Physics & Physical Science Demos, Labs, & Projects for High School Teachers

Optics with Jello Lenses

Posted on: June 3, 2009


cosbyHow do you see light’s path through a lens?  We did this experiment at the DAMOP teacher’s workshop at Penn State last year.  Make Jello in a flat bottom pan, about 3/4″ deep.  Use half the water so the Jello is firmer than normal.  You’ll have to experiment with the color and tell me which works best, I haven’t done this on my own yet.

Obviously the Jello is made the day before.  Now cut the Jello into the shapes of the lenses.  You can make prisms, double concave, convex, whatever you like.  You can float the pan in warm water to release the lenses from the pan.  Don’t do it too long, just enough for the Jello to lift out undamaged.

Now shine a laser pointer through the Jello.  You will be able to see the path of the laser and follow as the light is bent by the lens.  Set up a series of lenses and have fun.  When you are done, you can eat the experiment.

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6 Responses to "Optics with Jello Lenses"

How about using watch glasses as a “bowl” to form the jello?

If I understand your question, you have the wrong idea. You want the top and bottom of the lens to be flat. The sides are the curved portion, you aim the laser pointer in from the side. Check out the image here:

The idea is that you make the lenses shown here out of Jello rather than purchasing acrylic or glass. You will be able to see the path of the laser through the Jello just like in the photo.

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I have to disagree that you shouldn’t use the watch glass to make a slightly curved lens.

I have done this lab activity many many times, and I have used so many kinds of molds it’s funny! I have used the watch glass, but it creates only a very slightly convex lens that I chose not to use it again. Instead, I use the pvc plumbing caps you can get at any hardware store. I create a little “nest” out of paper towels so the caps don’t turn over in the fridge while setting. Then I get some thick waste laminate from a Kinko’s – it’s way thicker than the kind my school buys, and kinko’s looks at you funny when you ask for their laminate trash, but they’ll give it to you free when you tell them why! You let the kids put the upside down jello caps onto the laminate and use it as a magnifying glass on books and papers and even outside to look at bugs!

I have done all kinds of jello mixes also. All different colors with many different colors (or wavelengths) of lasers. You will see such remarkable differences in the internal optics! I have a great green laser that will not go through the red colored jello and the red laser is equally stopped by the blue and green jello.

Some very interesting notes:
1. The sugar-free and regular jellos will create different refractive indices due to the large molecular size of the sugar and the tiny size of the fake sugar.
2. Using Petri dishes works so wonderfully since you can use the dish itself as the lens cutter – creative cutting creates many lens conficurations from the same plate.
3. CLEAR jello is not easy to find – I had to use regular Know gelatin to make it, and of course the kids don’t really want to eat the flavorless lenses…but the total internal reflection is amazing!
4. Bubbles that set inside your jello create mini-air lenses that can also refract in interesting ways. I’m trying to find a way to create a specifically shaped lens “hole” inside a set-up of jello, but I’m not quite able to get it yet.

The jello lens activity is such a wonderful one – open your mind and let the creativity pour itself in!

I have done this a few times with kids, although I have used Knox (clear, unflavored) gelatin.

I find that cutting the gelatin from a pan as described above is a great way to make lens shapes, but it is difficult to get them symmetric and the cutting tool leaves irregular grooves that make it difficult for the light to refract properly. If you do cut the shapes, you may want to “polish” the cut edges with a warm surface. My preference is to use sheets of aluminum warmed in hot tap water or heated with a hair dryer. I have used aluminum roof flashing. They come in sheets of about 6×9 inches and can be bent easily. They are very cheap at Home Depot. You can sharpen one edge of the aluminum, bend it and press it into the gelatin to cut it. This is to be done in advance – don’t leave the sharp tools out around young kids.

I have been trying to concentrate on molding the gelatin instead of cutting it. Petri dishes make good bi-convex lenses. If you want to make the lens thinner, you can cut a middle strip out of the gelatin circle (think of a clock face and cut stright lines from 11 to 7 and 1 to 5) then push the straight cut edges back together. The cut edges should work OK if they are mated to each other (an internal surface that has the same index on each side)

The watch glass mentioned above should work fine if you cut the resulting lens in half and rest it on-edge on a flat surface.

The most fun is to cut free flowing shapes (here you don’t need the cut surfaces to refract so cutting is OK). Think of a candy cane that has a square cross-section. Now shine a pen laser or strong light through the end of the strip of gelatin. Light will reflect many times within the tube (total internal reflection) and light will come out the end of the tube.

For sources, consider laser levels from hardware stores. They sometimes come with a diffractive-like piece of plastic that breaks one beam into many beams. If you want all the beams collimated, you will need to have several pen laser sources or splurge for a laser ray box shown in the picture above.

One thing I have not tried yet is to make gelatin with tonic water. The quinine in tonic water makes it fluoresce and should be an interesting effect when shining a laser through the gelatin. But I do not know if tonic water has anything in it that will keep gelatin from becoming a gel. If someone experiments with this, please respond back with your results.

P.S. I have a colleague who purchased the domain name jellooptics.com but he hasn’t done anything with it yet. Maybe when he stumbles across this thread, it will prompt him to get off his a** and do something with it.

Oh! One more thing. Keep in mind that Jello and plain gelatin are foods made from animals. If plain gelatin is left unrefrigerated for 2-3 days, it starts to decompose and smell really bad. From personal experience, do not put it in a container and forget about it. I suspect Jello is similar, but because it is flavored the smell may be disguised for a longer period.

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