Atomic structure

Energy levels

Energy levels for an atom (Helium)

Energy bands of solids

Light consists of particles: photons

Energy of a photon:

fhE

secerg1063.6 27 h

Lasers

A laser is a device that can produce a very narrow intense beam of monochromatic coherent light.

The atoms are excited from state to state . The atoms quickly decay either back to or to the intermediate state , which is metastable with a lifetime about (compared to for ordinary levels). With strong pumping action, more atoms can be found in the state than are in the state. As soon as a few atoms in the state jump down to , they emit photos that produce stimulated emission of the other atoms, and the lasing action begins. A ruby laser thus emits a beam whose photos have energy 1.8 eV and a wavelength of 694.3 nm.

In a ruby laser, the lasing material is a ruby rod consisting of with a small percentage of Aluminum (Al) atoms replaced by chromium (Cr) atoms. In a process called optical pumping, the Cr atoms are excited by strong flashes of light of wavelength 550nm, which corresponds to a photon energy of 2.2 eV.

32OAl

0E 2E

0E 1E

1Es103 3 s103 8

0E0E1E

Electrons are waves

In 1924 Louis de Broglie suggested that particles may exhibit wavelike properties. He shows that the wavelength of the matter waves would be

The hypothesis was confirmed in 1925 by experiments which showed that electrons passing through crystals form wavelike diffraction patterns with a configuration corresponding to a wavelength suggested by de Broglie.

Where m and v are mass and velocity of the particle and h is the Planck constant.

mv

h

particles waves electronsDouble-slit experiment for

Electron microscope

The size of the smallest object observable by a microscope is about half the wavelength of the illuminating radiation. In light microscopes, this limits the resolution to about 200 nm.

It is relatively easy to accelerate electrons in an evacuated chamber to high velocities so that their wavelength is less than m. The short wavelength of electrons coupled with the possibility of focusing them has led to the development of electron microscopes that can observe objects 1000 times smaller than are visible with light microscopes.

10103

Spins

The electron can have two different states due to some intrinsic property that behaves like an angular momentum so that we call this property “spin”.

The electron has spin quantum number s = ½. In the external magnetic field, the spin of the electron may line up with the field direction, called “spin up”, or against the field, called “spin down”. A state with spin down has slightly lower energy than one with spin up.

Nuclei have spins too. The spin up state has lower energy in the external magnetic field.

Magnetic Resonance Imaging (MRI)

The information of nuclear spins can be utilized to measure the intensity of H atoms in organisms.

Radioactivity

Radiation therapy; Food preservation

Radioactivity is the result of the disintegration or decay of an unstable nucleus. It was found in every case to be unaffected by the strongest physical and chemical treatments, including strong heating or cooling or the action of strong chemical reagents.

Radioactive decay law: teNN 0

Half-life : 2/)( 0NN

Nucleus Half-life5730 yr

yr

138 days

162.3

C146

Po21084

U23892

Po21484

sec

9105.4

Isotopes

Radioactive dating

Most elements have isotopes differing from each other by the number of neutrons in their nuclei.

Tracers

A mass spectrometer can distinguish isotopes from each other. Many elements have isotopes that are radioactive. These isotopes are easily identified by their activity. In either case, isotopes can be used to trace the various steps in chemical reactions and in metabolic processes.

The age of any object made from once-living matter, such as wood, can be determined using the natural radioactivity of . All living plants absorb carbon dioxide ( ) from the air and use it to synthesize organic moleciles. The vast majority of these carbon atoms are , but a small fraction, about , is the radioactive isotope .The ratio of to in the atmosphere has remained roughly constant over thousands of years, in spite of the fact that decays with a half-life of about 5730 yr. The ratio of the two isotopes within the living organism thus remains nearly constant as well. When a organism dies, carbon dioxide is no longer absorbed and utilized. Because the decays radioactively, the ratio of to in a dead organism decreases over time.

C146

The decay of , because of its long half-life of years, is useful in determining the ages of rocks on a geologic time scale.

C146

2CO

C126 C126C146

C146 C146C126

U23892

9104.5

-12101.3C126

重力電磁力 強作用力

弱作用力

希格斯玻色子 (Higgs Boson)

夸克 玻色子

夸克

大強子對撞器 （ Large Hadron Collider)

To smash protons moving at 99.999999% of the speed of light into each other and so recreate conditions a fraction of a second after the big bang. The LHC experiments try and work out what happened.

安裝探測器 (CMS)

法國與瑞士的邊界

Uncertainty principle: paradise lost?

The classical Newtonian view of the world is a deterministic one.

According to quantum mechanics, the position and velocity of an object cannot even be known accurately at the same time, which is termed as uncertainty principle. Quantum mechanics allows us to calculate only the probability that an object will be observed at various space. Quantum mechanics says there is some inherent unpredictability in nature (Copenhagen interpretation).

• Movie:

The Mechanical Universe (disk 49)