Speed of Light
Kyla, do you remember how the wavelength of the electromagnetic wave became shorter when
the charge was vibrating faster?
Right, but the wave seemed to move with the same speed whether the vibration was fast
or slow. Only the wavelength changed when the oscillations became faster.
Yes, all electromagnetic radiation  from radio waves to xrays  travel at the speed of
light. In empty space this speed is approximately 300,000 kilometers per second! We can
even predict the wavelength of an electromagnetic wave if we know the time it takes for
the charge to oscillate once, returning to its original location. This time is called the
"period", T, of the wave. By multiplying the period with the speed of light (c), we can
determine the wavelength of any wave.
Oh, I get it! One gray dot takes a time "T" to go up and down, completing one cycle of
the wave. The other dot stays on the crest of the wave and moves a distance, "d", equal to
one wavelength. But I have a lot of questions: Is the period "T" for the wave the same as
the period for the charge?
Yes.
Why is d/T equal to the speed of the wave?
Well, the gray dot that moves a distance "d" has the same speed as the wave because it is
carried along with the wave. Since the time it takes to move that distance is equal to
"T," the speed is "d/T".
That is just like a car moving 100 miles in two hours at constant speed. The car must be
traveling 100/2=50 miles per hour. But how do we know that the speed of light is always
almost 300,000 kilometers per second?
Good question. It's just a fact of nature that the speed of electromagnetic radiation
moving through empty space always has the same value.


What is meant by the "frequency" of the wave?
The frequency, "f," is the number of completed periods in one second. If the period is 1/2
second, the frequency will be two wavelengths per second (1/2 second for one wavelength,
so two wavelengths in one second). In general,
So our formula can also be written as
The speed of the wave is equal to the wavelength times the frequency.
Oh, I see; now we can figure out the wavelength of any wave if we know the frequency, and
vice versa. My favorite radio station is at 90.1 MHz, so the wavelength of those radio
waves must be
,
or
That's about 10 feet from peak to peak!
