# Experiment: Measuring the Speed of Light

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Consider: You’re a deer on a dark road. A pair of headlights is suddenly approaching you. Fast. You stare at it intently… wondering - how fast is the light moving? Now, if you’re that deer, chances are this wouldn’t be your first concern. Or maybe it would. Maybe that’s why they take so long to move away… long calculations about the speed of light flood their minds..

Do you have anything to say? Wish to discuss the experiment? Ask questions or criticize the method? Post and debate here!

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Edited by Cap'n Refsmmat

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I did this experiment, mooeypoo, with my class of science students, and we got a value of about 2.3 x 10^8 ms^-1. I'm fairly sure we didn't do too much that was wrong, and although it might be a bit low, and of course the magnetron probably doesn't have the frequency quoted, I thought to myself "shouldn't the speed of light be slower in chocolate than in air anyway"?

the speed of light in a medium other than vaccuum (oxymoron, sorry) is dependant upon the medium's density, hence we get refraction bending toward the normal upon movement from a less dense to a more dense medium and vice versa. The speed of light in water is about 2.2^8 m/s, so in chocolate, which has the same approximate density as water, shouldn't it be around the same value, roughly?

I wonder if people have been biasing their results due to their "knowledge" that light travels at 3 x 10^8 m/s?

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Hmm.. this is REALLY interesting actually.

As an instinct reply, I have to say that since the chocolate is relatively VERY small (compared to the wave), I would guess that the change in speed is very miniscule too.

I laid out possible causes for the margin of error that I had in my experiment - for one, I can't be sure of where EXACTLY to place the chocolate so that the waves are 'hitting it' in their "x-axis" points of contact. That's important, and it probably sways the result much more than the change in speed.

But the idea that we usually ignore the differences is interesting. I will have to check, though, what the differences are to be sure if that's a major cause. The difference between 2.2*10^8 and 2.3*10^8 may sound small, but they're quite large when you think about it.. I *think* it's more logical that we have calculation/experimental errors than an actual change in the speed.

And as for 3.0*10^8 -- This, I would guess, is a historical/convinience reason. When you speak about the speed of light generally, it's much easier to say 300,000m/s than to start saying 299,792,458m/s

But.. again.. good point. I think it's a good place for our Physics experts to come into the picture... Klaynos? Martin? Anyone?

Maybe they can shed some light on the matter, in whatever speed...

~moo

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Although not the most accurate I found it vary intresting. Plus its probably the cheapest and easiest way to calculate the speed of light. Plus I would have to agree that the error was probably a error in calculations.

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I wondered to myself last night about this experiment and how it could be improved. I was thinking that since the chocolate bar is liquidified it spreads, and since it is opaque, one cannot see the exact point of meltage (neologism, I know).

Perhaps the experiment could be improved by the use of a transparent substance, perhaps a gel? so when it starts to boil the cracks in the gel stay where they are and can be seen from outside

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It seems to me though, the greatest error resides in measuring the distance between nodes. It's not all that easy to locate exactly where melting first occurred, nor is it precise to measure with a metric ruler.

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It seems to me though, the greatest error resides in measuring the distance between nodes. It's not all that easy to locate exactly where melting first occurred, nor is it precise to measure with a metric ruler.

there certainly is an element of uncertainty in the measurement of the distance between nodes when using chocolate. Mostly because as soon as the chocolate melts it flows, and also because it doesn't ONLY melt at the nodes, which makes the melted area larger than desired.

These are the reasons I suggested using a gel in which the first boiling, rather than the first melting can be observed, AND because the gel will crack, the positing of the first few bubbles will be recorded for later observation.

Your comment about using a metric ruler is totally nonsensical and I can't really say anything more about it.

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Hrmmm what would be good would be a clear gel or solid that when it undergoes a phase change it becomes opaque...

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then use egg albumen

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egg is a good idea i think

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ooh! sounds like an excellent idea for an attempt for a remake I'll write it down for the corrections/remake episode

thanks guys!

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youre quite welcome

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Oh, I thought of something that might be a problem -- an eggwhite (probably better than using the entire egg) isn't that "stable" -- can I be sure it's not moving around in the plate inside the microwave OR when I take it out, as gently as I can?

If it starts moving, even a bit, then the chocolate becomes more accurate.

Plus, I didn't use a single chocolate bar partly for this reason of the chocolate turning 'fluid' when hot -- with separate parts of chocolate, I can see a bit clearer which were melted and which weren't. Far from accurate, but it worked quite well.

Don't forget, I got about 6% error... that's pretty damn good. Would an egg supply better results?

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When in Doubt, Try it out!

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Fill a tray with egg white?

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When in Doubt, Try it out!

Haha Good point

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just spread some out thinly on a glass or piece of black card and go for it.

Iv used it Many times for Hot-Spot detecting in MWs it works well

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mooeypoo your statement that you got a 6% error is based on the assumption that the speed of ligth should be that of the speed of light in a vaccuum, which is not necessarily true in the case of a bar of chocolate. the use of a thin layer of egg white is probably going to remove that uncertainty at the least

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That's a good point, I didn't think about that..

