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Comment by kazinator | Hacker News Reader

If you tune two strings together, listening for beats, you are also guided by a moiré pattern. The whistle you hear when tuning an old AM radio is also moiré beats caused by the overlap of the local oscillator frequency and the incoming carrier.

Another example of moiré for accurate indication: optical flats:

https://en.wikipedia.org/wiki/Optical_flat

This application shows how an optical flat resting on a gauge block indicates a tilt: showing that the adjacent object being tested is taller or shorter than the block by a minute amount. https://commons.wikimedia.org/wiki/File:Optical_flats_in_use...

Interference patterns are basically moiré. Anything using interference for fine calibration is using moiré.

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For anybody who wants to dig into the theory behind all this, these are examples of aliasing (the same word you see in "anti-aliasing" regarding fonts, computer graphics, etc.). I've been fascinated by moire patterns since I was a kid 45+ years ago. Here's one of my dabblings that required a bunch of browser-specific hacks to turn anti-aliasing off so the aliasing would show (if you change window size, you'll notice different effects after it redraws): http://www.nukacourt.com/moire.html

> The whistle you hear when tuning an old AM radio is also moiré beats caused by the overlap of the local oscillator frequency and the incoming carrier.

Also when you receive Morse code on a radio using a Beat Frequency Oscillator, which is tuned close to but a little off from the IF frequency.

It's also how a Theremin works, using the capacitance between your hand and the pitch and volume antennas to pull the frequency of an oscillator. If you'd like to play with a Theremin I might be selling mine - I haven't touched it for months...

I don't think optical flats operate based on moiré gratings. The former is an interference/diffraction effect while the latter, is you underpinned earlier, is a sort of spatial heterodyning effect. For instance, you can generate moiré rulings (which effectively an optical illusion) with two angularly misaligned combs, but the comb spacing is order of magnitude larger than optical wavelengths, leading one to conclude it's not a wave interference phenomenon.

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