Placed in the correct configuration relative to a metal fork, a metal knife appears transparent, with some observers experiencing a bistable percept in which transparency alternates with reflective appearance

The Fork-and-Knife Illusion. Blaise Balas, Benjamin Balas. Perception, January 20, 2021. https://doi.org/10.1177/0301006620988847

Abstract: We describe a transparency illusion that can be observed with an ordinary metal knife and fork. Placed in the correct configuration relative to the fork, the metal knife appears transparent, with some observers experiencing a bistable percept in which transparency alternates with reflective appearance. The effect is related to other illusory percepts that follow from careful placement of mirrored surfaces, but to our knowledge, it is unique in that the key feature of the illusion is how the mirrored surface (in this case, the knife) is perceived rather than how a mirror induces altered perception of other objects and surfaces. We describe conditions that do and do not affect the strength of the illusion and point out its connections to previously reported phenomena.

Keywords: visual illusions, transparency, material perception