AMASE: Speed of Light

Posted on Thursday, March 26th, 2020


[Guelph Physics logo. Music.]

[Cut to Joanne sitting at the kitchen bar.]

Joanne: Faster than the speed of light! Have you ever heard that expression? Well, it's at the heart of our Ask Me Anything! Science Edition question today from Mara, grade 3 student in Fergus, ON.

[Mara enters the kitchen and sits at the bar.]

Mara: Hi. The other day my mom asked me to move faster than the speed of light of course, I asked how fast does light go? Since my mom loves to do experiments, we decided to measure it ourselves.

Joanne: That's right and you can do this experiment at home in your own kitchen all you need is a microwave, a ruler and some chocolate chips.

Mara: mmmmm... chocolate chips. Let's do it!

Joanne: Alright. So light is a wave which means that if we know the frequency.

[Joanne holds up a white board with examples of frequency graphs and wave lengths.]

Mara: Which is how quickly the waves move up and down this is high frequency this is low frequency.

Joanne: That's right and if we also know the wavelength...

Mara: Which is the distance between peaks. This is short wavelength this is long wavelength.

Joanne: Right and if we have those two numbers frequency and wavelength and we multiply them together we get the speed of the wave.

Mara: Hold up just one second what does that have to do with chocolate?

Joanne: Good question, that's how we're gonna measure the wavelength. We're going to take these chocolate chips, put them in our microwave and melt them just for a few seconds until they just start to melt. When we do that and take them out, you'll see an interesting pattern and that's going to tell us the wavelength of the microwave light.

Mara: Really? Let's do this! To the MICROWAVE!

[Cut to Mara at the microwave. Beeping sound.]

[Cut back to Joanne and Mara at the kitchen bar, with the plate of chocolate.]

Joanne: Okay, so you see how we've got a pattern here you've got stripes of where it's melted it and where it's not melted happening on the plate. So, can you measure how far apart those stripes are for me please?

[Mara uses a ruler to measure the distance between the melted lines. Music.]

Joanne: What have you got?

Mara: About six centimeters.

Joanne: Okay so that's actually half the wavelength so the full wavelength would be?

Mara: 12 centimetres

Joanne: 12 centimetres. Okay so we need the frequency now which we get from the microwave either inside the door or on the back. Our microwave here is two thousand four hundred and fifty megahertz which is two thousand four hundred and fifty with six zeroes after it. So, we take that really big number and we multiply by the wavelength which is:

Mara: 12 centimetres.

Joanne: And 12 centimetres in metres is 0.12 together that gives us a speed of 2.9 times 10 to the 8th metres per second or about 300 million metres per second which is what we expect to measure for the speed of light.

Mara: That's a fancy number. And crazy, crazy fast.

Joanne: It is crazy fast so at that speed it takes only about 8 minutes for the light from the Sun to reach us all the way here on planet Earth.

Mara: HOLY PLUTO. Which makes me wonder. What if we lived on Pluto?

Joanne: Hmmm. Good question. So Pluto's a lot further away which means it would take a lot longer for the light to reach us on Pluto. It would be in fact about 4 to 6 hours to reach us on Pluto.

Mara: Cool! So, if we could turn the sun off it would be four to six hours before anyone on Pluto noticed?

Joanne: Yup.

Mara: Is anything faster than light?

Joanne: Nope! So, when I asked you to move faster than the speed of light the other day, I was actually asking you to do something physically impossible.

Mara: Hey no fair!

Joanne: I'm sorry I was just trying to get you moving. Okay one more thing you know in a thunderstorm when we see the flash of lightning a little bit before we hear the thunder?

[Cut to video of lightning. Thunder sounds.]

Joanne: Well, that's because light moves so fast so as we know it moves at 300 million metres per second, so we see the light almost instantaneously, but it takes sound longer to get to us. So, if you time in seconds how long between the flash and the boom of thunder and divide that number by three you get the distance from you to the lightning in kilometres

Mara: Kewl. Thanks for doing an experiment with me.

Joanne: It was my pleasure and thanks for the great question! If you have a question for us, just let us know!

Mara: Thanks for watching Ask Me Anything Science Edition!

[Mara takes a big handful of chocolate chips.]

Joanne:  You're still on camera.


In our first video of the series, 3M National Teaching Fellowship recipient Dr. Joanne O'Meara and junior scientist Mara explain a little bit about the speed of light and how you can test it in your very own kitchen!

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