Wikipedia picture of the day on April 20, 2018: A tape head...

Wikipedia picture of the day on April 20, 2018: A tape head cleaner cassette made of clear hard plastic. The plastic is birefringent and demonstrates internal stress as coloured patterns (photoelasticity) when photographed using cross polarisation. The polarising light source (an LCD monitor) has been rotated such that the polarising filter on the lens is oppositely oriented and so cuts out all the direct light, leaving a black background.

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