Measuring the Speed of Light at Home

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..


Note: If you have been to this post before, you might’ve missed quite a big error that was spread around through the text. Accidentally, I wrote 300*10^8m/s instead of 3.0*10^8m/s which is quite a large difference (and is absolutely wrong, too!). I apologize for the mistake, and hope that if you came back, at least I can correct it so that readers will have the correct number. The speed of light is 3.0 * 10^8 m/s – or 300,000,000 m/s. Thanks to Gonelli for the sharp attention :)

Of course, any educated deer knows that light moves at approximately 3.0*10^8 meters per second, which is very fast. It is wise, therefore, to move away very fast. But, as luck goes, by the time that realization passes the deer’s mind… well, you know. Poor deer.

But, that’s life. It’s not the only disturbing thing that may happen with speedy objects rushing through the air. It once struck me that I had no idea how baseball announcers knew the exact speed of a flying fastball.How about the speed of a baseball that’s just been hit out of the park?How fast is it traveling when it hits that car window in the parking lot?It’s moving so fast you couldn’t possibly measure its speed without fancy instruments.So, if its so hard to measure a baseball’s speed, how in the world can we measure the speed of light?

How can scientists be so sure of its speed with such certainty?And why should we believe them?

So.. no more! You don’t have to take their word for it. From now on, you can measure it for yourself with this – pretty nifty – home experiment.

Materials Used in this Experiment:

  • Long chocolate bar – $2.00, $3.50 in NYC
  • Microwave Oven: $100
  • Plate: Clean, preferably. (Microwave safe if you like your microwave)
  • Calculator: Not a must (but see the section titled Time for experiment)
  • Ruler: $0.11 if you buy by the thousands.
  • Personal, intimate knowledge of the speed of light: Priceless

Time for experiment:

About 10 minutes (with a calculator.15 without one. Could be a lot more if you forgot your fifth grade math).

Accuracy:

There will be some inaccuracy in your result (my own calculations had a percent error of 6.3%). The size of this error is important, because it can help us understand what external factors played a role in our experiment.If you stop to consider your results, why you got them, and what they mean, it’ll help you understand how light works.

Since this experiment is done at home (which is not conducive to precise measurements, unless you live in a physics lab, in which case I am willing to relocate for a domestic partnership) there are several external conditions that could distort the measurements and lead to errors.This is by no means a complete list, but here are some external factors that I thought of:

  • Uncertainty about the exact position of the plate inside the microwave oven. I had no way of knowing how high, or low, to put the chocolate inside the microwave so the waves would hit said chocolate at the x-axis and produce a relatively useful waveform on the chocolate. However, the microwave is relatively small, and I tried to align the plate with the wave generator (or what looked like one).
  • There is air in the microwave oven. This type of experiments is best done in a vacuum, to eliminate any interference from “external” sources like resistance, humidity, etc. The electromagnetic (light) waves themselves might not be affected, but the chocolate certainly is.
  • My measurement was relatively generous, on the broad side. If the chocolate had melted at two distinct points, I could have taken a more accurate reading. On top of that, my measuring devices – a ruler and two imperfect eyes – were of limited accuracy.Real labs with real scientists have very accurate measuring devices which decrease the percent error.

To try and eliminate such mistakes from affecting real experiments, real scientists never stop after only one experiment. The tests are repeated over and over again, and sometimes even over again, again, and all results(failures and successesses(whatever)) are recorded and analyzed. My experiment was only a demonstration, but if I’d have done it a few more times, and taken an average of the distance between entry/exit points of the wave, my accuracy level would probably rise.

Despite all this, I have to say: measuring the speed of light in non-lab conditions, using a bunch of Hershey’s Kisses and a plastic ruler, and getting only a 6.3% percent error, well, that ain’t too shabby.

Practical Applications:

There are many applications to knowing the speed of light, and mentioning all of them would be a crazy, crazy task even for me. However, for the sake of consistency, here are a few interesting scientific applications that use the speed of light:

  • GPS Systems can pinpoint signal location on earth with calculations based on the speed of light.The GPS device (in your hand, for instance) receives a locator beam from multiple satellites at various locations in orbit. At any given time, GPS satellites are located over different parts of the Earth, and they all send out a signal synchronized to each other according to an atomic clock.In other words, they all broadcast at exactly the same time.But because the satellites are at different distances from you, your GPS unit will receive these synchronous signals at different times.Why?Because the signals, despite being broadcast at the same moment and traveling at the same speed (of light!) arrive at your location in a staggered manner – sooner from closer satellites, later for farther ones.Calculating the relative delays of the signals lets your GPS unit figure out where you are to an exceptional degree of accuracy.
  • Communication in Space: The speed of light affects communication in general (since light itself, as we said, is an electromagnetic wave, just like radio waves but at a different frequency) but the most noticeable effect is on communication in space. When Houston ground control (as in, “Houston.. we have a problem!”) communicated with Apollo 8, the first Apollo mission to orbit the moon, they had to wait about 3 seconds until their messages reached the astronauts.
  • Distances in Space: Because distances in space are so vast and the speed of light is constant in a vacuum and thus the same number always, distances to far solar systems, planets and other stellar objects are often referred to in “light years.” In colonel general, the units of “parsec” (“parallax second”) and kilometers are used, but to convey the sheer size of those distances, they are expressed in terms of the speed of light.
    This method of measurement is defined as the distance a light beam travels in a certain amount of time. For example, expressing the distance to Alpha Centauri system as 4.3 light years (meaning light from there takes 4.3 years to reach the Earth) is much more comfortable than trying to say, write, type or remember it as a distance of 4.06802721 * 10^13 km.

