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

hp =

Who’s right??

Waves explain diffraction & interference

Photons explain photoelectric effect & Compton scattering

Impossible to explain interference with particles

With 2 slits openno light goes here

Block off one slit

Now lightcan go here

Impossible to explain PE-effectand Compton scattering with waves

Electron KE (electron Volts)

red

0.5 1.0 1.5

yellow

blue

violet

Make an intereferencepattern 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:

hp =

particles:

hp =

Wave-particle duality is

a universal phenomenon

Ordinary-sized objects have tiny wavelengths

0.2kg30m/s

hp =

hmv

= 6.6x10-34Js

0.2kg x 30 m/s =

6.6x10-34Js6.0 kg m/s = 1.1x10-34m =

Incredibly small

the wavelength of an electronis not so small

9x10-31 kg 6x106 m/s

hp =

hmv

= 6.6x10-34Js

9x10-31kg x 6x106 m/s =

6.6x10-34Js5.4x10-24 kg m/s = 1.2x10-10m =

-

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

py

py

y

x

y

Waves thru a narrower slit

wider

py

py

y

When the slit becomes narrower, thespread in vertical momentum increases

x

y

Heisenberg Uncertainty Principle

y py > h

Uncertaintyin 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, usea shorter wavelength

But then the momentumchange is higher

x px > h

Localize a baseball

0.2kg x px > h px > hx

Suppose x= 1x10-10m

px > 6.6x10-34Js

1x10-10m

vx >

= 6.6x10-24kgm/s

A very tiny uncertainty

About the size of a single atom

px m

6.6x10-44Js

0.2kg = = 3.3x10-23 m/s

Localize an electron

x px > h px > hx

Suppose x= 1x10-10m

px > 6.6x10-34Js

1x10-10m

vx >

= 6.6x10-24kgm/s

Huge, about 2% of c

About the size of a single atom

px me

6.6x10-24Js

9x10-31kg = = 7x106 m/s

-me=9x10-31kg

uncertainty is inherentin the quantum world