Measure the Speed of Light using a Chocolate Bar

Step 1. check the Shopping List:

This is a very short shopping list for a super-cool activity that is used in college-level physics labs!

• A large 1 lb. bar of chocolate (I use Hershey's, but any kind should do)
• A ruler, pencil and paper
• A microwave oven and a plate

---

Step 2. Do the activity yourself:

Measure the Speed of Light using a Chocolate Bar

When you warm up leftovers, have you ever wondered why the microwave heats the food and not the plate? (Well, some plates, anyway.) It has to do with the way microwave ovens work.
Microwave ovens use dielectric heating (or high frequency heating) to heat your food. Basically, the microwave oven shoots light beams that are tuned to excite the water molecule. Foods that contain water will step up a notch in energy levels as heat. (The microwave radiation can also excite other polarized molecules in addition to the water molecule, which is why some plates also get hot.)

One of the biggest challenges with measuring the speed of light is light travels really fast… too fast to watch with our eyeballs.  So instead, we're going to watch the effects of microwave light and base our measurements on the effects the light has on different kinds of food.  What's really cool about this experiment that you can see the size of the wave for yourself by measuring the burn marks in the chocolate. Microwaves use light with a wavelength of 0.01 to 10 cm (that's the size of the wave itself).

Key Concepts

Energy can take one of two forms: matter and light (called electromagnetic radiation). Matter is what stuff is made from, like a chair or a table, and we'll talk a lot more about matter when we get to chemistry.
Light is energy that can travel through space and through some kinds of matter, like glass. Another word for light is “electromagnetic radiation”. Light can have high energy, lower energy, or anything in between… kind of like high energy kids (the ones who race all over the playground), lower energy kids (the ones reading a book in a corner), and kids whose energy is somewhere in the middle.
Scientists usually refer to the light energy you can see with your eyes as “visible light”, or just “light”, and it has middle-of-the-road amounts of energy – not high, not low. Just average. That kind of electromagnetic radiation is called “light”.

Lower energy electromagnetic radiation can have wavelengths longer than a football field, and those are called “radio waves”. These aren't the kind of waves that a guitar string makes when you pluck it. Radio waves are not sound waves. They are waves made out of electricity and magnetism (which we'll discuss later) that travel through space. Sound waves need something, like air, in order to travel because it does it by vibrating molecules. Electromagnetic waves work differently, but it's a little more complicated than we're going to discuss now, so just remember that light waves are different than sound waves. If you've ever seen a lightning storm, you know this is true, because you see the lightning way before you hear the thunder. Which wave do you think travels faster? Light or sound? Other examples of lower energy waves are the kind found in your microwave oven called “microwaves” (surprised?) Your TV remote uses infra-red electromagnetic radiation, which has a little more energy than microwaves. What about high energy waves? If you've ever been curious about why the dentist puts a heavy lead apron on you before x-raying your teeth, it's because they're about to use high-energy electromagnetic radiation called “x-rays” to see through your mouth tissues to get to the bones and teeth. Since high-energy rays can destroy living tissue, you have to wear that apron. Lead stops most high-energy electromagnetic radiation in the x-ray range. Black holes, supernovae, and quasars in the deep reaches of space emit deadly x-rays and even higher-energy gamma rays.

Experiment 

Video  http://www.superchargedscience.com/lnc813-16.htm

Materials:

• chocolate bar (extra-large bars work best)
• microwave
• plate
• ruler
• calculator
• pencil and paper

 

1. First, you'll need to find the ‘hot spots' in your microwave. 
2. Remove the turntable from your microwave and place a naked bar of chocolate on a plate inside the microwave. 
3. Make sure the chocolate bar is the BIG size – you'll need at least 7 inches of chocolate for this to work.
4. Turn the microwave on and wait a few minutes until you see small parts of the chocolate bar start to bubble up, and then quickly open the door (it will start to smoke if you leave it in too long). 
5. Look carefully at the chocolate bar without touching the surface… you are looking for TWO hotspots, not just one – they will look like small volcano eruptions on the surface of the bar.  If you don't have two, grab a fresh plate (you can reuse the chocolate bar) and try again, changing the location of the place inside the microwave. 
6. You're looking for the place where the microwave light hits the chocolate bar in two spots so you can measure the distance between the spots. Those places are the places where the microwave light wave hits the chocolate.
7. Open up the door or look on the back of your microwave for the technical specifications.  You're looking for a frequency in the 2,000-3,000 MHz range, usually about 2450 MHz. 
8. Write this number down on a sheet of paper – this tells you the microwave radiation frequency that the oven produces, and will be used for calculating the speed of light. (Be sure to run your experiment a few times before taking actual data, to be sure you've got everything running smoothly.  Have someone snap a photo of you getting ready to test, just for fun!)

Going further: You can experiment with other easy-to-melt foods, like cheese, buttered bread, chocolate chips, peanut butter, or marshmallows! Just pop in the first food type on a plate (without the turntable!) into the best spot in the microwave, and turn it on.  Remove when both hotspots form, and being careful not to touch the surface of the food, measure the center-to-center distance using your ruler in centimeters.
TIP: If you're using mini-marshmallows or chocolate chips (or other smaller foods), you'll need to spread them out in an even layer on your plate so you don't miss a spot that could be your hotspot!

How to Calculate the Speed of Light from your Data

Note that when you measure the distance between the hotspots, you are only measuring the peak-to-peak distance of the wave, which means you're only measuring half of the wave.  We'll multiply this number by two to get the actual length of the wave (wavelength).  If you're using centimeters, you'll also need to convert those to meters by dividing by 100.
So, if you measure 6.2 cm between your hotspots, and you want to calculate the speed of light and compare to the published value which is in meters per second, here's what you do:

2,450 MHz is really 2,450,000,000 Hz or 2,450,000,000 cycles per 1 second

Find the length of the wave (in cm):

2 * 6.2 cm = (12.4 cm) /(100 cm/m) = 0.124 meters

Multiply the wavelength by the microwave oven frequency:

0.124 m * 2,450,000,000 Hz = 303,800,000 m/s

The real (published) value for light speed is 299,792,458 m/s = 186,000 miles/second = 671,000,000 mph. How did you do?

Questions to Ask

1. What would happen if you used cheese instead of chocolate?
   
2. Does it matter where in the microwave the chocolate is located? Does placement of the chocolate affect the wavelength?
   
3. Can you explain what the burn marks on the chocolate bar are from?