Learn About "Fork-and-Knife Illusion" From A New Scientific Paper Written By A Middle-schooler

Middle schooler Blaise Balas was playing with her food, as adolescents are wont to do. When she placed her metal knife vertically between the tines of the fork, she noticed something: from a certain angle, the knife appeared to become transparent.

It hadn't actually become transparent, of course. But there was something about the optical illusion that piqued not just her own interest, but her father's as well. Benjamin Balas is a scientist who studies optics and vision, and together, the father-daughter duo authored a new scientific paper in the journal of Perception about the so-called Fork-and-Knife Illusion. Here's the 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.

As Psychology Today notes:

This is not the first illusion in which a mirrored surface (like a knife) takes on a transparent appearance. For example, research in the 1990s on phantom limbs by V.S. Ramachandran and D. Rogers-Ramachandran found that placing a mirror that occludes one's phantom arm while reflecting the other arm could lead to pain reduction in amputee patients. In this case, the mirror takes on a transparent appearance because of the bilateral symmetry of the human body.

The advantage of the novel Fork-and-Knife Illusion is that you don't need a custom-built mirror box to produce it, but rather two common household items that are already usually found together: a metal fork and knife.


The novel Fork-and-Knife Illusion is not only a practical visual illusion you can try out at the next (in-person) dinner party but also a reminder that scientific discoveries can happen in the least expected situations. All it takes is some common objects, a playful context, and a keen observer.

So it's a pretty neat discovery — especially for a middle-schooler to make! In fact, it's even been named as a top-10 finalist for the Best Illusion of the Year Contest. Good work, Blaise!

The Fork-and-Knife Illusion [Blaise Balas and Benjamin Balas / Perception]

The Fork-and-Knife Illusion Makes Metal Appear Transparent [Nicolas Davidenko Ph.D. / Psychology Today]

Image: Public Domain via Pixabay