1849 Armand Louis Fizeau

1
1849 Armand Louis Fizeaus method
reflector 86.33 meters away (50
back-and-forth reflections)?8.630 km (5.36
miles) 720 cogs in the wheel which had
to turn 12 Hz to make light vanish
measured c 194,410 mi/sec
313,300 km/sec 1876 Cornu
reflector 23 km away measured c
298,000 ? 500 km/sec 1879 J.B. Léon
Foucault rotating mirror
reflector 605 meters away measured
c 186,380 mi/sec 299,910 ? 200
km/sec 1926 Albert A. Michelson
rotating mirror reflector 22
miles away (Mt. Wilson to Mt. San Antonio)
measured c 186,380 mi/sec
299,774 ? 4 km/sec 299,792,458 m/sec or
186,282 miles per second
2
1
Electrons in a TV picture tube hit the television
screen at a high speed and come to a stop. When
the electrons stop in the TV screen they produce
e
2
A) X-rays which travel mostly in direction 1 B)
X-rays which travel mostly in direction 2 C) no
electromagnetic waves at all
Remember our picture of a charge moving. The EM
wave is emitted perpendicular to the direction of
the motion of the charge. By the way, the
electrons also lose energy to the phosphor in the
screen, which causes it to glow, thusly producing
a picture.
3
Oscillating electric charge q (and the q its
absence leaves behind).
E field points down
strong
weak
zero
E field momentarily zero
Equilibrium position, where all charges could
rest stably.
E-field pointing down
E field points up
4
Also, this SIMPLE HARMONIC MOTION generates a
B-Field!
slowing to zero speed
As this positive charge moves up, the B-field in
this region points INTO the screen
maximum speed
5
B field is oscillating, continuously flipping the
magnetic field
in out in out in out
and at the same time
E field is oscillating, continuously flipping the
electric field
up down up down up down
6
Before the days of cable, televisions often had
two antennae on them, one straight, and one
circular. One antenna picked up electric field
oscillations, and the other picked up magnetic
field oscillations. Which antenna picked up the
magnetic oscillations? 1) the circular
antenna 2) the straight antenna
7
Carrier Frequency
Amplitude Modulation
8
Frequency Modulation
9
changing E field
Maxwell looked at this table and, appealing to
symmetry, postulated...
A changing E field creates a B field