Wave? Particles?? Physics 100 Chapt 22

Maxwell Light is a wave of oscillating E- and B-fields James Clerk Maxwell E B

Einstein Light is comprised of particle-like quanta called photons E=hf h p =

Who’s right?? Waves explain diffraction & interference Photons explain photoelectric effect & Compton scattering

Impossible to explain interference with particles With 2 slits open no light goes here Block off one slit Now light can go here

Impossible to explain PE-effect and Compton scattering with waves Electron KE (electron Volts) red yell ow blue violet

Make an intereference pattern with low intensity light One photon at a time goes through the two-slit apparatus The interference pattern emerges one dot at a time

Wave-Particle “duality -Light behaves like a wave when it propagates through space -And as a particle when it interacts with matter

Photon photography

Louis de Broglie If light behaves as particles, maybe other particles (such as electrons) behave as waves h p =Photons: hphp  = particles: hphp  = Wave-particle duality is a universal phenomenon

Ordinary-sized objects have tiny wavelengths 0.2kg 30m/s hphp  = h mv = 6.6x Js 0.2kg x 30 m/s = 6.6x Js 6.0 kg m/s = 1.1x m = Incredibly small

the wavelength of an electron is not so small 9x kg 6x10 6 m/s hphp  = h mv = 6.6x Js 9x kg x 6x10 6 m/s = 6.6x Js 5.4x kg m/s = 1.2x m = - About the size of an atom

Send low-momentum electrons thru narrow slits See a diffraction pattern characteristic of wavelength =h/p as predicted by de Broglie

Light thru a small hole “Diffraction” rings

Matter waves (electrons through a crystal) “Diffraction” rings

Waves thru a narrow slit pypy pypy yy x y

Waves thru a narrower slit wider pypy pypy yy When the slit becomes narrower, the spread in vertical momentum increases x y

Heisenberg Uncertainty Principle  y  p y > h Uncertainty in location Uncertainty in momentum in that direction If you make one of these smaller, the other has to become bigger

Heisenberg tries to measure the location of an atom For better precision, use a shorter wavelength But then the momentum change is higher  x  p x > h

Localize a baseball 0.2kg  x  p x > h  p x > hxhx Suppose  x= 1x m  p x > 6.6x Js 1x m  v x > = 6.6x kgm/s A very tiny uncertainty About the size of a single atom pxmpxm 6.6x Js 0.2 kg = = 3.3x m/s

Localize an electron  x  p x > h  p x > hxhx Suppose  x= 1x m  p x > 6.6x Js 1x m  v x > = 6.6x kgm/s Huge, about 2% of c About the size of a single atom pxmepxme 6.6x Js 9x kg = = 7x10 6 m/s - m e =9x kg

uncertainty is inherent in the quantum world