Please, I really need some help!

Bananaz32 · Post by **Bananaz32** » Thu Apr 12, 2007 10:53 am

I am working on the science project, "How to measure the speed of light in jello" I finished the experiment and got that the speed of light in jello was 2.88*10^8 m/sec, but I don't know if that is correct. I was hoping someone could confirm my finding. I have searched the internet to find the answer, but I can't find it. Please, Please help me!!

deleted-2131 · Post by **deleted-2131** » Thu Apr 12, 2007 3:41 pm

This is an interesting question. If your data says that the speed of light in Jello is 2.88*10^8 m/s, then yes, that value is correct. I think what you are wondering is how accurate that value is (how close is it to the true speed of light in Jello). Is this correct?

If this is correct, you can do a simple percent error calculation: (experimental value - "true" value) / true value time 100. You're doing well if your percent error is less than 10%.

So now the issue is trying to find out the "true" speed of light in Jello. Let me say this: since the speed of light in a vacuum is 3.00*10^8, and Jello is more dense than air, I would say that your value is on the right "side" of 3.00*10^8 (its a slower speed, not a faster speed). This is a good sign, because waves slow down when moving from less dense to more dense mediums. Now, according to some research done at the University of Texas-Austin, the index of refraction of Jello is 1.38. Since the speed of light in a medium is c/n, the speed of light in Jello should be 3.00*10^8/1.38 = 2.17*10^8 m/s.

Now, a couple of notes: the University of TX number was for clear gelatin. Since Jello has added sugar and other "stuff" it probably scatters the light differently and has a different density than does the clear gelatin.

Moral of the story: According to the University of Texas - Austin, the speed of light in clear gelatin is 2.17*10^8 meters/sec. This value probably isn't the exact same as the speed of light in your Jello because of slight differences between Jello and clear gelatin. However, it is similar enough to compare your value against. To compare your speed to the "true" speed, use a percent error calculation.

If you have any other questions (or I have totally confused you), please let me know. It would also be nice if I could know what grade you are in.

good luck.

Bananaz32 · Post by **Bananaz32** » Thu Apr 12, 2007 4:56 pm

You answered my question perfectly thank you!

btw i am in 12th grade.

Louise · Post by **Louise** » Thu Apr 12, 2007 5:34 pm

Terik Daly wrote: snip

Now, a couple of notes: the University of TX number was for clear gelatin. Since Jello has added sugar and other "stuff" it probably scatters the light differently and has a different density than does the clear gelatin.

Moral of the story: According to the University of Texas - Austin, the speed of light in clear gelatin is 2.17*10^8 meters/sec. This value probably isn't the exact same as the speed of light in your Jello because of slight differences between Jello and clear gelatin. However, it is similar enough to compare your value against. To compare your speed to the "true" speed, use a percent error calculation.

good luck.

I think there are more variables here that need to be considered.

It also depends on how "thick" you make your jello. A lot of optics projects that use jello seem to make it extra jelled (less water) or do a whole range of mixtures to vary the refractive index. I found a bunch of different reported values for the refractive index of jello. I don't think it is a good idea to compare to any one value, unless you are sure the group in question used exactly the same jello/water ratio.

This idea is in fact one of the variations for the sciencebuddies project- vary the refractive index of the material by changing the jello recipe.
https://www.sciencebuddies.org/mentorin ... ?from=Home

Since you are measuring the refractive index (and calculating the speed of light from that) you should probably compare with reported refractive indices, if you are going to compare at all.

If you absolutely must compare to some number, you could try one of the materials listed with the closest refractive index and see if you get similar results. [That is, the refractive index of glass or water or anything in that list is much better characterized than the refractive index of jello] Use that as your standard. I mean, do the experiment, and calculate the refractive index for water @ 20C or whatever. If you are off by 20% from the tabulated value, that is probably a good sign your jello is also off...

Louise

deleted-2131 · Post by **deleted-2131** » Thu Apr 12, 2007 6:48 pm

Excellent idea Louise!

Bananaz32,

Basically what Louise is suggesting is that you use the same bleep you used to determine the refractive index your jello to determine the speed refractive index of a well characterized substance. Then do a percent error calculation for that.

If you are way off for the well-characterized substance, chances are you're way off for your jello. This bleep helps control lots of the variables involved with the jello.

I would highly reccomend taking Louise's advice if you are ready and able to do so.