Astronomer studying star formation, like myself, use telescopes that can see though the pretty optical exteriors of nebulae into the interiors, where very cold dust radiates in the submillimetre and microwave regimes.
Microwaves, fall on the electromagnetic spectrum, between radio waves and infrared waves. They are usually around the size of a few centimetres and you may well be very familiar with them as they are produced by the microwave oven that might just be sitting in your kitchen.
Microwave ovens use a particular microwave frequency to excite molecules of water. Since water is present in lots of food and drink, this means that microwaves heat up lots of useful stuff – and they do it quickly. The fact that microwaves are now readily available to most of us in the western world and they are only a few centimetres in length, means that you can measure the speed of light in your very own home.
What You Need:
The quickest and tastiest way to perform this little experiment is with marshmallows, but chocolate chips also work. You’ll obviously need a microwave oven as well, and a large, microwaveable dish. You will need a ruler, too.
What to Do:
Get your large, microwaveable dish and place a layer of marshmallows at the bottom of it.Remove the turntable from the bottom of the microwave oven. If you don’t, then this experiment will not work at all. If your microwave doesn’t have a turntable, it means that the turning mechanism is elsewhere and you’ll need to find a regular microwave oven to try this experiment.
Cook the marshmallows on a low heat for a couple of minutes, or until you see parts of the marshmallows starting to bubble. When you do, remove the dish and take a look at the marshmallows.
You ought to see that they have not melted evenly. In fact you should be able to see a regular pattern has formed, drawn out in melted-mallow. It depends on your microwave oven, but you should see a melted/unmelted pattern across the dish in some direction. When I tried it at home, my oven created long melted strips next to long unmelted strips (see above).
This regularity is caused by the same mechanism that heats up the food you place into your microwave oven. The appliance generates microwaves which very quickly form standing waves (see animation above) inside the cavity inside, where you put food. As the food rotates around, it passes through the standing wave nodes and this excites the water molecules, heating the food.
Measure the Microwaves:
Take your ruler and measure the distance between the melted parts of the marshmallows. You should find that there is an even pattern of melting and that the distance between them is something like 5 or 6cm. Why? Because that is the distance between the nodes of the standing waves.
Without the rotating mechanism, the food does not move around and cook evenly, instead it just heats at the nodal points. Using your marshmallows you have created a ‘map’ of the microwaves in your microwave oven!
Find the Frequency:
Finally you need to know the frequency at which your microwave oven operates. It is usually written on the back somewhere in small writing. Most standard microwave ovens operate at 2450 MHz. If you cannot find the value on the back of the oven, you can take it for granted that 2450 MHz is about correct.
Measure the Speed of Light:
Now you have what you need to measure the speed of light. You just need to know a very fundamental equation of physics:
Speed of a Wave (c) = Frequency (f) x Wavelength (L)
The distance between the melted sections of the marshmallow is in fact L/2, because there are two nodes for each wave (see animation). So if you have measured 6cm and your oven operates at 2450 MHz, then your measured speed of light is (0.12 x 2450,000,000) 294,000,000 metres per second.
The agreed value of the speed of light through a vacuum is 299,792,458 metres per second. See how accurately you can measure it? what could you do to make the experiment better, and thus get a closer answer?
Now You Can Eat the Gooey Melted Marshmallows:
…and make yourself sick. Yay!
Brilliant! Cubs and Scouts will love this.
http://mangsbatpage.433rd.com/2008/05/yummy-science-experiment-eating-speed.html
Super! Goodness gracious! If only we could clone folks like you can make them all science teachers!!!
Technically, you are not measuring the speed of light, but the wavelength. You then CALCULATE the speed of light. But still cool, and full of marshmallow goodness!!!
Did you have too much time on your hands while doing this expirement? If so, we need a lot of people with your talent of “What ifs” to be bored.
Of course, I always knew how clever you are. Please will you do the marshmallows thing next time you come to stay.
But will it work with the new Apple Microwave ovens?
http://technoracle.blogspot.com/2009/04/i-can-finally-talk-about-my-apple.html
Just have found your site (searching for people with the words PhD and Astronomy on Twitter), and I have to say, it’s amazing!
Hi, I tried this too with my microwave, but my result are nowhere as clear as yours. I am starting to think my microwave has a stirrer device hidden somewhere in addition to the turntable. Since I am writing an article for a Dutch website I was wondering whether I could use your images. Please contact me by e-mail…
Hi! I did your microwave experiment with a group of first graders as part of a program i did for IYA. They loved it AND i can say they seemed to grasp the concept well. One of the 6year olds ask me if this would work with chocolate colored marshmallows? Thanks again! Gregg
As interesting as this experiment is, unfortunately it does not prove anything about the speed of light.
That frequency you see printed on the back of a microwave? The manufacturers don’t measure it directly. Instead, they measure the wavelength of the antenna they built inside their microwave, and take the published value of the speed of light and work that equation the other way to derive the frequency.
So when you take the same measurement, and use that printed value that was actually derived from the “unknown” variable, and plug it back into the same equation, of course you wind up with the same “unknown” variable the manufacturer started with. You’ve learned nothing new, sorry.
Realist: you have missed the point! This is science activity for your kitchen that reasonably accurately results in you determining the speed of light using a microwave. The frequency on the back is pretty well defined and the result is usually fairly accurate to all intents and purposes.
I’m afraid you are re-enforcing the stereotype of scientists lacking a sense of humour. It uses marshmallows for goodness sake!
This experiment is a pretty good experiment. I will try and enjoy it during Science festivities in France.
Thank you !
>> Realist, what you say is definitively wrong, this experiment prove the reality of the speed of… photon not light at 2.45GHz. Measurement of the frequency of a magnetron is done with electronic system (antenna) not with any information about speed of light! sorry for you…
In fact this experiment proves : speed of photon and standing waves in a cavity. Very very good!
Very helpful, I used this for an assignment in science. Although with chocolate melt I had to use the melts that tops melted into them. Thanks a heap.
Really glad I found your site…if you have more wonderful experiments such as this to share please email me. This is wonderful stuff for our home school classroom:-) Thanks so much, from Beautiful North Idaho!
Great!!
I have a group of students that will definetly benefit!
Thank you
hi, you have done a wonderful work indeed. i will try to measure the speed of light with this method thank u very much.
i did this project but i still dint understand how thats the speed of light
Nice project, but It is hard to calculate exact speed of light from this experiment, cause wavelength (phase speed of electromagnetic field) inside cavity or waveguide can very be different from speed in open space (usually measured wavelength inside of cavity will be shorter)
BRILLIANT marshmallows!
DUUUUUUDE!!! They should give you your own TV show. I would watch it… all-the-time.
Hey thanks will use on my year 11 physics class should save me writing my own experiments and the school will pay for my lunch 2.
Great!
cool good work want more my class will enjoy
This is awesome! I have boxes and bags of all kinds of food stuff that I didn’t know what to do with.
Hello Microwave!