Help with data analysis

[kevintian]

Hi everyone! I recently did a modified version of this Science Buddy project for my school science fair: http://www.sciencebuddies.org/science-f ... p009.shtml
My hypothesis was if the speed of light in gelatin was affected by the temperature of gelatin. I heated a plate of gelatin to different temperatures (40,50,60 degrees) using a microwave and then measured the angles of refraction and incidence and all that good stuff for 3 trials. However, I found that my data showed a clear decline in the speed of light (the index of refraction increased) as the temperature increased. I was confused because I always thought that light traveled faster through liquids than solids, and as I heated up the gelatin, I could see clear signs of it becoming for liquid like (rather than the solid form that came out of the freezer). So basically, my question is why did the light decrease as the temperature increased, or what did I do wrong? Also, it would be nice if someone could tell me a practical application for the experiment, as I can't think of one. Thanks in advance!

[John Dreher]

Welcome to the forum.

I'm not at all sure what I would expect to see for such a complicated material as a gel. For pure water the speed of light increases by a very small amount as the temperature rises. (I've attempted to attach a GIF of a graph showing this effect.) To me. this seems reasonable as the density of water (away from the freezing point) decreases as the temperature rises, meaning there are fewer electrons per cm traversed to affect the propagation speed (in the sense of reducing the phase velocity). But for gelatin, as you heat it it becomes more like water and less like the gel -- at a wild guess this means the density increases with temperature for this mixture, thus having an opposite (and at a guess much larger) effect than for pure water. Again, I repeat that this is pure guess work. You have something infinitely better: data. So you have the answer, regardless of theory, which in this case is likely to be complex and, possibly, unknown even to an expert in such things (not me). Live with it. Love it!

Good luck with your science fair!

[John Dreher]

Another try at attaching the file, this time as a PDF.

[kevintian]

Thanks for your reply! For our science fair conclusion, we are supposed to include a theory as to why our experiment turned out the way that it did, and I thank you for that great answer. With that said, I still cannot think of a real-world application for this experiment, can you?

[dcnick96]

Hello. What a great experiment. Here is one example I can think of.

First, let's just talk about light refraction, regardless of temperature. Have you seen the movie "Castaway" with Tom Hanks? To survive on the island, he must spearfish. Since light refracts when entering the water, the fish is not located where it appears to be. It is offset a little. A spear fisherman (Tom Hanks in the movie) must account for that when he is throwing his spear. Otherwise, he will miss every time.

Ok, now let's talk about temperature. Say a spear fisherman learns to fish in the tropical warm waters near the equator. But, he moves to Alaska and still must spearfish to survive. The distance difference he has learned in the tropics may not work in the colder waters of Alaska. He will have to adjust his aim to account for the cooler temperatures.

I hope this helps. If you would like more examples, a simple Google search of water and light refraction should provide more examples.

Good luck!