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

• Diffraction – the bending of waves at an aperture or obstacle

Double-slit pattern of water/sound waves

• Interference – double-slit, speckle

screen Optic axis

Path difference = d sin d

y

D

Ipeak=4Io

Iaverage=2Io

• Polarization

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

Electron diffraction

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

Types of Lasers• 3-level vs 4-level

• Ruby, first, a 3-level pulsed laser

• He-Ne, gas, 4-level

• Argon ion, gas

• CO2 – molecular gas laser

• Neodymium – solid-state

Diode laserDye – liquid laser

Laser Applications• Communications

• Coding information, fiber optics

• Medical uses

• Materials processing• Science

Laser Tweezers

Pout Pin Pout Pin

Pleft Pright

Pnet

• Holography