AKA the ‘Yoko Ono’ Effect
Polarized Beetle 1
Polarized Beetle 2
The beetle reflects circularly polarized light. A filter in front of the camera lens blocks that polarization, but the light reflecting from the mirror flips polarization and passes through the filter.
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Borrow a pair of 3-D movie eyeglasses, look in a mirror, close one eye. Though which eye are you looking at the image?