But to change the percentage you need relatively large changes in the speed between the air and the chocolate -- the wave, after all, traveled about an inch in chocolate, probably even less --- does that really make that much difference in terms of the speed?

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Don't forget, I got about 6% error... that's pretty damn good.

It's good — anything to within 10% is for something like this — but it's not the error. Unfortunately this is often taught in physics class, or at least is not corrected. Error has to do with uncertainty concerning the experiment, not the difference between your answer and the accepted one. You might estimate that your measurement of the wavelength is to within ±0.5 cm. which would be ± 1.2e7 m/s, or 7.7% error. In that case, your answer and the accepted on agree to better than the error. Alternately, (and more deliciously) you could do the experiment many times and find the standard deviation of the measurements.

You might get better results if you used a chopped up bar or smaller kisses (or just chocolate chips), spread out a little more.

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I don't agree that you should use the speed of light in chocolate. The standing wave is in the air. The wave in the chocolate would not generally be a standing wave, it will be two traveling waves, and will not give you nodes and antinodes, since the return wave will be out of phase. And the amount of power in that wave will be small.

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It's good — anything to within 10% is for something like this — but it's not the error. Unfortunately this is often taught in physics class, or at least is not corrected. Error has to do with uncertainty concerning the experiment, not the difference between your answer and the accepted one. You might estimate that your measurement of the wavelength is to within ±0.5 cm. which would be ± 1.2e7 m/s, or 7.7% error. In that case, your answer and the accepted on agree to better than the error. Alternately, (and more deliciously) you could do the experiment many times and find the standard deviation of the measurements.

I was also going to post this when I commented on wondering the refractive index of chocolate, and whether it matters due to the amount of wave in the air compared to the chocolate...

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That's a good point, I didn't think about that..

But to change the percentage you need relatively large changes in the speed between the air and the chocolate -- the wave, after all, traveled about an inch in chocolate, probably even less --- does that really make that much difference in terms of the speed?

The speed of light is related somehow (not sure exactly how) to the density of the medium. The speed of light in water is about 2.2 (i think?) x 10^8 m/s, and since chocolate has a density much closer to that of water than that of air or vaccuum, we might expect the value to be in the mid to low 2's rather than 3, although there is further complication in that the wave probably doesn't travel entirely through chocolate

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I don't agree that you should use the speed of light in chocolate. The standing wave is in the air. The wave in the chocolate would not generally be a standing wave, it will be two traveling waves, and will not give you nodes and antinodes, since the return wave will be out of phase. And the amount of power in that wave will be small.

that sounds interesting. However, your predictions defy my limited knowledge of wave mechanics. If you are right, hopefully the albumin adaptation will help to prove it.

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The speed of light is related somehow (not sure exactly how) to the density of the medium. The speed of light in water is about 2.2 (i think?) x 10^8 m/s, and since chocolate has a density much closer to that of water than that of air or vaccuum, we might expect the value to be in the mid to low 2's rather than 3, although there is further complication in that the wave probably doesn't travel entirely through chocolate.

It's far more complicated than just density being important. But it can be simplified into refractive index, which is the ratio of the speed of light in a vacuum / speed of light in a material. It's also wavelength dependent.

It is possible to work out the refractive index of a material, but unfortunately I don't have easy access to a microwave spectrometer at the moment. Give me a few months and when I start my studentship I'll ask one of my friends the feasibility of it, I suspect we'll need a 1m square of very flat chocolate though....

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that sounds interesting. However, your predictions defy my limited knowledge of wave mechanics. If you are right, hopefully the albumin adaptation will help to prove it.

Take a rope, tie it to a pole or tree, etc., i.e. something solid. Shake it to make a standing wave. Now, let the tips of it hit the ground. What happens to the wavelength? Does the position of the antinode(s) shift much? (the answer should be essentially nothing/no) You're driving the wave in a medium at a certain frequency, and that dictates the wavelength. Not the interaction with the ground, even though you are coupling some energy to it.

If the microwave were largely filled with chocolate, then your scenario would apply. That would be letting much of the standing wave hit the ground, rather than the tips.

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It's far more complicated than just density being important. But it can be simplified into refractive index, which is the ratio of the speed of light in a vacuum / speed of light in a material. It's also wavelength dependent.

I'm aware that it's more complicated than that, and unfortunately I have to teach my students annually about refractive indexes without going into any of the interesting details. Anyway Ijust thought that the density might give us a first approximation

Take a rope, tie it to a pole or tree, etc., i.e. something solid. Shake it to make a standing wave. Now, let the tips of it hit the ground. What happens to the wavelength? Does the position of the antinode(s) shift much? (the answer should be essentially nothing/no) You're driving the wave in a medium at a certain frequency, and that dictates the wavelength. Not the interaction with the ground, even though you are coupling some energy to it.

If the microwave were largely filled with chocolate, then your scenario would apply. That would be letting much of the standing wave hit the ground, rather than the tips.

well i guess we wont know until someone does the remake. Certainly it seems we have a good case for reducing the quantity of the test material to a minimum to reduce any changes in the speed of the waves to another minimum

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