LIGHT

Beam Show

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) m/s103 8 fv

EvidenceDouble slit experiment

Double Slit Demo

Double Slit

Demo

InterferenceConstructive (bright points of light)

+ + + + +

=

Destructive (dark areas)

+ + + + +

=

The Diffraction Grating

…colour sorter

Candle light

Candle Light

LightAmplification byStimulatedEmission ofRadiation

Laser light is coherent, monochromatic and non-dispersive

Fluorescent Dye: emits a broad spread of wavelengths

Add Dye

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

Interferometer Demo

The experiment failed to detect any time difference for the two paths!

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

PE Demo

PE Demo

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…

Fibre Optic

Fibre Optic

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:

pj@soton.ac.uk 023 8059 2159

www.lightexpress.soton.ac.uk