Speed of Light
3 people who have played a
significant role in calculating the
speed of light
Galileo-1620
In the early 17th century, many scientists
believed that there was no such thing as the
"speed of light"; they thought light could travel
any distance in no time at all. Galileo disagreed,
and he came up with an experiment to measure
light's velocity: he and his assistant each took a
shuttered lantern, and they stood on hilltops one
mile apart. Galileo flashed his lantern, and the
assistant was supposed to open the shutter to
his own lantern as soon as he saw Galileo's light.
Galileo would then time how long it took before
he saw the light from the other hilltop.
Galileo-1620
Used the ECHO idea to try to calculate the
speed of light.
Used a lantern with shutters and a timing
device
Galileo-1620
Realized he was actually measuring his
reaction time, he stated that the speed of
light is so fast that no one will ever be
able to calculate its speed.
Roemer-1670’s
Danish astronomer Ole Roemer was making
extremely careful observations of Jupiter’s moon
Io. Io makes one complete orbit around Jupiter
every 1.76 days; the time it takes to make each
orbit is always the same, so Roemer expected
that he could predict its motion quite precisely.
To his astonishment, he discovered that the
moon didn't always appear where it was
supposed to be. At certain times of the year, it
seemed to be slightly behind schedule; at other
times, it was slightly ahead.
Roemer – 1670’s
When the Earth is at different locations, it
is different distances away from Jupiter
and therefore it takes light a different time
to arrive from Jupiter to Earth.
Roemer – 1670’s
•When the Earth moved away from
Jupiter, the moon appeared to stay
behind the planet 22 minutes longer
than when the Earth was moving
towards Jupiter.
Io
Eclipse is shorter
than it should be.
Eclipse lasts longer
than it should
Roemer – 1670’s
Knowing the distance that the Earth is
away from the sun and the time delay, he
used velocity = distance/time.
Roemer – 1670’s
Roemer’s method calculated a value that
was very close to the accepted value of
the speed of light. Roemer’s method
calculated the speed to be 2.1 x 10^8
m/s.
Roemer’s Method
Take the radius of the earth’s orbit and
double it to get the diameter (this is the
distance the light travels)
Roemer’s Method
Take the radius of the earth’s orbit and
double it to get the diameter (this is the
distance the light travels)
Get the time it takes to travel that
diameter of the earth’s orbit
Roemer’s Method
Take the radius of the earth’s orbit and
double it to get the diameter (this is the
distance the light travels)
Get the time it takes to travel that
diameter of the earth’s orbit
Use
Velocity = distance/time
Michelson-1900
Michelson-1900
Albert Michelson went back to Galileo’s
idea of an echo but used greater distances
and a better timing device.
Michelson-1900
The rotating mirror was on Mount Wilson
and the fixed mirror was miles away on
Mount San Antonio.
Michelson-1900
This is a picture of the site of the
experiment on Mt. Wilson looking at Mt.
San Antonio. The pillar on the right is
where the octagonal mirror was located.
Michelson-1900
Michelson slowly increased the frequency
of the octagonal mirror until he saw
flashes in the viewing telescope.
Michelson-1900
When this occurred, the mirror was
rotating a perfect 1/8 of a turn in the time
it took the light to go there and back.
Michelson-1900
Knowing the frequency at this point, find
the period (1/f). This is the time to go
once around, but it actually only went 1/8
of a rotation so take 1/8th of the period
Michelson-1900
1/8 of the period is the time value (t).
Michelson-1900
Knowing the distance between the two
mirrors and the time, he calculated the
speed of light using the echo equation.
Michelson-1900
Velocity = 2 (X)/ t
His value was 2.99 x 10^8 m/s which we
can round to 3 x 10^8 m/s.
Michelsons Method
Take 1/frequency to get the period
Michelsons Method
Take 1/frequency to get the period
Take 1/8 (8 sided mirror) of the period to
get the time it takes for ‘light echo’
Michelsons Method
Take 1/frequency to get the period
Take 1/8 (8 sided mirror) of the period to
get the time it takes for ‘light echo’
Get the distance to the other mountain
top
Michelsons Method
Take 1/frequency to get the period
Take 1/8 (8 sided mirror) of the period to
get the time it takes for ‘light echo’
Get the distance to the other mountain
top
Use Echo equation
V = 2 (distance) / time
SPEED OF LIGHT
3 x 10 ^8 m/s