7. Light as a particle
• Photon
• Energy packet
• Photoelectric effect
• Double slit- revisited
anode
photocathode
A
*
*
*
8. 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
9. Questions?
Why do no electrons (or photons)
ever arrive at some locations?
Clearly 1 2
( ) ( ) ( )
tot
P 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
11. How is light produced?
• Chemical energy - fire
• Electrical energy - lightning
• Nuclear energy - stars
– Incandescent
– Electric discharge – continuous vs discrete
spectrum
– Fluorescence
12. Special Properties of Laser Light
• Monochromatic
• Intense
• Pencil beam – collimated
• Coherent
• Can be cw or pulsed
13. 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
14. Types of Lasers
• 3-level vs 4-level
• Ruby, first, a 3-level
pulsed laser
• He-Ne, gas, 4-level
