Download Measuring the Speed of Light: Roemer

Measuring the speed of light:
Galileo
Galileo’s assistant
1667
And God said, Let there be light: and there was light.
~ Genesis 1:3
I was unable to make sure whether the facing light
appeared instantaneously. But if not instantaneous,
light is very swift.
Galileo
PS 110A Hatch Ch. 11 - 2
Measuring the speed of light:
Roemer (1676)
2 x 108 m/sec
Io makes one complete orbit around
Jupiter every 1.76 days. During the year
it sometimes appeared ahead of
“schedule” and sometimes behind. Must
be the time to travel farther.
PS 110A Hatch Ch. 11 - 3
Measuring the speed of light:
Armand Fizeau (1849)
3 x 108 m/sec
PS 110A Hatch Ch. 11 - 4
So what is light? Newton thought light
was particles because it cast sharp
shadows
This ( ~) speed accepted today using modern
techniques (short pulse propagation time, e.g.,
laser techniques).
PS 110A Hatch Ch. 11 - 5
Particles reflect and refract (as do waves)
when certain assumptions are made about an
interface
A wave would diffract and produce
“soft edges” whether or not you have
a pinhole.
PS 110A Hatch Ch. 11 - 6
PS 110A Hatch Ch. 11 - 7
What happens when waves
strike multiple slits?
Constructive
Interference
Destructive
Interference
What happens when particles
strike slits?
Single slit scatter pattern
Double slit scatter pattern
PS 110A Hatch Ch. 11 - 8
PS 110A Hatch Ch. 11 - 9
Thomas Young
-diffraction
Diffraction: let’s experiment!
He showed that light
had wave properties, it
just has a very short
wavelength
Light exhibits single slit diffraction
single-slit diffraction (fingers)
diffraction of a laser beam (Demo)
P1: What happens to the light spread as
(~1801)
the slit gets smaller, i.e., closer to the
wavelength of light.
PS 110A Hatch Ch. 11 - 10
Thomas YoungYoung-interference
PS 110A Hatch Ch. 11 - 11
What will particles do?
Pattern when you
change slit width?
P2: Pattern when
you change slit
separation distance?
Light exhibits interference
PS 110A Hatch Ch. 11 - 12
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So light has wave properties.
What is waving?
Interference:
laser beam
A “field” associates attributes with points in
space.
Now light through two slits!
A temperature field:
Young’s double-slit (Demo)
effect of changing slit
separation (and width)
Hot
Warm
Cold
Demo: two waves model
difference
PS 110A Hatch Ch. 11 - 14
Electric and Magnetic fields
PS 110A Hatch Ch. 11 - 15
Light as an electromagnetic
wave
+
E.&M. fields tell how a magnet
points or charged particle responds
No need for a propagation medium!
PS 110A Hatch Ch. 11 - 16
Accelerating Electrons
PS 110A Hatch Ch. 11 - 17
Color
Newton saw the rainbow created by a prism.
He wondered “was the color in the glass or
in the light”?
Electromagnetic radiation is given off whenever
electrons accelerate.
It, in turn, causes electrons elsewhere to accelerate.
(TV, microwave oven)
PS 110A Hatch Ch. 11 - 18
PS 110A Hatch Ch. 11 - 19
Color and frequency
(wavelength) of light
The electromagnetic spectrum
Newton demonstrated that the colors were in
the light and not in the prism.
Thomas Young proved that color was
associated with wavelength.
Same speed,
different frequency
u
u
u
PS 110A Hatch Ch. 11 - 20
What we knew about light in
1900
PS 110A Hatch Ch. 11 - 21
Maxwell (1864)
Light behaves as an electromagnetic wave
Light diffracts, refracts, interferes, and reflects
The frequency of light waves is perceived as
color
Blue light diffracts less than red light.
PS 110A Hatch Ch. 11 - 22
What can we conclude?
the equations that showed that the electric
and magnetic fields could “wave”
wave” & predicts
speed of light
PS 110A Hatch Ch. 11 - 23
Light emission
Planck ~ 1900
Objects glow with a characteristic
color depending on temperature. The
emission spectrum doesn’
doesn’t match
what would be expected using
classical electromagnetic waves,
energy must be a multiple of an
elementary unit E = h ν
Light has wave
nature
P3: What type of
wave is it? What is
the medium needed
to travel in?
Demo: polaroid sheets
PS 110A Hatch Ch. 11 - 24
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Does light have a
“particle” nature?
The photoelectric effect
If so, it will look “lumpy”
Wave absorption should be
continuous
Particle absorption should be as a
packet
Shine light on metal surface
and electrons are given off.
Energy in a normal wave is
proportional to amplitude, not
frequency.
i.e. will a little or big wave
knock you over at the beach
-does brighter light mean more
energy in electrons?
PS 110A Hatch Ch. 11 - 26
photoelectric effect - what do we
find?
PS 110A Hatch Ch. 11 - 27
Photoelectric effect
The effect cannot be explained by thinking
light behaves as a wave!!!
Let’s experiment with light!
visible light
brighter visible light
longer time
dim UV light
BUT
The effect can be explained by thinking
light behaves as a stream of particles!!!
PS 110A Hatch Ch. 11 - 28
The Photoelectric Effect
PS 110A Hatch Ch. 11 - 29
Why the “big deal”?
It can only be explained if light is interacting like a
particle with the electrons in the metal!
Each individual photon has an energy of hf where h =
Planck’s constant (very small) and f = frequency.
E = hf =W + kinetic energy of electron
Consider making a print with uv light
PS 110A Hatch Ch. 11 - 30
Photoelectric effect shows that our
classical (usual) physics doesn’t work.
How can a wave make individual
spots?
PS 110A Hatch Ch. 11 - 31
Photon
Wave Particle Duality
Light is both a wave and a particle.
• it is a particle of light
It behaves like a wave when unobserved
– Energy
– No mass
– No charge
It travels through both slits like a wave
It is detected like a particle
It hits the screen as individual dots
• number of photons makes up the
brightness (amplitude) of light beam
PS 110A Hatch Ch. 11 - 32
If this bothers you, you are in
good company!
“All these 50 years of
pondering have not
brought me any closer to
answering the question,
‘what are light quanta?’
These days every Tom,
Dick, and Harry thinks he
knows it, but he is
mistaken.”
~ A. Einstein
PS 110A Hatch Ch. 11 - 33
Photos: let’s experiment
P4: What would happen if we shined a
very dim light through the slits in
Young's double-slit experiment and just
detected the light striking
the screen one photon at
a time?
PS 110A Hatch Ch. 11 - 34
PS 110A Hatch Ch. 11 - 35
Thought Experiment
wave-particle duality
P5: What will I see in the double slit
Light is not just a wave nor just
a particle, light is light:
something which exhibits
characteristics of both.
experiment if I put some sort of
detector at one of the slits that allows
me to see which slit each photon goes
through? Why?
But…how can a stream of photons
interfere?
PS 110A Hatch Ch. 11 - 36
PS 110A Hatch Ch. 11 - 37
Wave Particle Duality
Light is both a wave and a particle.
It behaves like a wave when unobserved
It travels through both slits like a wave
It is detected like a particle
It hits the screen as individual dots
The wave nature contains a probability of the
photon (particle) striking at a certain position.
PS 110A Hatch Ch. 11 - 38