OpenStax College Physics for AP® Courses, Chapter 24, Problem 10 (Problems & Exercises)

This is College Physics Answers with Shaun Dychko. After combing your hair, the comb will have some electrons on it due to the rubbing against your hair— that's the static charge on the comb— and if you were to move the comb up and down quickly that would be producing a change in magnetic field or... a change in electric field I should say and that could also create an induced changing magnetic field and there we have an electromagnetic wave now which is light. So assuming we want to produce a light with color red... that looks like from this figure to be about 700 nanometers so that's 700 times 10 to the minus 9 meters and the question then is with what frequency would the comb have to be moved up and down to produce this light? So the speed of light equals the product of the wavelength and the frequency and we can solve for f by dividing both sides by λ and so the frequency then is c divided by λ. So that's 2.998 times 10 to the 8 meters per second divided by 700 times 10 to the minus 9 meters which is 4 times 10 to the 14 hertz. This is 400000 billion oscillations complete oscillations up and down... like round trips per second which of course is not practical for well, a human to do but it's just interesting to see that.