “use a lamp, not the sun” jokes and “DIY laser lab” memes. The vibe: half the crowd geeking out over double slits, half calling for diagrams, and everyone squinting at desk lamps like it’s finals week. It’s Optics vs Eyeballs, with one burning question: are we witnessing elegant wave interference—or just finger magic and risky squinting?
Key Points
- •A narrow gap between two fingers can produce fine interference fringes when viewed against room light or sunlight.
- •The article argues the observed pattern is due to diffraction by a semi-infinite screen, not single-slit diffraction.
- •Key differences include how fringe spacing relates to gap width between the two phenomena.
- •Diffraction is framed via the Fresnel–Kirchhoff approach, derivable from the Helmholtz equation and described by Kirchhoff’s integral theorem.
- •The double-slit interference geometry (D sinθ ≈ Dθ) is reviewed as background for understanding the finger-gap effect.