Huh. Never seen one of those, but it wouldn’t be hard to do.
The hidden image is printed with an uncolored polarizing ink. Light can pass through the ink, but is polarized. You don’t see anything, because light passes through the hidden image unimpaired.
The “lense” you’re looking through is another polarizer, oriented 90 degrees from the polarized image. “Crossed” polarizers block the light – the first polarizer only passes light in (say) the left-right polarization. The “lens” only passes light in the up-down polarization. So the image appears black, since the light can’t get through.
Try turning the lens 90 degrees. The image will disappear.
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