Interference Pattern Formed in a Finger Gap is NOT Single Slit Diffraction｜hydraenids
Hacker News
February 13, 2026
AI-Generated Deep Dive Summary
Creating an interference pattern using a narrow gap between two fingers is a simple yet fascinating experiment that challenges the common misconception of it being single slit diffraction. By peering through the gap at room light or sunlight, one can observe a fine interference pattern within the shadow area formed by the fingers. This phenomenon, contrary to popular belief, is not single slit diffraction but rather "diffraction by a semi-infinite screen," where the relationship between the gap width and the period of the stripe pattern differs significantly from traditional single slit experiments. The author successfully captured this pattern using a compact digital camera, demonstrating its visibility through photography and visual observation.
The principles behind this phenomenon involve understanding diffraction as the bending of waves around obstacles or apertures. According to Kirchhoff's integral theorem, light intensity distribution can be determined by solving complex integral equations, but for practical purposes, the high school physics concepts of interference patterns suffice to explain the observed phenomenon. Unlike single slit diffraction, which produces a distinct central bright band with diminishing intensity on either side, the finger gap experiment results in a series of evenly spaced interference stripes due to the nature of the semi-infinite screen.
This discovery holds significant implications for tech enthusiasts and educators alike, offering a low-cost method to explore wave properties and optical interference. It also highlights the potential for creative problem-solving in experimental physics using readily available materials. By challenging conventional understanding, this experiment encourages further exploration into alternative diffraction techniques and their real-world applications.
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Originally published on Hacker News on 2/13/2026