Finding the speed of light with peeps

What do you get when you combine a microwave and a pan full of those pastel colored marshmallows known as peeps? NPR’s science reporter, Adam Cole demonstrates in this segment the squishy mess can actually help you calculate the speed of light.


What do you get when you combine a microwave and a pan full of those pastel-colored marshmallows known as Peeps?

As NPR science reporter Adam Cole demonstrates in this segment, the squishy mess can actually help you calculate the speed of light.

Let's take a look.

It's the week after Easter, and that means homes all across America are overflowing with stale Peeps.

Now, you could either eat those Peeps, or you could use them to find the speed of light.

♪ Finding the speed of light with Peeps ♪

The speed of light is one of those numbers that people have chased for centuries.

Galileo wanted to know how fast light moves...

Buonasera! he proposed an experiment.

[ Speaks Italian ]

He would uncover a lantern, and when the light reached his assistant, the assistant would quickly uncover his own lantern.

Using a water clock, Galileo would keep track of the time between uncovering his light and seeing his assistant's light.

Up close, there was no noticeable delay except for the split second it took his assistant to react.

Then, they started moving away from each other.

Galileo figured the delay would get longer and longer because the light would need more time to travel the greater distance.

But even when they were a mile apart, there wasn't a noticeable pause.

[ Speaks Italian ]

He couldn't have guessed that light had made that trip in 1/100,000 of a second.

Centuries later, in 1849, a French scientist whose name I can't pronounce...

Armand Hippolyte Louis Fizeau.

...came up with an even better way to measure the speed of light, which was pretty impressive, considering that people weren't even using light bulbs yet.

He took a toothed wheel and spun it faster and faster.

He shot a beam of light between the teeth and reflected that beam of light off a mirror 5 miles away.

By the time the light got back, a tiny fraction of a second later, the teeth had moved over just enough to block the light.

No light made it back through.

But if he made the wheel spin a little faster, the teeth would move over one whole position, and the light could get through.

So when Fizeau saw the light, he knew the wheel was spinning so fast that a single tooth was moving over one place in the time it took light to travel 10 miles.

With some quick math...

Armand Hippolyte Louis Fizeau.

...determined that the speed of light was 700 million miles per hour, which we now know was only 5% off the actual value.

Today, I can find the speed of light in my kitchen, and that brings us back to Peeps.

♪ Finding the speed of light with Peeps ♪

And a microwave.

♪ And a microwave

And just a little bit of physics.

♪ And physics ♪ Just a little bit

Microwaves travel at the speed of light.

And like all waves, their speed is determined by how fast they go up and down -- that's the frequency -- and their wavelength.

Frequency is written on the side of my microwave, right here -- 2,450 megahertz.

So we've got the frequency.

Now, to calculate the speed, we just need to know the wavelength.

And we can find the wavelength by microwaving Peeps.

I'll set it on low so the Peeps don't just explode.

And let it run for a minute or two.

Inside, microwaves are bouncing around.

In some spots, there isn't much energy at all, and the Peeps in those areas stay cold.

In other spots, there's lots of energy, and the Peeps get really hot.

Those hot spots are half a wavelength apart.

So, in theory, if I measure the distance between melted Peeps, I can find the wavelength.

And sure enough, some of the Peeps are really gooey while others haven't melted at all.

Poking around, I found a rough location for a handful of hot spots and measured the distance between them.

Those average out to 2.43 inches.

So that's half a wavelength.

Multiply by 2, and we get a wavelength of 4.86 inches.

So, now, to find the speed of the microwaves, which, remember, is the speed of light, I just have to do the math.

Frequency times wavelength -- 2,450 megahertz times 4.86 inches -- gives us about 12 billion inches per second, or 676 million miles per hour.

And that's pretty close to the actual speed of light.

[ Ding! ]

♪ We just found the speed of light with Peeps ♪

Joining me now is Adam Cole, the creator and host of NPR's science YouTube channel, Skunk Bear.

How many different ways are there of measuring the speed of light?

Well, there's been a lot throughout history.

As soon as people figured out that light was moving, which was, for a long time, mysterious, they wanted to know if it traveled instantaneously from one point to another or if it actually had a speed.

And so there's been a lot of contraptions created since, basically, the 1600s to try and experimentally determine this.


People used rotating mirrors and toothed wheels, like we saw in the video.

People looked at the ticking clock that is the moons of Jupiter eclipsing to try and have a measurement of time and light passing between that planet and ours.


So there's been a lot of different methods and some that were more theoretical, using the constants of physics to try and figure out the speed of light.

So, this Peeps method --

It's probably the most serious of them all and the most precise.

That's right.

We're talking about wavelength and frequency.

What's the difference?

The difference between wavelength and frequency?

Well, wavelength is the distance between -- if you think about a wave in the ocean -- the distance between two troughs.

When a wave goes up and down, the distance from this low and this low is the wavelength.

And the frequency is how fast that wave goes up and down.

So you can think of it as the speed of a person walking is determined by... the length of their step and the number of steps they take per second.

And you multiply those together, you get the speed of them walking.

Multiply a wavelength and frequency, you get the speed of a wave.

Okay, so, this is an experiment that you would recommend that people try at home.

I would.

I hope that they do.

Were the marshmallows delicious after?

You know, I think I didn't have a lot of love for Peeps before this experiment, and after, I definitely don't.

All right.

Adam Cole from the YouTube channel for NPR, Skunk Bear.

Thanks so much.

Thank you.