Graded Index Gelatin

I decided to try to make some GRIN (Graded Index) lenses out of gelatin. My inspiration was the GRIN gelatin blocks made by Groot Gregory, PHOTON Projcts Industry mentor, for OSA Educator's Day in Rochester, NY last year.

Graded Index means that the index of refraction varies across the material. GRIN optical fiber has a core index that is high in the center and decreases (usually parabolically) to the cladding. Slices of GRIN material can be used to make lenses. To bring light to a focus, you need to slow the light at the center of the beam relative to the edges. This can be done by making the lens center thicker (as with a conventional lens) or making the index of refraction higher (as with a GRIN lens.)

Using Groot's method I made extra stiff gelatin with LOTS of sugar. Of course I forgot to write it down but I think I softened 2 packages of gelatin in 1/3 c cold water, then added 1/2 c boiling water plus about 3/4 c sugar. (I need to ask Groot for the "real" recipe.) I put this in large prescription medicine containers so I'd get cylinders of gelatin. Once they were solid, I removed them from the containers (dip in hot water a few seconds).

Grin lenses

Then (here's the secret) I placed them in a bowl of cold water overnight. Apparently the sugar diffuses out, changing the index of refraction from the "outside in." Since the process occurs at the ends, as well as around the cylinder, I cut the ends off with a heated knife. I didn't do a very good job, and the knife left scratch marks. So, I heated a plate and "sat" each end on the plate for a while until it melted smooth. They still weren't very straight (I'll have to work on that process.)

Here you can see what happens when a laser is directed through the cylinder, top, middle and center. The laser is to the right of the cylinder. In the end photos, it's clear the beam isn't straight, but curves downward toward the far end.

I tried to show the focusing ability of this "lens" with an expanded beam but it seemed to be position dependent. I think maybe the gelatin that was resting on the bowl didn't lose as much sugar as the rest of the cylinder- does that make sense? Anyway, the first photo shows the expanded laser beam shining on a box on my table. It was a laser pointer, so expanding it emphasizes the fact that it's actually a rectangular spot (the yellow rectangle, where the camera sensors were saturated.) The middle picture shows the "lens" inserted and you can see that the yellow spot is much smaller. It wasn't round, though. The photo on the right is a close up of the "lens" trying to show how the beam was focused. The beam is narrower on the left end than on the right, but it's not symmetric.

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