This extinction illusion (Hermann grid variation) has been making the rounds on social media courtesy of a Facebook post by psychological scientist Akiyoshi Kitaoka. There are 12 black dots in this grid. Most people cannot see them all at once.
The illusion itself first made an appearance in a 2000 journal article by Ninio & Stevens. Check out their paper for other illusions.
The larger lesson for your students: Our senses, including vision, allow our brains to create a representation of the world around us. Our senses do not allow our brains to generate a perfect replication of the world.
For students who want to know why the illusion works, well, that’s a little more challenging. The short answer is that our retinas are hard-wired to send the clearest, sharpest signals to the brain. Receptor cells that get the strongest signal block out the weaker signals. Those dots in the periphery get blocked by the grey that surround them thanks to the sparser rods in the periphery.
For the longer answer, read up on lateral inhibition. Wikipedia provides a nice summary. (Yes, lateral inhibition is also used to explain Mach bands.)
If you want to wade into this even deeper with your students, Wesley Jordan (St. Mary’s College of Maryland) has created a class activity that should help students understand lateral inhibition.
Ninio, J. & Stevens, K.A. (2000). Variations on the Hermann grid: An extinction illusion. Perception, 29, 1209-1217.