Measurement of the Speed of Light
with a Pulsed Laser
Attempts to measure the speed of light date back at least to Galileo during the Late Renaissance.
experiment (reported in 1638), he flashed a lantern to a friend on a distant hill who responded by flashing
his own lantern back as soon as he saw the light.
Alas, the experiment failed because the time necessary
for light to travel from one hill to the other was much shorter than the time it took for each person to
perceive and respond to the flash.
But the idea of Galileo’s—to measure the time between sending and
receiving a flash of light over a known distance—is perfectly valid and indeed is the principle of this
The apparatus used here replaces the persons used by Galileo with electronic circuits, which have much
more rapid response times that can be much more precisely measured.
The flash of light is created by a
pulsed laser diode, a device very similar to a laser pointer, except this laser is switched on and off—
—at a very high rate: 1 million times per second (1 MHz).
The pulsed beam can be bounced off of
a mirror and the return pulses detected by a very responsive (“fast”) detector.
Each pulse of the laser
creates two signals, one from the laser itself (the “trigger”) and one from the detector when it picks up the
These two signals can be compared to each other with a fast oscilloscope, and the time
between them can be measured as a function of the distance between the laser/detector apparatus and the
By varying the total distance from the laser to the detector
and monitoring the time difference
on the scope, one can determine the speed of light
If the time difference between the two signals
depended only on the time it took for the light to travel that distance, then
would be given by
Now, if you have studied much physics, you may be bothered by one fact: by common assent, since 1983
the speed of light (in vacuum) is
to be 299,792,458 m/s.
Thus, an experimental measurement of
it seems out of line; the unit of distance, the meter, is taken to be 1/299,792,458 of the distance that light
travels in exactly 1 second.
So, how can we say we are honestly
the speed of light when we
use the meter to express the distance traveled by light in this experiment?
Indeed, what is the point of the
These are fair questions, and they raise fundamental issues that are important in every experiment. First,
any experimental design must confront the question of
how does one know that the experiment
actually makes a good measurement?
One of the most common ways to check is to measure a quantity
that is already known, for example, a spectroscopist might check the hydrogen or mercury spectrum with
her instrument to see that it gives the expected spectral lines before turning her attention to an unknown
Second, every real measurement has some uncertainty: there are limits to how well any
measurement can be made.