circularly polarized light

Definition: Light whose wave travels in a spiral, instead of the usual two-dimensional wave-form. * If a wave-front of light is held back (retarded) by 1/4 wavelength and rotated 90< or 270<, the combination of the original wave-front and the retarded one creates one whose electric vector rotates as it propagates. * Mica is an example of a quarter-wave retardation material.
Auto-focus in spin
This optical obscurity is important with modern auto-focus (AF) systems as they cannot read light which is plane-polarized at certain angles: circular polarisation allows use of the photographic effects of filtering polarized light without impairing the workings of AF sensors.

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