When I turn the torch light on, to us, it
comes on instantaneously
but that's because it travels really really fast.
how fast? 300,000 kilometers per second
now that's fast enough to go around the
world seven and a half times in one second
but when did we discover this and
how did scientists have the means to
measure that speed? Well stay tuned: this
video will look at some of the history
behind measuring the speed of light.
Now prior to the 17th century there were
a number of views about the nature of
light but they did lack experimental
evidence. Some,  like Pierre Gassendi
thought that light was traveling in bits
or what he referred to as corpuscles,
whereas others such as Rene Descartes
believed that light travelled pretty
much like the waves on the ocean. There
was the idea that light traveled with a
certain speed in essence it was thought
to be instantaneous so there was no real
way to actually measure the speed of
light. So the first key experiment is the
one made by a guy by the name of Olaf
Rømer in the 1670s. Now
Olaf Rømer was a Danish scientist who
went to the observatory in Paris
to observe the eclipses of one of the moons of Jupiter called Io.
So what I've got here is a diagram that is actually from Olaf Rømer's work that shows the path
of the earth and the path of Io,  Earth as
it moves around the Sun and Io as it
moves around Jupiter and all I've done
is just that added little icons here of
the earth Jupiter and IO
to help us understand what's going on so
he was interested in the eclipse of Io
so Io moves around Jupiter like so with
a known period of around 42.45 hours
as it goes around now how was he able to
measure that well if you were to let's
say be in this position you would see it
appear out of the shadow here and then
go around and then when it next
appeared out of the shadow that will give
you the period. Now if he were to measure
on the other side of the orbit of the
earth around the Sun then he would also
look at the Eclipse as Io entered the
shadow and again as it again entered in
the shadow they're following times.
Throughout his time of measuring the
earth of course is moving around like so
at some point he's measuring the period
around Earth and a lot of times as algae
and so forth at L and K if he was in
this section he wouldn't be able to see
anything because Jupiter would be in the
way. A similar issue here is if the earth
was between
k and F and the Sun basically is in the
way and so he wouldn't be able to
measure the period of Io at all.
But the problem that Olaf Romer
discovered is that he found that his
values for the period varied.
They weren't always 45.45 hours. Sometimes they were smaller
sometimes they were larger and he worked
out that they were larger when you were
at position F and a precision K and
smaller at position G and at position L
and what he was able to infer is that
the light that is traveling from Io to
the earth well it has to travel a longer
distance to get to K and that was the
reason why there was a delay the light
just takes longer to get to K than it
was let's say at L and similarly over on
the other side so the first thing and
really critical thing is that light is
now finite that is it has a speed it's
not instantaneous because clearly light
has to travel a certain distance in
order to be perceived depending on where
you are around the orbit.
So that's the first thing and the second thing you he was able to do is to say well I can work out
the delay from the maximum distance. Now the maximum distance is 180 degrees.
In other words it's at E. Now he couldn't
actually see Io but using mathematics he
was able to determine that the delay
here from there to there. In other words
the maximum delay you'll ever get is 22
minutes now that means that it took an
extra 22 minutes for the light to travel
from this position here to this position
here and if you know the diameter of the
orbit you can now work out the speed of
light now Olaf Rømer didn't work that
out. Christian Huygens used
Olaf Rømer's results and determined the
speed of light using this data to be
around two hundred and twelve thousand
kilometers per second.
Now that is of course significantly lower than what we do know as the speed of light today but
that's predominantly due to the fact
they did not have an accurate
measurement of the diameter of the
Earth's orbit around the Sun so it's
still pretty good considering that we're
dealing here with scientists working in
the 17th century
now Rømer presented his findings but
many were skeptical and it wasn't until
1727 with the work of James Bradley
that he was supported. Bradley was an
English scientist who in 1727 also used
astronomical methods determine the speed
of light. In this case he examined the position of stars.
So what did James
Bradley do? James Bradley was looking at
the positions of the star now you all
look up into the night sky and the stars
in the same position but they actually
do vary slightly and that's simply
because of a issue called parallax. Now
the earth of course moves around the Sun
like so over 365 days. That means if you
were to examine the position of let's
say a particular star its position will
be different depending on what time of
year you are looking at if I were to
move the earth six months later then the
position of the star in the night sky
would be slightly different so as a
result if you were to watch a star over
the period of 365 days you would find
that it actually news in a small circle
now what James Bradley predicted is that
if you were to say see the star in this
position in December then you would see
this position over here in June and
similarly speaking you would have the
intervening months in those two
positions but that's not what he got he
found that the were slight delays and as
a result he was able to infer that there
was a delay a change simply because as
the light was traveling to the earth
from the star then the earth would move
and so because the earth is moving,
once the light appears it's actually coming
from where it was at a that time and
then what he did was mathematically
determined the difference in these
angles now these angles weren't aren't
very big we're talking about very small
angles in fractions of a minute. Using
this concept called stellar aberration
that is the light is traveling towards
the earth but the earth is moving at the
same time you're getting a difference in
where the star is supposed to be to
where the star is seemed to be, he was
able to determine the speed of light
considering that the earth speed has a
particular value of 4v and as a result
he worked at the speed of light to be
around 303 thousand kilometers per second
Now that is particularly good
considering he was using only
astronomical means by measuring the
deflection of the stars of where they ought to be
and where they were
appearing to be.
