How many rectangular prisms do you count?

This illusion can be constructed entirely out of line segments! I won’t list them all here, but you can count that there are 31 of them: 16 for the faces at the front, 9 for the long edges, and 6 for the faces at the back.

This illusion breaks apart when you adjust some of the parameters, so I highly encourage you to play around with the Maple Learn document I used to create it: Optical Illusion: Impossible Prisms.

The first change you can make is reducing the length of the prisms. By bringing the back faces closer to the front, it makes it easier to follow the edges with your eyes and see how there isn’t a one-to-one correspondence between the vertices.

Next, I’d recommend opening up the Visualization Commands section and changing some of the colours. This really helps distinguish the line segments. By now you might be able to see where the trick is: by overlapping edges of the prisms, you can blend the different faces together.

And if you’re still having some difficulty seeing how it works (rightfully so!), one final adjustment you can make is by opening the Visualization Definitions section and changing the obtuse angle that the edges of each face form:

Now, I usually try my best to cite the original creators of works like this – for this particular illusion I couldn’t find the source. However, this appears to be based on D.H. Schuster’s Impossible Trident (also known as the Devil’s Fork), since the illusion also arises from simple shapes blending one into another.

So, what's the answer? How many prisms are there?

If you count the front faces, there are 4, but if you count the faces at the back, there are 3. Maybe we could agree that the expected answer is 3.5? Personally, I think that an acceptable answer is 2, since only the top and bottom prisms are technically complete (and being technically correct is the best kind of correct).