Measure the speed of light using marshmallows

This lesson is developed by Conor Coyle. Originally from Monaghan Conor moved to Dublin for university where he studied Applied Physics for four years. After this he spent a year in Chongqing China teaching, my itchy feet didn’t stop there, after China Conor moved to France where he worked as a waiter and studied French part-time. He then returned to Dublin where he is now a final year postgraduate researcher working on low temperature plasma physics for biomedical applications. Conor likes cooking and make a mean chilli con carne. (which he hasn’t made for me yet! Naoisé)

Items needed for this experiment : Marshmallows, Plate, Microwave, ruler, marker, brain engaged

What you need to do:

  1. Cover the plate in marshmallows, remove the turntable from the microwave and cook for 45-60 seconds.
  2. Take the marshmallows out and note the melted spots on the marshmallows. Using ruler measure the distance between the spots.They should find that the distance between spots repeats over and over. This distance will correspond to half the wavelength of the microwave, about 6 cm

Question for students: why did only spots melt not the whole thing?
Answer: A standing wave forms in the microwave, the nodes are the hot points and it is at these points that strong heating effects are felt

What is the purpose of the turntable in a microwave?

As heating only happens at the nodes, the turntable allows an even heating through out all the food

How would using larger or smaller marshmallows effect results?
Smaller marshmallows would allow you to locate more precisely where the nodes are forming, thus smaller marshmallows lead to more accurate result, larger marshmallows more inaccurate result.

Would a microwave oven work in a vacuum?
Yes, microwaves are part of the electromagnetic spectrum and do not need a medium to travel through, unlike sound waves that will not work in a vacuum. In space no one can hear you scream, is a very accurate statement as sound will not travel in space.


Note from Naoisé: I like this lesson because It gives us a practical way to look at what can be a very boring and aloof topic! Take a look at the two pictures attached to this post .. which interests you more?! You don’t need to have hi-tech equipment either, we only have a basic small kitchen in the school. Again you always remember what you do!