How can you use a plate of shredded cheese to determine the wavelength of the standing wave generated in a microwave oven?
Sprinkle the cheese evenly over a large plate and put it in the microwave and start heating it. Before the cheese fully melts, take the plate out and you should see patches of melted portions and patches of unmelted portions, evenly spaced over the plate. Why is this? The microwaves generated by the oven are standing waves, which means they oscillate so that there are, permanently, nodal positions where there is no intensity between points of maximum or minimum intensity like so
These points are evenly spaced apart since the microwave is periodic. The distance between patches of unmelted cheese is approximately the distance between nodes of the microwave. That distance corresponds to half the wavelength of the microwave, which should come out to be about 6 cm.
To see a video demonstration of this, watch https://www.youtube.com/watch?v=kp33ZprO0Ck.
standing wave GIF retreived from http://i.stack.imgur.com/bzoQv.gif