Science Buddies: "Ask an Expert"

Hello I am doing this project (https://www.sciencebuddies.org/science- ... ml#summary) for my Science Fair and I have a few questions that I hope can be answered.

First what is gelatin? Is it jello, an kind? And where can I buy it so i can use it in my experiment?

Secondly, how do you actually find the speed of light using Snell's law? I don't completely understand how you actually calculate the speed of light and it seems like you're just finding the speed of light in he substance rather than the actually speed of light, can you explain this? I also understand that you also need to use the index of refraction of a material and some angles to use Snell's law, but how?

This also brings me to my third question, which is what is the index of refraction of gelatin, in the make it your own page (https://www.sciencebuddies.org/science- ... eityourown), it gives the indices of several other materials, but the index of gelatin is never given, so could you tell me what it is?

Thanks in Advance for answering my questions, and that is all for now. Thank You!

Hello, looks like a cool experiment you're doing!

Jell-O is a brand of gelatin that is usually flavored and colored to eat as a dessert. The gelatin you want for this experiment should be unflavored and uncolored. Fortunately, plain gelatin can be found in almost any supermarket. Look for brands such as Knox. Gelatin is a substance made from animal bones and ligaments. You can read more about what gelatin is here: http://www.madehow.com/Volume-5/Gelatin.html

Speed of light varies depending on what medium the light is traveling through. For this experiment, you are just measuring the speed of light in gelatin. To do that, you must first determine the index of refraction for gelatin using this part of Snell's law:

sinθ1/sinθ2=n2/n1

θ1 and θ2 can be found through your measurements, and n1 (the index of refraction for air) is 1.00029. So to find n2 (the index of refraction for gelatin), simply divide sinθ1 by sinθ2 and multiply that quotient by n1.

Now to find the speed of light in gelatin. The definition for the index of refraction is:

n=c/v

where n is the index of refraction you just calculated, c is the speed of light in a vacuum (approximately 2.997 x 10^8 m/s), and v is the speed of light in gelatin. So to find v, just divide c by n.

Hopefully these answers clear things up for you.

-Karen

Hello thanks, now I have some other questions.

First is the gelatin that I bough correct(see below), I know it has to be unflavored and uncolored but it only says it is unflavored so what about uncolored. Also how much do I need?
Next how do I make this gelatin?
And does the gelatin have to be outside the container for this experiment?
Also my hypothesis is:
If the speed of light calculated from the gelatin is the most accurate, then the gelatin can most accurately calculate the speed of light without sophisticated expensive equipment because out of all the other substance it was the accurate compared to the other substances.
but since this experiment is calculating the speed of light in a material should I change my hypothesis to:
If the speed of light calculated from the gelatin is the fastest, then the light travels the fastest in gelatin because out of all the other substances it was the fastest.
Thanks in advance.

Hi Pokeandhan,

The picture of the gelatin didn't come through, unfortunately. But, the gelatin that I've used when doing this project comes in packages that look like this:

http://www.amazon.com/Knox-Gelatine-Unf ... ds=gelatin

That said, there are several different brands of gelatin, so the packaging you have might look different. It's OK if the packaging for your gelatin doesn't look exactly like this one. The unflavored gelatin that I've seen at the grocery stores has always been uncolored, so I suspect that the gelatin you purchased will work for this project.

The exact amount of gelatin that you make isn't critical for the project, but you don't need a lot. If you have a square container that is ~2" (5 cm) on a side, that should work OK. A square that were a bit larger (say, 10 or 15 cm on a side) would also be OK. You could also make the gelatin in a larger container and then cut the gelatin down to a smaller size.

Sometimes the back of the gelatin powder container will have directions for making it. If not, I usually make gelatin this way:

Sprinkle 0.25 oz (which for me is one packet) into 1/4 cup cold water. Let stand for about a minute. Then add another 1/4 cup boiling water and mix until the gelatin is all the way dissolved. Stick it in a refrigerator and let it sit until firm. This will make 1/2 cup of gelatin. You can double or triple the recipe if you need more than that. (You can make the gelatin less firm by adding 1/2 cup boiling water instead of 1/4 cup.)

The experiment would work best if it is outside the container. You can loosen the gelatin to help it come out by doing what Note B in step #2 of the procedure says to do.

Your second hypothesis is an improvement over the first. If the speed of light is greatest in the gelatin, then the light travels faster in the gelatin than in the other materials you test.

Post back if your have other questions!

Thank you! But I have another question what do you do if the sin of angle 2 is negative?

Hi pokeandhan,

The sin of an angle can be negative. If you are doing the project correctly, then it may be that you are measuring the angle in a different direction. Can you please post the values you are using so that I can check what you are doing?