Transcript Light Fantastic - University of Southampton

LIGHT
Beam
Show
Beam Show
University of
Southampton
School of Physics and Astronomy
Light: at the forefront of Physics
 Light as a wave
 Relativity
 Quantum mechanics
 Particle physics
Light is a Wave
 (wavelength)
Evidence
Double slit
experiment
v  f  3 10 m/s
8
Double
Slit Demo
Double Slit Demo
Interference
Constructive (bright points of light)
+
+
+
+
+
=
Destructive (dark areas)
+
+
+
=
+
+
The Diffraction Grating
…colour sorter
Candle
Light
Candle light
Laser light is coherent,
monochromatic and
non-dispersive
Light
Amplification by
Stimulated
Emission of
Radiation
Fluorescent Dye: emits a broad
spread of wavelengths
Add Dye
Add Dye
The Speed of Light
For water waves the motion of an observer
will “change” the speed of the waves
*
c
*
c+v
v
We should be able to see this effect using
the motion of the Earth
Michelson and Morley
In 1887 Michelson and Morley did an
experiment to detect the ether using
the Earth’s motion relative to the ether
Interfero
meter
Demo
Interferometer Demo
The experiment failed to detect any
time difference for the two paths!
Relativity
Light is an electromagnetic
wave – there is no ether
The speed of light is the
same for any observer!
This means nothing can travel with light –
nothing can reach v=c!
1
E
2
1 v
 mc
c
2
2
Rest mass = energy
Space - Time
A flash of light causes a spherical wave front
even if you move relative to source
*
This only makes
sense if space and
time mix!
t 
x 
1  v c t  vx c 
1  v c x  v  t 
2
2
2
2
2
Light and Matter
Photo-Electric Effect
Light can provide energy to kick electrons out of a
metal
Light
e-
Metal
If light intensity is lowered so there is less energy,
we expect the evicted electrons to have less
energy… but they don’t… we just see fewer
electrons of the same energy…
The energy in light comes in lumps!
The size of the energy lumps
depends on frequency
Light
E  hf
h  6.6 10
34
Js
e-
Metal
If we reduce the frequency too much
then the electrons don’t receive enough
energy to escape the metal surface
PE Demo
PE Demo
Light Quanta
The energy in light comes in lumps
In a sense we can think of light as
particles in our detector - photons
Quantum Mechanics is the theory of the
dynamics of these quanta
Dirac’s Legacy
Electrons can absorb photons
etime
But in relativity observers do not agree on
time ordering of events… so can we have:
e-
time
What does it mean for an electron to travel
backwards in time? We only measure charge:
-q
=
+q
time
It looks like a +ve charge electron moving
forward in time
We have discovered anti-particles!
Particle Physics
eenergy in
photon
New
Particles
e+
time
Electron positron annihilation to a photon
allows us to convert their energy to look for
all the particles that make up nature
At the LHC collider, currently
being built, we will produce
100 billion collisions per
second!
How can we communicate so much data?
In 1990 Tim Berners-Lee invented
the Web to transfer our data!
And laser light now carries the
information around the world…
Fibre
Optic
Fibre Optic
the Light
fantastic
was brought to you by:






www.lightexpress.soton.ac.uk
Nick Evans
Pearl John
James Gates
Keith Wilcox
Matt Praeger
Sunil Patel
Thank You
For more information, please contact:
Pearl John – The Light Express Coordinator:
[email protected] 023 8059 2159