Mid Morning Experiment: Speed of Light
By Mark Steer
What to do

Don't leave your chocolate bar in the microwave for too long. We did. It exploded.
 On a plate, heat a chocolate bar on full power until it starts to melt in two or three places – this will probably only take about 20 seconds.
 Remove the chocolate from the microwave and measure the distance between the melted spots.
 Assume your microwave is a standard one which has a frequency of 2.45 gigahertz (you could check this in the back of the manual if you know where it is).
 Multiply the distance between the spots on the chocolate bar by 2 and then by 2,450,000,000 (which 2.45 gigahertz expressed as hertz).
 Fall off your chair as your result is astoundingly close to 299,792,458, metres per second  the speed of light. (Remember that if you’ve measured the distance in cm then you’ll have to divide your answer by 100).
What happens

Working out the wavelength
Because the chocolate bar stays still in the microwave, the waves keep hitting it in the same places  the bits which get hot and melt. The distance between the melted spots is the distance of half a wavelength since the microwave passes through the bar both on its way up and way down. Multiplying the distance by two gives us the entire length of one wave.
Why the frequency is important
Microwaves, like light waves, are a form of electromagnetic radiation which travel at light speed. In your oven they have a frequency of 2.45 gigahertz, which means that they bounce up and down 2.45 billion times every second. We have already worked out the wavelength  distance a wave travels when it goes through one up and down cycle  therefore we can work out how far the wave would go in 2.45 billion cycles or, in other words, how far it would go in a second.

If you found that the distance between the spots was 6cm then 6 x 2 x 2,450,000,000 (2.45 billion) would give you a result of 29,400,000,000 cm per second, or 294,000,000 metres per second – a result which is blinking similar to the real speed of light that physicists spent the best part of half a century trying to measure.
Incidentally, the cooking power of microwaves was first discovered when a scientist stood too close to a microwave experiment with a chocolate bar in his pocket. It melted, the scientist was intrigued and the microwave oven was born.
Champagne < Prev  See all  Next > Flaming Candles
This experiment has been adapted from Mick O'Hare's excellent book How to Fossilise your Hamster see more at www.newscientist.com/hamster.
How about trying one of our other top tens:
Top Ten Christmas Experiments
This experiment has been adapted from Mick O'Hare's excellent book How to Fossilise your Hamster see more at www.newscientist.com/hamster.
How about trying one of our other top tens:
 Top Ten Crazy Xmas Gifts
 Top Ten Geek Holidays
 Top Ten Stupid Science Studies
 Top Ten Workrelated Ills
 Top Ten Killer Vegetables
 Top Ten Weird Drinks
 Top Ten Grim Parasites
 Top Ten Things Science Hasn't Explained
Hub image: Dan Shirley
Share this