Our next scientist did not use astronomical means to determine the speed of light but what is referred
to commonly as time of flight experiment
And in 1849 Hippolyte Lewis Fizeau
developed a really cool experiment using
mirrors and a very good  cogwheel to
determine the speed of light. So here we
have our diagram that represents Fizeau's
experiment. We have a light source over
here then what we have here is what we
call a half silver mirror and what
happens is that the light would pass
through this half silver mirror through
a separation in the cold and then strike
the mirror over here now then the light
of course would return via the
separation of the COG and then it would
go off the side halves of a mirror and
you would be able to observe the light
passing through this time. Now the cog of
course is spinning all the time which
means at some stage the light would pass
through the gap but then be blocked by
the time that the light returns.
Now here is a diagram that also looks at the same
situation from a two-dimensional
perspective so let me repeat that -  this
thing here is turning with a particular
frequency. The light as this turns is
past going down and back and this
distance is a certain distance we'll
call L and that value in
terms of what Fizeau did was a value of
eight thousand six hundred and thirty
meters. So how was he able to determine
the speed of light using this setup?
Well let's break it down. The first thing you
need to appreciate is that the speed of
light going to and fro fro, is simply
equal to twice the distance because it's
going there and back divided by the time
it takes to go there and back. So how is
time related to the spinning cog? Now
this cog had 720 teeth. He had a friend
who was a clock maker and was able to
produce a really accurate cog with 720 teeth.
The speed of the cog is going to be
equal to simply 2 π r f, that is,  the 2
π r is the total circumference of our
cog multiplied by the frequency. But the
distance between this cog here and this
cog here is simply equal to the total
circumference divided by the number of
cogs which is going to be in so
therefore our total time going as the
spinning around is simply equal to the
distance, that's this distance here, divided
by the velocity, which is this velocity over here
Which means you end up getting
an value of equaling a time 1 over n f
but Fizeau was looking at the time that
it goes from the blocking the view to
seeing the views he was getting the
right speed not for this distance here
but only half that distance here so
therefore the time that he was
interested in was not 1/nf but
ended up being 1/2nf.
So now we have the total time that Fizeau used and
that's the time that we need up here.
We get now the speed of light is equal to 2
L divided by this 1/2nf
and that of course is equal to 4Lnf.
Now Fizeau had a length of eight
thousand six hundred and thirty meters
he had of course seven hundred and
twenty teeth and the frequency that he
ended up getting was 12.6 Hertz
or 12.6 revolutions
per second
and as a result he calculated the value
of the speed of light as
313,000 kilometers per
second. So this is the first real
experiment to determine the speed of
light by what we refer to as
the time of flight methodology. Now it's clearly not
super accurate but reasonably good -
in terms of working out something that is
not astronomically based.
Fizeau's experiment was not accurate so Foucault
set about to improve Fizeau's
experiment by replacing the cogwheel
with a rotating mirror and reducing the
path length from around 8.6 km to
about 20 meters; though he mainly did
this because of technical limitations. So
here we have Foucault setup and in
Foucault's case we have not a rotating cog
we have a rotating mirror and so what
happens is is this mirror spins and as
light travels down through this
half-silvered mirror and bounces back
again the light would reflect at a
different angle depending on the
frequency of the spin here.
And as a result he was able to determine the angle between the light source and also
what he saw and based on the number of
spins that he was getting and what the
path length he had here, he was able to
work at the speed of light.
So let's have let's see how that works out Well we know that the time is still equal to
2L divided by C and this is the value he is
interested in. And so what is the time?
Well the angle that he actually
determined over here remember this is he
measured this just to the angle so he
was able to work out the angle here that
is equal to Omega T and that's simply
circular velocity he so omega is angle
over T so this is the time he which is
the time he now that means
we have t being 2L over C so we got to L
over C multiplied by omega of course he
had the angle so C is equal to 2L Omega
over theta. Now the value he ended up
getting was 298,000
thousand kilometers per second.
So his value of 298,000 kilometers was very good,
good enough in fact that in subsequent
experiments which involved shining light
through moving water he was able to
conclusively show that light traveled
slower in water
Albert Michelson used the same setup as
Foucault but he improved on it basically
by improving the optics and increasing
the baseline considerably and in 1883 he
published a value of
299,853 kilometers per second give or take
sixty kilometers per second. Now it was
about this time that James Maxwell in
1861 in fact published his theory of
electromagnetic waves and determined the
speed of an electromagnetic wave at the
value of three hundred thousand
kilometers per second
now this align so well with the speed of
light evidence that he was able to state
that light was a form of electromagnetic wave.
Now I'll discuss more on Maxwell in
another video. The last 100 years or so
there have been a variety of experiments
conducted to further improve the
precision of the speed of light with
experiments determining the constants in
Maxwell's formula the use of standing
waves in microwaves and the use of
lasers and cesium clocks. The precision
now is so high that we are able to
define the unit of distance by it and
the definition of the meter is now
defined as the distance light travels in
a vacuum during the time interval of 1
over 299 792 458 of a second so that
makes the speed of light exactly
299,792,458 meters per second in a
vacuum. So our understanding of how light
travels has developed remarkably over
the last few centuries getting
ever-increasing accuracy as technologies
 improved and in the last few
hundred years we've learnt so much more
about the nature of light but that's the subject of other videos
well I hope that
helps you understand the concepts thanks
for watching
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at a high school level I'm Paul from
High School Physics Explained, bye for now
