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

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

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.

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

The main questions in the beginning of 1800s What is light ? What is electricity ? Electric and magnetic field ? 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! … and then in the 1900s people came to the question of atoms (particles) again.

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!

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!

The experiments that lead to quantum physics Discovery of the electron ( charge quantization). Photo-electric 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

Faraday: electrolyses and cathode rays 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. Electrical conduction through gases: cathode glow, colored glow (depending on the gas in the tube) ~1850

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 !

Charged particles are deflected in magnetic field Another proof that cathode rays are charged particles Determine the sign (-)

Thomson measured the q/m for the cathode rays: crossed field experiment r= mv/qB No deflection :  F B = F E  v = E/B q/m = v/rB J.J.Thompson found:  the q/m for cathode rays was 1000 times higher than that of the hydrogen ion => either q is very large, or m is very small !  Cathode rays do not depend on the cathode material

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

The discovery of the electron and Thomson’s “raisin cake model” of the atom Measured q/m for cathode rays (1000 times larger than for H ion) Charge is quantized (multiples of e) Subatomic particle – the electron

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 workedquantum tunnelingelectric field 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!

Photo-electric effect: year 1900 Hertz discovered: Phillip Lenard (Hertz’s student) studied the photo-electric effect

Photo-electric effect: experiment

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 eV 0 = K max = hf –  

Is light particle or wave? phenomenonwaveparticle reflectionyes refractionyes interferenceyesno diffractionyesno Photoelectric effect noyes

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.

Rutherford found that: 3 types of rays:  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

Rutherford’s experiment

The Rutherford atom Measured the distance of closest approach ~ m

Key discoveries in the beginning of the 20 th century Electrons – subatomic particles that carry the smallest possible charge. Atoms exist and they have a small extremely dense positively charged nucleus. The electrons balance the charge to get an electrically neutral atom. Light behaves both as a wave and a particle ( photo-electric effect) Radioactivity: some atoms are unstable and emit different types of rays