Modern Physics

1.The Nature of Light: Particles vs. Waves

  1. Young’s Double Slit Model
  2. In 1801, an English physicist named Thomas Young performed an experiment that strongly inferred the wave-like nature of light. Because he believed that light was composed of waves, Young reasoned that some type of interaction (interference) would occur when two light waves met. Since light shows interference patterns, it must be a wave.
  3. This model was accepted for a long time – until Einstein came along.

  1. Einstein’s Photoelectric Effect
  2. The photoelectric effect is one of the key experiments that supported early quantum theory.
  3. Light, prior to the early 20th century, was considered to be a wave phenomenon. In most ways, this idea reflects reality well—for instance, light is bendable when passed through a lens. The energy of a wave is given by amplitude of the wave squared, so a light wave of a certain frequency should be able to have any value for energy as long as there is a bright enough light source.
  4. The photoelectric effect is observed when electromagnetic radiation strikes the surface of a metal and the resulting energy transfer causes the metal to emit electrons.However, when red light was shone on a metal surface, no electrons were ejected even when the brightest red light sources were used. On the other hand, when blue light was shone on the same metal surface, electrons were ejected even when the source of light was weak (and brighter blue lights ejected more electrons).The energy didn’t seem to depend on the amount of light hitting the metal but instead the frequency of light that hit the metal.
  5. Max Karl Ernst Ludwig Planck was a German theoretical physicist whose work on quantum theory won him the Nobel Prize in Physics in 1918. He put us on the path leading out of this thicket of confusion when he theorized that light and other forms of energy comes in “packets” or discreet “bundles”. Light, in this theory, is considered to be a particle, which we now call a photon.
  6. Light can behave like a particle instead of a wave.
  7. The photoelectric effect was explained by Einstein when he conjectured that Planck’s bundles of energy (i.e. photons) were “knocking loose” the electrons—but only if the photons had enough energy to do the job (a two year old isn’t able to knock a football player off his feet, but a bull undoubtedly could).
  8. Einstein’s ideas gave further support to the theory that light energy really is not continuous with infinitely small increments of change (a wave), but is in fact “chunky.”
  9. His discovery led to thequantumrevolution in physics.
  10. The Nobel Prize in Physics 1921 was awarded to Albert Einstein "for his services to Theoretical Physics and especially for his discovery of the law of the photoelectric effect".

  1. Dual Nature of Light (Wave-Particle Duality)
  2. Light behaves as both a particle (photon) and a wave.
  3. Explains why light can travel in a vacuum (as a particle).
  4. Explains why light can produce an interference pattern (like a wave).

2.How do atoms give off a unique spectrum of light?

  1. Photons of light are emitted when electrons fall down from a higher orbit to a lower energy orbit and are absorbed when the electron jumps up to a higher orbit from a lower orbit.
  1. The wavelength of the light (frequency and color) is related to the energy of the jump.
  2. When the electrons of an element emit photons, an emission spectrum is formed.
  3. When the electrons of an element absorb photons, an absorption spectrum is formed.
  1. Each element has a unique pattern.

  1. To the right is shown the emission spectra of the first four elements in the periodic table: Hydrogen, Helium, Lithium, and Beryllium. The scale is in nanometers. Notice that the location, and pattern, of lines is unique to each element.

3.4 Fundamental Forces of Nature

  1. The Strong Nuclear Force is very strong, but very short-ranged. It acts only over (very small) ranges of order 10-13 centimeters and is responsible for holding the nuclei of atoms together. Since the range is so small, it is a limiting factor to the size of stable atoms.
  2. The Electromagnetic Force causes electric and magnetic effects such as the repulsion between like electrical charges or the interaction of bar magnets. It is long-ranged, but much weaker than the strong force. It can be attractive or repulsive, and acts only between pieces of matter carrying electrical charge.
  3. Weak Nuclear Force is responsible for radioactive decay (including Beta decay and Carbon dating) and neutrino interactions. It has a very short range and, as its name indicates, it is very weak.
  4. The Gravitational force is weak, but very long ranged. It is always attractive, and acts between any two pieces of matter in the Universe since mass is its source.

4.Nuclear Stability and Nuclear (Radioactive) Decay

  1. Nuclear Stability
  2. Since the protons in the nucleus repel each other, strongly because of their close proximity, there must also be an attracting force (called the strong nuclear force) which holds the nucleus together. This attracting force is very short range and virtually the same between neutrons and protons.
  3. The addition of neutrons in a nucleus tends to separate the protons and provide additional binding potential. There are, however, competing forces, and the balance between neutrons and protons for a stable nucleus is a complicated matter.
  4. Atoms with the same number of protons but different numbers of neutrons are called isotopes.
  5. In general, for heavy nuclei, a few more neutrons than protons are needed for a stable isotope.
  6. Quarks:
  7. Within the atomic nucleus, elementary particles called quarks carry charges 1/3 and 2/3 the magnitude of the electron’s charge.
  8. There are six flavors of quarks, named up, down, strange, charm, bottom and top.
  9. Each proton and each neutron is made up of three quarks.
  10. Since quarks always exist in such combinations and have never been found separated, the whole-number-multiple rule of electron charge holds for nuclear processes as well.

