The last post celebrated the role mystery can play in language learning. Optical illusions present us with a genre of mystery that (a) holds almost everyone’s attention for a little while and (b) lends itself to using lots of high-frequency language structures.
In addition to whatever words are relevant to a particular illusion, the discussion of most optical illusions can involve such handy phrases as these:
It seems that, I think that, in my opinion, it looks like, at first I thought, on the other hand, etc.
In many languages, expressing these comments involves “complex” grammar that can be demonstrated or practiced in a fun context using illusions.
Lots of optical illusions have to do with numbers–how many faces, animals, lines, dots, colors, etc. do you see?–so discussion of these illusions involves useful expressions of quantity, including, besides the numbers themselves, these:
How many, (much) more than, (much) less than, (many) fewer than, (a) few, some, lots of, etc.
Finally, discussion of optical illusions can summon many phrases that are useful for discussing images in general or spatial relationships in general:
In the top/bottom left/right corner, in the foreground/background, in the middle, near the edge, behind/in front of/next to, etc.
The peculiarities of your target language may allow you get extra mileage out of a single illusion. In many languages, for instance, the questions (and answers to) How many are there? and How many do you see? involve quite different grammar.
As for the mechanics of dealing with illusions in class, you have all the options that you have with any other piece of content: Q & A as a whole class, in small groups, or between partners; quickwrites, etc. You might set up a gallery of several illusions throughout the room and have students roam while discussing the pieces. You might have one or more students give another detailed instructions for how to locate or otherwise discover a certain element of the illusion.
I have used optical illusions quite a bit in language lessons with both adults and younger learners, always with great pleasure and profit!