A Peep into the Speed of Light


In the week after Easter, we had a lot of old Peeps lying around. No one seemed that interested in eating them, so we used them to measure the speed of light. As you do. (NPR)

What a great science experiment! For other cool science experiments, check out the collection on the Nat Geo Kids YouTube channel.

Teachers, scroll down for a quick list of key resources, from marshmallows to microwaves.

Discussion Ideas
The speed of light is 299,792,458 meters per second, or 670,616,629 miles per hour. According to polymath mathematician John von Neumann, “In mathematics, you don’t understand things. You just get used to them.” Get used to that number!




On to the experiment! We did this experiment in our office, and the text in red reports our results.


  • Like all waves (of electromagnetism, water, or anything else), microwaves are defined by their frequency and wavelength. How do you determine the frequency of the radiation in your microwave oven?
    • You look at the manufacturer’s sticker! Look for the abbreviations MHz or GHz. (The hertz is the unit of measurement for frequency. A megahertz is a million hertz. A gigahertz is a billion hertz.) You might need to read the small print, or find a coworker/student with good eyes to help.
      • We have two microwaves in the Nat Geo Education office, and had to look on the back of one and the inside panel of the other to find the frequency info. Like most modern microwaves, they have the same frequency as the microwave in the video—2,450 MHz, or 2.45GHz.
      • So, 2,450,000,000. Remember this number—it will be important later!


  • How do you determine the wavelengths of the radiation in your microwave? (This is the fun part!)
    • 1. Take a plate full of Peeps or anything else that melts—chocolate chips, chocolate bars, cheese? (Some people recommend egg whites, but we like the idea of c standing for chocolate as well as the speed of light!)
    • 2. Heat up your Peeps on a flat plate the microwave on fairly low heat. Don’t heat it too much—you want it to warm unevenly.
    • 3. Take out the plate and measure the distance between hot/gooey areas. The average distance between these areas is about half a wavelength.
    • 4. Do the math! Find out the wavelength by multiplying the average distance by two.
      • We used chocolate chips, and the average distance between melted chips was about 6.1 centimeters. So, the wavelength was about 12.2 centimeters.


  • OK, now calculate the speed of light! Just multiply your frequency (2,450,000,000) and your wavelength. You should get pretty close to the speed of light in inches per second or centimeters per second.
    • Use this site to convert inches-per-second to miles-per-hour.
    • Multiply your answer by .01 to convert centimeters-per-second to meters-per-second.
      • It works! Using data from our own chocolate-chip experiment:
        • 12.2(2,450,000,000)=29,890,000,000. That’s centimeters-per-second.
        • 29,890,000,000(.01)=298,900,000. That’s meters-per-second. That’s pretty darn close to the constant 299,792,458!!!!!


TEACHERS’ TOOLKIT (for reference)

NPR: Kitchen Science: We Used Peeps To Calculate The Speed Of Light

Nat Geo: Nat Geo Kids Intro to Cool Science Experiments

LiveScience: Why The Speed of Light Matters


TEACHERS’ TOOLKIT (for the experiment)

  • chocolate, egg whites, cheese, marshmallows or something else that will melt easily and unevenly
  • flat-bottomed, microwave-safe plate
  • oven mitts
  • microwave oven
  • ruler
  • calculator or basic math skills

2 thoughts on “A Peep into the Speed of Light

  1. Speed of Light was determined by measuring the speed of a Laser beam. Laser beam gets off from a two-dimensional flat surface of a narrow cylindrical tube, but light of celestial sources like the Sun, emanates from three-dimensional spherical surface. Moreover, Laser light travels in the form of a beam – retaining the same intensity; whereas, light emanated from the spherical Sun expands – concurrently decreasing the intensity. There are much more dissimilarity between a Laser light and the Sunlight. Therefore, the speed of light cannot be decided by measuring the speed of a laser beam. In reality, Light of celestial sources contacts an object at an unimaginable speed.

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