Full disclosure: this “illusion” isn’t actually that great. But it’s not bad. And it’s sciencey, so what more do you want, right? This one’s all about light refraction and how it’s affected by different mediums. Party on.
Not A Medium Water; A Water Medium
This Demo Science science demo is pretty simple. All you’ll need is a clear drinking glass, enough water to fill said glass about halfway, and a drinking straw.*
Step one: clear your desk and have your students gather round; they may want to take notes, so make sure they have the proper supplies at the ready. Place the glass in the middle of your desk and add water until it’s about half full. Then, place your drinking straw into the glass and, obviously, the water.
Have the kids observe the straw from all sides, even the top if they really want to (though you’ll have to decide if you want them clambering up on your desk to take a peek). Tell them to move around your desk as they need to and to make notes about what they see and what they think is the explanation.
Dost Thine Eyes Deceiveth You?
Your students will see that the straw appears to be bent, broken, or otherwise askew at the point where it meets the water. However, unless you did something severely wrong, that is not, in fact, the case. It is, in fact, an optical illusion. (Again, not a great optical illusion, but an optical illusion nonetheless.)
The straw looks out of whack because light rays move more slowly through mediums like glass and water than they do through air. Because of this, the light showing straw in the water reaches the observer’s eyes after the light showing the straw in the air does; because of this, the straw appears broken.
* Plain ol’ plastic straws will work fine. However, as this is a fairly simple experiment, I find that students pay more attention and seem a little more impressed if you jazz up the visuals a bit. Using a glass straw makes the refraction even more dramatic, or, you can have some custom coil winding done and get yourself a cool metal twisty straw for super crazy refractionary illusionary funtime fun.