Wave equation c = l*f

Procedure

Opener

Introductory Question: How does a microwave work?
It makes water molecules vibrate faster. We did thermodynamics before this section, so they already knew that faster vibrations meant more heat. All food has some amount of water in it. The real reason it works is that the ovens are tuned to emit a very particular frequency which is a resonance frequency of vibration for the water molecule. Microwaves are just light.
 

Development

Lecture: c = l*f
All waves obey the above equation (replace c with wave speed if it's not light). l is the wavelength, or distance betweeen two crests or troughs and f is the frequency or number of time the wave goes up and down per second.
Because our students haven't mastered algebra yet, we teach the triangle method for three symbol equations:
     c
    l    f
Simply cover up the quantity of interest. For example, l = c/f.

In the microwave, a standing wave is set up, which is a wave where the nodes are stationary (they should already know this vocabulary). The chips heat up and melt at the anti-nodes.

Problems:
1) What is the wavelength of radio waves of frequency 100,000,000,000 Hz?
    ->3e8/1e11 = 3e-3m = 0.3 cm.
2) Visible light has a wavelength of about 0.0000005 m. What is its frequency?
    ->3e8/5e-7 = 7e14 Hz

Lab: Give the students a paper plate covered in chocolate chips. Chocolate chips are good because they won't spread the heat too far, which is the problem with using sold food. The turntable has to be removed because otherwise the standing waves will touch different parts of the food as it rotates, and the experiment won't work. Make sure to only let the kids microwave the chips until they are just beginning to melt.

The oven will set up standing waves and the chocolate chips will melt at the anti-nodes. The wavelength is twice the distance between melted spots, and should be about 12cm. The frequency is on the back of the microwave usually, and is 2.5e9 Hz.

Closure

That's basically it, they measure the distance between melted spots and double it to find the wavelength of the microwaves. The frequency is 2.5e9 Hz, so they should get 3e8 m/s for c. They probably will only get within a factor of two, but that's fine.
 

The big thing here is that they can see where the waves are.