MOIRÉ. This has always fascinated me. When you walk by curving vertical railings they pattern strangely. Watered silks show a water ripple effect. Umbrellas in street lights can also display these optical effects. Moiré patterns “are large-scale interference patterns that can be produced when an opaque ruled pattern with transparent gaps is overlaid on another similar pattern. For the moiré interference pattern to appear, the two patterns must not be completely identical, but rather displaced, rotated, or have slightly different pitch.”
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