Safety Filters

Miffy noticed my first entry in TerryBob's September challenge for Photoelasticity, and wondered how I got the black background. I didn't do anything special, but tonight thought I'd experiment.

I found these safety pins in a hard plastic box which made a reasonable subject. Once again I used the iPad with a white background, but this time rotated the polarising filter, and sure enough, at one angle it gave a black background, while a 90 degree rotation gave white. Magic!