Comment by rprospero

First off, thank you for pointing this out. I occasionally teach introductory physics and I had never realized that people could interpret the graph in that manner. I'll try to make nip this in the bud when I present this material in the future.

Your alternate graph, however, shows another common misconception that is matched in your text. Specifically, the amplitude of the wave varies, but the intensity does NOT. The amplitude being graphed can be though of as the strength of the electric field pointing.... how about to the north? Then, when the amplitude is negative, all that's happening is that the field is point to the south, instead of the north. The intensity of the light is the energy contained within the electromagnetic field, which is independent of which direction the field is point. Thus, where you have dark spots in the graph where the amplitude goes negative, the intensity of the light is just as strong as it was at the peaks.

The obvious next question is what happens when the amplitude is zero? Well, the changing electric field produces a magnetic field. This magnetic field reaches its maximum value where the electric field goes to zero and goes to zero itself at the minima and maxima of the electric field's amplitude. Thus, the total energy electromagnetic energy, and, therefore, the intensity of the light, is constant across the whole graph.

That's why I'm cautious about using your proposed diagram. I've known many students with the misconception that light alternates between bright and dark as it travels and that lines up exactly with what you've drawn.