1. The end of classical physics: photons, electrons, atoms
PHYS117B

2. "What is the world made of?"
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 !
We’ll 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.
Maxwell’s 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 = 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 gases: cathode 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 =>
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 . That’s how old TV’s worked
Heated cathode: thermionic emission ( discovered by Edison in 1883). Heat the cathode => 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 (Hertz’s 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
refraction
interference no
diffraction
Photoelectric effect

19. Thomson’s “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. Rutherford’s experiment

23. The Rutherford atom
Measured the distance of closest approach ~ m