More About Visualizing Electromagnetic Waves
You mean the wave we just saw when the charge on the spring vibrated isn't the whole story?
But that looks crazy! Why does the wave get bigger farther away from the
So I can't tell how big the force is by looking at the line of force. How do I know the
direction of the force on a test charge?
So how do we know how big the force is?
Well, think of the orbits experiment. When the force button is turned
on you can see how big the force is as you put the electron in various places near the proton. We
used some of the basic laws of electromagnetism to calculate the strength and direction of the
force on a (test) electron that you put down at rest. Arrows are used to indicate the strength as
well as the direction of the force. A long arrow means a big force, while a short arrow means a
So how about the wave we saw on a wiggling line of force from a vibrating charge? What
do the force arrows look like along that wave?
So the force the wave would exert on a test charge really gets smaller farther away from
the vibrating charge?
That's right, Alex, and far enough away we can use the arrows on the ray to visualize the
wave and forget about the lines of force, as in the picture below. The arrows show the
strength and the direction of the force a test charge would feel if it were put at
different points along the ray at a given instant of time. Notice that the forces are all
perpendicular to the ray!
I've seen that picture of an electromagnetic wave before, but with a curvy line drawn
through the arrow tips. Like this --
Isn't that curvy line a line of force?
No, it isn't a line of force at all. It's just a curve through the arrow tips -- a
different way of visualizing the wave. Sometimes the arrows are even left out completely.
Why is that a good way to visualize an electromagnetic wave?
Because then the picture becomes much more like a wave on a rope when you shake one end
and the other is held fixed.
even though they have the same wavelength. The second wave is more intense or, in the case of
light waves, "brighter".