General Properties of Light
Light as a wave
• Speed
• Wave properties: wavelength, frequency, period, speed, amplitude, intensity
• Electromagnetic wave
EM Spectrum
102 104 106 108 1010 1012 1014 1016 1018 1020 1022
Wavelength (m)
Frequency (Hz)
3 x 104 3 x 10-4 3 x 10-8 3 x 10-12 3
Energy (eV) 4 x 105 4 x 101 4 x 10-3 4 x 10-7 4 x 10-11
Radio waves Infrared Gamma rays X-rays Ultra- violet
Visible Light
Long waves
• Interference – double-slit, speckle
screen Optic axis
Path difference = d sin d
y
D
Ipeak=4Io
Iaverage=2Io
Light as a particle
• Photon
• Energy packet
• Photoelectric effect
• Double slit- revisited
anode photocathode
A
*
*
*
Double Slit Experiment with Electrons
Detections come in lumps = quanta
Intensity exhibits interference
Reducing intensity of the light, so that only one electron arrives at the slits at a time
Questions?
Why do no electrons (or photons) ever arrive at some locations?
Clearly 1 2( ) ( ) ( )totP x P x P x
How can we get interference when only 1 particle at a time is going through the slits? What does that particle interfere with?
Suppose we actually check which slit each particle went through and know with certainty – what happens then?
How can this be?
Resolution comes from QED with Feynman
How is light produced?
• Chemical energy - fire
• Electrical energy - lightning
• Nuclear energy - stars– Incandescent– Electric discharge – continuous vs discrete
spectrum – Fluorescence
Special Properties of Laser Light
• Monochromatic
• Intense
• Pencil beam – collimated
• Coherent
• Can be cw or pulsed
Prototype Laser
• Energy levels• Absorption, spontaneous emission• Stimulated emission• Population inversion• Metastable state• Pump – optical, electron excitation, atom-
atom collisions, chemical• Resonant cavity – gain, cavity modes,
beam