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University of Southampton
School of Physics and Astronomy
Light as a wave
Relativity
Quantum mechanics
Particle physics
Light: at the forefront of Physics
LightAmplification byStimulatedEmission ofRadiation
Laser light is coherent, monochromatic and non-dispersive
Fluorescent Dye: emits a broad spread of wavelengths
The Speed of Light
*c
v
We should be able to see this effect using the motion of the Earth
For water waves the motion of an observer will “change” the speed of the waves
*c + v
Michelson and Morley
In 1887 Michelson and Morley did an experiment to detect the ether using the Earth’s motion relative to the ether
RelativityLight is an electromagnetic wave – there is no etherThe speed of light is the same for any observer!
This means nothing can travel with light – nothing can reach v=c!
Rest mass = energy
2
2
21
1mc
cv
E
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!*
tvxcvx
cvxtcvt
22
222
1
1
Light and MatterPhoto-Electric Effect
Light can provide energy to kick electrons out of a 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!
Lighte-
Metal
The size of the energy lumps depends on
frequency
If we reduce the frequency too much then the electrons don’t receive enough energy to escape the metal surface
Lighte-
MetalJs106.6 34
h
hfE
Light QuantaThe 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 LegacyElectrons can absorb photons
But in relativity observers do not agree on time ordering of events… so can we have:
e-
time
time
e-
What does it mean for an electron to travel backwards in time? We only measure charge:
It looks like a +ve charge electron moving forward in time
We have discovered anti-particles!
-q
=
+q
time
Particle Physics
Electron positron annihilation to a photon allows us to convert their energy to look for all the particles that make up nature
e+
e-
New Particles
time
energy in
photon
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…
Thank You
the Light fantasticwas brought to you by:
Nick Evans Pearl John James Gates Keith Wilcox Matt Praeger Sunil Patel
For more information, please contact:Pearl John – The Light Express Coordinator:
[email protected] 023 8059 2159
www.lightexpress.soton.ac.uk