Waves, Light & Quanta
Tim Freegarde
Web Gallery of Art; National Gallery, London
Young’s double slit diffraction

2 d

a

 d
x
s
 d
amplitude
intensity
2
Single slit diffraction

amplitude
y

a
x
intensity

s
3
Diffraction grating
y

a
x
d

s
• place secondary sources along wavefront
• ...and trigger when wavefront arrives
• apply to sinusoidal waves by taking into account
the phase with which components arrive
INTERFERENCE
• combine by adding the amplitudes
• contributions may therefore interfere
constructively or destructively
4
Grating spectrometer

scope.pari.edu
Michelson interferometer
• interference by division of amplitude
δx
beamsplitter
detector
source
6
Michelson interferometer
• interference by division of amplitude
δx
δx
beamsplitter
detector
source
7
Michelson interferometer
• interference by division of amplitude
δx
δx
beamsplitter
• FTIR: Fourier transform infrared
detector
source
chemistry.oregonstate.edu
• sodium doublet
optique-ingenieur.org
Newton’s rings
beam-splitter
diffuser
r
lens
s
plate
• apply to sinusoidal waves by taking into account
the phase with which components arrive
• combine by adding the amplitudes
www.sciencebuddies.org
• contributions may therefore interfere
constructively or destructively
9
Michelson stellar interferometer

2 d
1
2
a

 s1
 s2
 d
x
 d
intensity
10
Michelson stellar interferometer

d


a
11
Michelson stellar interferometer
Mount Wilson Observatory www.mwoa.org
12
Coherence
NON-POINT OBJECTS
• many independent emitters, e.g. different atoms in star
• spatial extent (angular width) determines fringe visibility
• partial visibility when phase difference partially defined
• coherence is a measure of extent to which the phase difference is known
• degree of coherence
 12 
E1  E2
E12  E22
• (approximate expression – requires complex numbers)
• complex degree of coherence
 12 
E1  E2
E1 E1  E2 E2
13
Light and optics
RAYS
• straight propagation paths
• least time (Fermat’s principle)
focus
• reflection, refraction, lenses, telescopes, microscopes
directrix
WAVES
• Huygens’ description of propagation, reflection, refraction
• polarization, colour (wavelength, frequency)
• diffraction, interference, beats, interferometers
• Maxwell’s electromagnetism, Einstein’s relativity
PHOTONS
• Planck, Compton, Einstein
14