The end of classical physics: photons, electrons, atoms

1
The end of classical physicsphotons, electrons,
atoms

• PHYS117B

2
People have long asked, "What is the world made
of?" and "What holds it together?"
3
What is the World Made of?

• Why do so many things in this world share the
  same characteristics?
• People have come to realize that the matter of
  the world is made from a few fundamental building
  blocks of nature.
• The word "fundamental" is key here. By
  fundamental building blocks we mean objects that
  are simple and structureless -- not made of
  anything smaller.
• Even in ancient times, people sought to
  organize the world around them into fundamental
  elements, such as earth, air, fire, and water.

4
What is fundamental ?

• Today we know that there is something more
  fundamental than earth, water, air, and fire...
• The atomic idea has been around for a long time
• By convention there is color,By convention
  sweetness,By convention bitterness,But in
  reality there are atoms and space.   -Democritus
  (c. 400 BCE)
• There is a principal difference between the way
  the ancient philosophers approached the world and
  what scientists do.
• The EXPERIMENT is the ultimate judge of any
  theory !
• Well discuss some fascinating experiments that
  brought the present scientific idea of atoms,
  nuclei, elementary particles

5
The main questions in the beginning of 1800

• What is light ?
• What is electricity ?
• Electric and magnetic field ?
• and then in the 1900s people came to the
  question of atoms again.
• Maxwells theory explained electric and magnetic
  phenomena. It combined electric and magnetic
  field into ONE theory of the electromagnetic
  field. It also showed that light is an
  electromagnetic wave!

6
But accepting that light is a wave was difficult

• In most everyday experiences light behaves as
  rays or stream of particles
• To see the wave properties you need to deal with
  sizes that are comparable to the wavelength!

7
Wave nature of light

• OK, if you use slits that have small width and
  small spacing you will see it !
• Last time we used
• Slit width 0.040 mm
• Slit spacing 0.25 mm
• Just when everybody
• Was convinced that
• light is a wave
• EXPERIMENTAL facts that
• showed that light comes in quanta!

8
The experiments that changed physics

• Discovery of the electron ( charge quantization).
  
• Photo-eclectic effect (light is a particle)
• Atomic emission and absorption lines (atomic
  energy levels) angular momentum is quantized
• The discovery of the nucleus (Rutherford
  scattering experiment)
• Electron diffraction ( wave-particle duality)
• X-rays, radioactivity

9
Faraday electrolyses and cathode rays
1850
Electrical conduction through gasescathode
glow, colored glow (depending on the gas in the
tube)
Faraday Water is not fundamental. Atoms exist.
Charge is somehow associated with atoms.
Electricity is not some unique type of substance.
It flows through liquids and gases.
10
Cathode rays
Reduce the pressure cathode glow extended
towards the anode. Electricity behaves as stream
of particles ( shadow of cathode glow)
Collimate the beam of cathode rays, place a
collecting electrode gt Measure current !
Cathode rays have charge !
11
Charged particles are deflected in magnetic field

• Another proof that cathode rays are charged
  particles
• Determine the sign (-)

12
Thomson measured the q/m for the cathode rays
crossed field experiment

• r mv/qB
• No deflection
• FB FE
• v E/B
• q/m v/rB

13
How to measure q and m separately ?Millikan
oil-drop experiment

• mg q E
• q m g/E
• m ?
• Use density of oil
• Measure diameter
• D 1 micron TOOO small to see
• Let the drop move (not a free fall
• Measure terminal speed depends on radius of the
  drop
• Charge is quantized ! Year 1906
• All charges are multiples of a certain minimal
  value, e

14
How are electrons emitted from the cathode ?

• Cold cathode in Crooks tube high voltage some
  of the electrons are expelled from the cathode.
  The phenomenon is now known as field emission,
  which is a form of quantum tunneling in which
  electrons pass through a barrier in the presence
  of a high electric field . Thats how old TVs
  worked
• Heated cathode thermionic emission ( discovered
  by Edison in 1883). Heat the cathode gt some of
  the electrons have enough thermal kinetic energy
  to overcome the attraction from the nuclei.
• BUT you could get electrons out of the cathode
  if you shine light on it!

15
Photo-electric effect year 1900
Hertz discovered
Phillip Lenard (Hertzs student) studied the
photo-electric effect
16
Photo-electric effect experiment
17
And the explanation came from Einstein

• The energy transfer is all or nothing process in
  contrast to the classical theory of continuous
  transfer of energy
• eV0 Kmax hf E0

18
Is light particle or wave?
phenomenon wave particle
reflection yes yes
refraction yes yes
interference yes no
diffraction yes no
Photoelectric effect no yes
19
Thomsons raisin cake model of the atom

• Measrured q/m for cathode rays
• Compared to H atom
• 1000 times larger for cathode rays!
• Smaller mass ? Or larger charge ?
• Fundamental charge unit measured in ionized gases
  (e) and by Millikan
• Cathode rays do not depend on the cathode
  material
• Subatomic particle the electron

20
How to probe the atom ? Radioactivity was just
discovered

• In the late 1800s the German physicist, Wilhelm
  Röntgen, discovered a strange new ray produced
  when an electron beam struck a piece of metal.
  Since these were rays of an unknown nature, he
  called them "x rays".
•              
• Two months after this discovery, the French
  physicist, Henri Becquerel, was studying
  fluorescence, when he found that photographic
  plates were exposed in the presence of some ores,
  even when the plates were wrapped in black paper.
  Becquerel realized that these materials, which
  included uranium, emitted energetic rays without
  any energy input.
• Becquerel's experiments showed that some
  natural process must be responsible for certain
  elements releasing energetic x rays. This
  suggested that some elements were inherently
  unstable, because these elements would
  spontaneously release different forms of energy.
  This release of energetic particles due to the
  decay of the unstable nuclei of atoms is called
  radioactivity.

21
Rutherford found that

• 3 types of rays a,b,g
• q/m for beta rays was the same as in cathode rays
• Measured the spectrum of alpha rays it turned
  out to be the same as for He

22
Rutherfords experiment
23
The Rutherford atom

• Measured the distance of closest approach 10-14
  m