Sugar GRIN (gradient index) lens

Light doesn’t always go straight! Lenses and mirrors bend light rays because glass has a higher refractive index (optical density) than air, so the rays bend due to the shape of the glass. But instead of having a constant refractive index and varying shape,  you can equivalently have a constant shape and a varying refractive index.

Adding sugar to water is an easy way to increase its density, and therefore its refractive index. Light bends towards areas of higher density, so if you can create a liquid that has a high density at the bottom and gradually gets lower as you go up, any rays entering it will bend downward!

Here, I poured a bag of sugar into the bottom of a fish tank, added a little water to make a super-concentrated solution, then freezed the whole thing, before pouring water on top. As the bottom melts, the sugar diffuses upward, creating a nice gradient refractive index.  The index should drop off rapidly, meaning that near the top of the tank, the beam doesn’t bend much:

Boiinnngg. Total Internal Reflection (TIR), or a mirror, can make it bounce off the bottom:

And just for fun, here’s what TIR at the top of the tank looks like: