EM wave tutorial
Topic/Concept: Relationship between wavelength, frequency, and speed for electromagnetic waves
Type of Activity: Short proof/thought experiment
Prerequisite knowledge required: Need to know what an electromagnetic wave is.
Resources required: Paper, writing instruments
Learning Objectives: Reinforce the relationship between wavelength, frequency, and speed for EM waves. Increase mathematical intuition.
Common misconceptions: People have trouble remembering the relationship between wavelength and frequency for light.
Detailed description of activity:
I had them derive the equation (wavelength) x (frequency) = (speed of light) using only the concept of an electromagnetic wave and velocity = distance / time. I spent the first 5- 10 minutes of class explaining what the term electromagnetic wave meant. The derivation can be done in 5 or so lines if done efficiently. To start out, I avoided using variables and presented them with an EM wave with wavelength 1 m. I gave them the speed of light and them asked them how long it would take for this wave to pass by an electron (or whatever you think would be clearest). I broke them into groups to answer this question, forcing each group to fully agree and understand an answer before I would admit that they were right. I also had little hesitation, however, about giving people ideas to get them thinking in the right direction since time is short. I then asked each group how many “humps”/waves would go past the electron in 1 sec. After they got this, I asked them to answer the exact same question using the exact same method but instead use the symbol lambda for the wavelength instead of 1 m and c instead of the actual speed of light. The net result: lambda*frequency = c. I then quizzed them on what happens to frequency and speed when the wavelength gets larger. I related this wave to shaking a rope. From the equation they should be able to answer that the frequency goes down, but I tried to get them to accept this answer on physical grounds using the rope analogy. To make a long wave on a rope, you shake it less often. I then went through almost every combination of high/low frequency/wavelength to drill home the physical sense of the equation lambda*frequency=c.
Assessment: Did they get the right answer in the end?
Associated files and images: None
Comments: None
By Christer Watson (8/99)