  1. The most common types of radioactive decay consist of Alpha (α), Beta (β-, β+), and Gamma (γ) particles, although other decay modes are not uncommon including the emission of neutrons.
  2. Alpha () Decay
  3. During alpha decay, an atom's nucleus sheds two protons and two neutrons in a packet that scientists call an alpha particle (it’s actually a He nucleus).
  4. Since the decaying atom loses two protons during alpha decay, it changes from one element to another.
  • Beta () Decay
  • Too many neutrons in the nuclei can result in radioactive decay with the emission of β- particles (electrons), where a neutron is essentially converted into a proton and an electron with the ejection of the electron. This process brings that nucleus back toward the line of stability.
  • Too many protons can result in the emission of a β+ particle (a positron), essentially converting a proton in the nucleus into a neutron. In this process, the element number would decrease by one, but the atomic mass would be essentially unchanged.
  • Since the number of protons changes, the atom will change from one element to another.
  1. Gamma () Decay
  2. The emission of γ particles (highly energetic photons) does not change the proton neutron ratio, but it does allow a nucleus in an excited state to transition to a less energetic state.
  3. Since the numbers of protons remains constant, the element does not change.
  1. Penetrating power of radiation
  1. Half-Life is the time it takes for half of a sample of radioactive material to decay. It doesn’t matter how much material there is, half of it will decay in the half-life time. See the graph below:
  1. Carbon Dating
  2. Carbon-14 is a naturally occurring radioactive isotope of carbon.
  3. Carbon-14 has 6 protons and 8 neutrons
  4. The most common isotope of carbon is carbon-12 and it has 6 protons and 6 neutrons.
  5. Carbon-14 has a half-life of about 5,700 years.
  6. How carbon dating works:
  7. Plants absorb carbon dioxide as a part of life processes. Some of that carbon dioxide contains carbon-14 and some contains carbon-12.
  8. Animals (including humans) eat the plants and absorb both the carbon-14 and the carbon-12.
  9. As soon as a living organism dies, it stops taking in new carbon. The ratio of carbon-12 to carbon-14 at the moment of death is the same as every other living thing, but the carbon-14 decays and is not replaced.
  10. The carbon-14 decays into nitrogen-14 while the amount of carbon-12 remains constant in the sample. By looking at the ratio of carbon-12 to carbon-14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of a formerly living thing fairly precisely.

5.Nuclear Fission, Fusion and Nuclear Energy including Einstein’s E = mc2 equation

  1. Fission is the splitting of a large atom into smaller atoms. The sum of the masses of the fission products will be less than the mass of the target nucleus (see below), releasing energy according to Einstein’s E = mc2 equation.
  2. Power plants and Atomic/Thermonuclear bombs use Nuclear Fission.
  3. The fission process also produces gamma photons and free neutrons, which can cause chain reactions.
  • Nuclear Chain Reactionsare series of nuclear fissions (splitting of atomic nuclei), each initiated by a neutron produced in a preceding fission.
  1. Fusion is when two smaller atoms fuse together to form a single larger atom. When the smaller nuclei are forced together, they will fuse with a yield of energy because the mass of the combination will be less than the sum of the masses of the individual nuclei.

That decrease in mass comes off in the form of energy according to the Einstein relationship.

Fusion of hydrogen atoms is how the Sun produces energy.

  1. Mass transforms into Energy
  2. During Fission, the original atom has slightly more mass than the combined mass of the resulting atoms after the split.
  3. During Fusion, the resulting atom has slightly less mass than the combined mass of the fusing atoms.
  4. The “lost” mass is actually transformed into energy according to Albert Einstein’s famous equation: E = mc2
  5. This mathematical equation shows the relationship between mass (m) and energy (E). The “c” in the equation stands for the speed of light, 3.0 x 108 m/s.
  6. Example: If you converted 1 kilogram of matter completely into energy, how much energy would you create?

E = mc2= (1 kg)(3.0  108 m/s)2= 9  1016 joules or 90,000,000,000,000,000 joules of energy

6.Atomic and Nuclear Phenomena in the medical field

  1. Nuclear medicine and radiology are medical techniques that involve radiation or radioactivity to diagnose, treat and prevent disease.
  2. Testing such as MRI, NMR, x-rays, etc., are safe and painless and don’t require anesthesia.
  3. Treatments for medical conditions such as cancer involve nuclear processes.

7.Quantum phenomena and its Applications

  1. Quantum mechanics is often the only tool available that can reveal the individual behaviors of the subatomic particles that make up all forms of matter (electrons, protons, neutrons, photons, and others).
  2. A great deal of modern technological inventions operates at a scale where quantum effects are significant. Examples:
  3. The most precise clocks in the world, atomic clocks, are able to use principles of quantum theory to measure time. Among other things, these clocks keep GPS systems in line.
  4. Lasers produce coherent beams of light in a very narrow range of wavelengths because quantum interference ensures the wavelengths are all uniform. Coherent light is where the electromagnetic waves maintain a fixed and predictable phase relationship with each other over a period of time.
  5. Diodes, transistors and microchips are indispensable parts of modern electronics systems and devices such as the Blu-Ray Player.
  6. The electron microscope and magnetic resonance imaging (MRI) are incredible technological tools that allow us to see things never before seen.
  7. The digital camera depends on the principles of the photoelectric effect.