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How to Measure The Speed of Light!

April 7, 2017

 

It's true! With a little bit of maths and a microwave, you can measure the speed of light in your own kitchen!

 

 

 

 

How?  Well, all you need is...

  • A Microwave

  • Chocolate to big 250g bars

  • A plate

  • A bowl

STEP ONE

 

Set the microwave up for the experiment by taking out the turn table and placing a bowl over the centre pillar of the microwave so that it will prevent your plate, that you will place on top of it, from rotating.

 

IT IS REALLY IMPORTANT THAT THE PLATE DOESN'T TURN WHEN DOING THIS EXPERIMENT!

 

STEP TWO

 

Set the chocolate side by side, on your plate so they're in a neat square.

 

STEP THREE

 

Place the plate in the microwave on top of your bowl the put the microwave on FULL power for 20 seconds.

 

When it comes out you should notice the chocolate is patchy with melted bits! 

 

A microwave produces a chain of waves on one side of the machine which reflects off the other side of the microwave then returns back to where it came over lapping with the original wave. 

 

In the cold patches (the area where the chocolate didn't melt) two waves have met in a way that cancels each other out. 

 

It is the places between these cold and hot spots that we can measure to calculate how long each wave length.

 

STEP FOUR

 

Measure the distance of two of the melted patches.  It should be about 12cm if you've done it right.

 

Now for the maths!

 

STEP FIVE

 

Find out the frequency of your microwave.  This is the number of waves it produces a second and should be on a sticker somewhere on your microwave - ours was on the back. Our microwave was around 2450 Megahertz (2.45 Gigahertz) which, from my research, is about the average. 

 

STEP SIX
 

The measurement you made using your ruler, multiply that by two!  Remember this number or even better write it down.

 

You need to double it to fine the length of the microwave because the distance between the peak and a trough is always half of a wave length.

 

STEP SEVEN

 

Multiply the wave length you calculated in the last step by the frequency of your microwave.

 

STEP EIGHT

 

Now you need to be really carefully with the units in the next calculation.  The speed of the wave is calculated by multiplying the wavelength (the distance one whole wavelength travels) by the frequency (the number of wavelengths made in a second): 

 

If you are working in megahertz you must multiply the number you calculated in STEP SEVEN by 1,000,000

 

If you are working in gigahertz you must multiply the number you calculated in STEP SEVEN by 1,000,000,000

 

THIS IS THE FINAL STEP.  You have measured the speed of light in cm per second!
 

AWESOME ISN'T IT!

 

You can go further and divide it by 100 (knock two 0's off) to find the speed in metres per second.  It should work out somewhere round 300 million metres a second.

 

Go further and divide it by 1000 and you'll have Kilometres per second which will be around 300,000 km/per sec.

 

 

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