LIGHT

Beam Show

University of Southampton School of Physics and Astronomy  Light as a wave  Relativity  Quantum mechanics  Particle physics Light: at the forefront of Physics

Light is a Wave (wavelength) Evidence Double slit experiment

Double Slit Demo

Interference Constructive (bright points of light) = Destructive (dark areas) =

The Diffraction Grating …colour sorter

Candle light Candle Light

Light Amplification by Stimulated Emission of Radiation Laser light is coherent, monochromatic and non-dispersive Fluorescent Dye: emits a broad spread of wavelengths

Add Dye

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

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 ! 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! *

Light and Matter Photo-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! Light e-e- 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 Light e-e- Metal

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 But in relativity observers do not agree on time ordering of events… so can we have: e-e- time e-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+ 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…

Fibre Optic

Thank You the Light fantastic was 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: