25: Speed of Light

Over 300 years of physicists trying to measure the speed of light, what method of collecting data was used?

v = d / t

How did Galileo attempt to measure the speed of light?

Galileo on one hilltop, while another person stood on another hill, holding lanterns.

• Opening each lantern as a signal, hoped to measure time for light to travel distance

Why did Galileo understand he couldn’t measure the time?

1. human reaction time 

2. poor methods to measure time

How did Ole Rømer accidentally measure “c”?

• Observed Jupiter’s moons.
  Did follow up measurements later in the year.

  • Noticed Earth & Jupiter were further apart.

There was a delay in the time he could see
things happen, since light had to travel the
extra distance between Earth & Jupiter!

How would Rømer and other physicists solve for the first measurement of “c” based on Rømer’s observation?

Others (including Huygens) used his work to calculate “c” (although the number was a little low)

What experiment did Armand Hippolyte Louis Fizeau conduct to measure “c”?

Fizeau shined a beam of light to go between the teeth of a spinning gear.

• Goes 8.5km, hits a mirror, and bounces back the way it came.

• The beam passes through next open gap in gear. This allowed him to measure the time it took the beam to get back

What number did Armand Hippolyte Louis Fizeau come up with for “c”?

Fizeau's result was 3.13e8 m/s (about a 4% error).

What experiment did Jean Léon Foucault conduct to measure “c”?

Foucault used a beam of light that passed through a lens to a spinning mirror, bounced off a stationary mirror back to the spinning mirror.

• When the ray bounced back, the spinning mirror had changed it's angle a bit so the beam bounced off at an angle.

• Foucault could calculate the time the mirror took to spin that extra little angle, along with the distance the ray had traveled.

What number did Jean Léon Foucault come up with for “c”?

Foucault got a value of 2.98e8 m/s, only 0.67% off from the current measurement!

What experiment did Albert A. Michelson conduct to measure “c”?

Had beam hit one side of a rotating eight sided mirror.

• Reflected to a mirror 35 km away. 

• Beam bounces back to rotating mirror. 

  • Rotating mirror must spin at exactly the right speed to move exactly 1/8th of a turn to reflect the light again.

What number did Albert A. Michelson come up with for “c”?

2.99792458e8 m/s, the most accurate measurement we have