Isn’t light brilliant?- I know, I didn’t laugh either.

Remember: True science is about experimentation and observation. If you use your brain to do some thinking, the world is at your feet!(Of course the world is at your feet anyway, but if you don’t think you won’t know what to do with it.)

(lots of thanks to Daniel, who helped me out with English, jokes, and a decent way of combining the two.)

Resources:

Speed of light:

Electromagnetic Waves:

Original idea (You can also find more information of how to conduct this experiment yourselves):

Similar Experiment:

18 comments
pragunbhutani
pragunbhutani

I don't know if you're brilliant or insane, so I'm going to settle by calling you 'insanely brilliant'!! :D

I think that that was by far, the coolest method of measuring the speed of light that I've ever seen and props to you for coming up with it!! 

Max Tello
Max Tello

ey yoo miss with the experiments and what not, this is all very cool, but my milk chocolate did not melt, it burned. would you be so kind as to enlighten on what class of chocolate you used so I can do the same and my microwave doesn't turn such a delicious treat into ashes. Thanks :)

gps comparison
gps comparison

Interesting home experiment... I'm going to ask my daughter to try this. Hopes she likes the experiment more than the chocolate, lol :)

KillaRandy
KillaRandy

Cool! Our Physics teacher shared this link too and made it our assignment!! Nice Video !! LOL!! XD

secretbaboy
secretbaboy

cool!! our teacher shared this link to be our assignment.

cliff manchaster
cliff manchaster

I love this post. I emailed this to my 9 year old nephew, he's a little scientist in the making. He is already asking questions about planets and stars, and if we could ever live on them. It's great that he is so excited about the learning process.

matt
matt

I love this website...another great post...I can't wait to show one of my friends this one!nm

Archi
Archi

Excelent. Your messages are really interesting. To have a good blog you should not only to post smth, but do it with your soul. You do your best.

mooeypoo
mooeypoo

... oh my god! You're right! I totally missed it, what a silly - and huge! - mistake!

Thanks, Gonelli.. I am correcting it in the post. You are ABSOLUTELY right. The speed of light is 3.0*10^8, and not 300*10^8 - there's a consistently missing dot there. Quite a big mistake... good that you caught it :)

Gonelli
Gonelli

Great video, I loved the experiment. But isn't the speed of light about 3.0*10^8 m/s, not 300*10^8 m/s ?

Daniel
Daniel

In place of deer, I would use consciousness (my brain). If I survived, it was two motor cycles and not a truck.

Mooeypoo:”True science is about experimentation and observation. If you use your brain to do some thinking, the world is at your feet”

I try, but what to do if I am stupid? I still don’t know whether light in your experiment are Dirac photons or Maxwell (HBT) waves.

Mooeypoo:” lots of thanks to Daniel, who helped me out with English, jokes, and a decent way of combining the two.”

Don’t mention it!

Daniel.

BadBoy
BadBoy

Ooh melted peaks of gooey chocolate heaven.

You make it megahertz so good;)

mooeypoo
mooeypoo

Hey Sharkey, thanks for the comment. You're right, the speed of light is not exactly 300*10^8m/s (I wrote the actual one in the video), but for the sake of simplicity I used the simpler one. I did correct, however, and added "approximately" ;)

Thanks!

sharkey
sharkey

You really need to correct your description of the speed of light as 300*10^8 meters per second.

Great video, I'm going to use it to explain this to my six year old son.

Fan
Fan

This is AWESOME!!!

mooeypoo
mooeypoo moderator

@Max Tello I used a row of Hershey kisses. Try to makes sure your chocolate bar doesn't have anything other than chocolate (don't use chocolate bars that have caramel inside them, etc) and use the lowest heat setting in your microwave oven.

There may be differences on how to accomplish this well because of the differences between microwave ovens -- try with pure chocolate (stripped of its wrapping of course) and perhaps the large size hershey kisses. Good luck!

kivatte
kivatte

@secretbaboy, you IDIOT!!!!!!!!!!!!! there's nothing cool if our teacher gives us assignment!

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