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The speed or velocity of light is
approximately 186,000 miles per second or 300,000
kilometers per second in a vacuum. All electromagnetic
waves--which include visible light--travel at that
speed. With such an enormous speed, it has been
difficult to devise experiments to measure it. The speed
of the electromagnetic waves slows down when they pass
through matter. According to the Theory of Relativity,
the speed of light is the fastest at which anything can
travel.
Questions you may have include:
-
How is the speed of light measured?
-
What is this speed's relationship to
matter?
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Can things go faster than light?
Measuring the speed of light
Since the speed of light is so great, it
is very difficult to measure.
Note that the terms speed of light and
velocity of light are used. Either one is acceptable,
but you must remember that speed means how fast
something is going, while velocity means how fast it is
going in a given direction.
It was thought that the velocity of
light could be measured that same way as for the
velocity of sound. A common method to measure the
velocity of sound is to calculate the time it takes for
an echo to return and then divide that by the distance
the sound travels there and back. Since distance equals
velocity times time:
c = d / t
where
But the velocity of light is so large at
186,000 miles/sec (300,000 km/sec), that in 1/1000 of a
second, the light would travel from Milwaukee to Chicago
and back (or from Los Angeles to San Diego and back).
That is over 90 miles (150 km) one way.
If you used a timer or shutter that
could measure in 1/100,000 of a second, it might be be
more practical.
One clever method that was one of the
first to measure the speed of light was to shine a light
through pinhole on a spinning disk. The light was then
reflected off a mirror that was some distance away.
Since light travels so fast, it would
normally be reflected back through the pinhole (provided
everything was lined up properly). By adjusting the
speed of the spinning disk and/or the distance, the
pinhole could be made to move enough that the light
would not pass through it. Then, by calculating the size
of the pinhole, the speed of the disk, and the distance
to the mirror, the speed of light could be calculated.
With modern electronics, the speed of
light can now be measured in a physics lab.
One example to measure the speed of red
light is to use equipment that includes a LED
(light-emitting diode) that emits a regular series of
pulses of red light that are only 20 nanoseconds in
duration.
(A nanosecond is one-billionth of a
second or 1/1,000,000,000 second. It is also written as
10-9 seconds, 10^-9 or 1e-9, where the -9 indicates the
number of zeros in the denominator.)
That means the pulse of light blinks on
for 20 billionths of a second or 20/1,000,000,000 second
and blinks 40,000 times per second. Having such a short
pulse allows the scientist to measure the difference in
time it takes to travel two different paths. If the
duration was longer, the distance traveled would have to
be longer.
By splitting the light beam with a
half-silvered mirror, one beam travels to a mirror 10
meters away and then back to a photodiode detector. The
other beam is reflected off a mirror only a few
centimeters away. The time difference for the two beams
is about 67 nanoseconds, which can be displayed on a
regular dual beam oscilloscope.
The total distance the light travels is
20 meters, which equals 0.02 kilometer (20/1000).
The speed of light is then:
c = 0.02 kilometers / 67*10-9 seconds =
298,500 kilometers per second
This is a fairly accurate reading and
pretty close to the actual speed of 299,792 km/s.
Speed through matter
The speed of electromagnetic waves
passing through transparent matter is slower than it is
in a vacuum. Glass is transparent to visible light,
radio waves will easily pass through non-metals, and
x-rays pass through most materials except lead.
Most measurements of the speed of light
are made in the atmosphere. Since the effect on the
speed when passing through air is so very small, the
speed of light in air is almost the same as it is in a
vacuum. The difference is negligible.
The reason electromagnetic waves travel
slower though transparent materials is the effect that
the electrons have on the waves. They act somewhat like
a "friction" on the waves.
The fact that light moves slower through
matter can be seen when visible light passes through
glass. If you shine a light at an angle through a piece
of glass, the light beam will be bent or refracted.
The ratio of the speed of light in
vacuum divided by the speed of light in the material is
called the index of refraction for the material. The
index of refraction of glass or other material indicates
how much slower the light travels through the material
than in a vacuum.
Typically, the index of refraction of
glass is from 1.2 to 1.5. That means the speed in a
material of index 1.5 is 66% of the speed in vacuum.
Although the speed of an electromagnetic
wave through matter is can be up to 50% less than the
speed through a vacuum, scientists were able to greatly
reduce the speed through matter in special situations.
This was first done in 1999.
Danish physicists performed an
experiment where they slowed light down to only 38 miles
per hour or about 57 kilometers per hour. They did this
by sending a beam through a material made of sodium
atoms cooled to near absolute zero (-273�C or -460�F).
They achieved this low temperature by using lasers to
slow down the atoms, through a special method used in
quantum mechanics called the Bose-Einstein condensate.
(Explanation of this goes away beyond the scope of this
course).
Speed is the maximum
The speed of light is supposed to be the
maximum speed at which matter can travel.
In fact, according to Einstein's Theory
of Relativity, strange things happen to matter as it
approaches the speed of light. Matter becomes compressed
as it gets within 90% of the speed of light, such that a
ruler would appear shortened. Also, the mass of the
matter starts to increase.
Another interesting phenomenon happens
when matter approaches the speed of light, and that is
that time for the matter slows down. In other words, if
you were traveling through space near the speed of
light, you would age more slowly than a person on Earth.
A year year trip might seem like 10 minutes to you,
while it would seem like a full year to everyone else.
Time travel or going at "warp speed" as
is seen in such TV shows as Star Trek is not physically
possible, as far as we know.
All electromagnetic waves, including
visible light, travel at 186,000 mi/sec or 300,000
km/sec in vacuum. The speed of light slows down as it
passes through matter. The speed of light is the maximum
speed for matter. |
