Physics AS

Overview of particle physics

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Using TIR in optic fibres

A ray of light can travel along inside a solid glass fibre. Each time it reaches the outer surface of the glass it is reflected back inside, since i is nearly 90.

Practical optic fibres

Optic fibres are made from glass or plastic and surrounded by a cladding of material with a slightly lower refractive index.

Rays that travel straight down the centre of the fibre have the shortest route and take least time. Oblique rays have further to travel, and take longer.

Transmitting data

Optic fibres carry data in digital form. A ray of light from a laser is modulated at high frequency to encode the data. Problems arise if rays can travel along other paths inside teh fibre.

Advantages if optic fibres

They are used for:

  • Endoscopy
  • Telecommunications networks
  • Cable televison
  • Links between computers.

They can carry vastky more data than an electric current. They are very difficult to bug. Signals can travel for many kilometers before becoming weak.

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Line spectra

The atoms of a gas at low pressure may be excited by a flame or by a high-speed electrons. The excited gas atoms fall back to their ground state and emit photons of distinct energies corresponding to the differences between energy levels.

The atoms of any given element all have the same set of energy levels. This means that each element produces a unique line spetrum that may be used to identify the element.


Ionisation means giving an atom enough energy that an electron is totally removed from atom. This is called the ionisation energy and can be provided by heating, or by collision with a high-speed electron, or by the atom 'capturing' a photon of sufficent energy.

To remove an electron completely from an atom requires energy to overcome the force of attraction between the negative charge on the electron and teh positive charge on the nucleus.

Fluorescent lamps

A fluorescent light tube contains mercury vapout gas at low pressure, and two electrodes. A large number of electronsare released from the cathode and accelerated by the p.d between anode and cathode. Some of these electrons ionise gas atoms by collision, producing more electrons.

Some of teh electrons collide with gas atoms which become excited and emit photons of UV light as they fall back down to the gorund state. These photons in turn excite atoms of a coating material on the inside of the tube, which emit visible light. Because the coating atoms are close together, there are so many 'allowed' energy transitions that all colours of the visible spectrum are produced.

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Electron diffraction

  • Electrons can be diffracted-this shows that, when they pass through a fine grid, they behave like waves.
  • A beam of fast-moving electrons is produced in a cathode ray tube.
  • The electorn beam passes through a thin layer of crystalline graphite.
  • A diffraction pattern of fuzzy, light-and-dark rings is produced on a screen.

To make the electrons go faster, increase the accelerating voltage. The diameter of the rings is decreased. This shows that the wavelength decreases as the electrons go faster.

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The electronvolt

1eV is the energy transferred when an electron moves through a p.d of 1volt.

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Alpha particle scattering

An atom is normally neutral-it contains equal amounts of positive and negative charge. The positive charge is concentrated in the tiny nucleus at the centre. The negative charge is spread out around teh nucleus. Evidence for this comes form Rutherford's scattering.

  • A narrow beam of alpha particles was aimed at a thin gold foil.
  • An alpha particle is made up of 2 protons and 2 neutrons, so has a positive charge.
  • The direction of the alpha-particles after they have passed through the foil was determined using a detector.
  • most of teh alpha-particles went straight through.
  • A few were deflected more than 90 degrees.

Most alpha particles pass straight through gold foil, this shows that atoms are mostly empty space.
A few are deflected back towards the observer, this shows that positive charge is concentrated in a tiny volume-the nucleus.

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Photons of electromagnetic radiation

Sometimes electromagnetic radiation behaves as if it were made up of particles rather than waves. These particles are called photons.


High-frequency radiaton(such as gamma rays and X-rays) has the most energetic photons. Low-frequency radiation(such as radio waves and microwaves) has less energetic photons.

There is no such thing as an anti-photon; antimatter emits photons identical to those emitted by 'normal' matter.

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Annihilation and creation

When a particle meets its antiparticle, they may annilate one another. A photon of electromagnetic energy is produced; this is annihilation.

The same process can operate in reverse: if aphoton has enough energy, a particle and its antiparticle pair can be produced; this is pair production.

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Quarks and antiquarks

Isolated quarks have not yet been observered; they are always grouped together in twos and threes as hadrons. Quarks like leptons, are belived to be fundamental particles of matter.

  • A meson is made up of a quark and a antiquark.
  • A baryon is made up of 3 quarks, the antiparticle is when all fo the quarks are changed into antiquarks.
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Exchange particles

Two electrons repel one another because they both have negative electric charge. We picture this force being transmitted by a photon passing between them. The photon is the exchange particle that transmits the force.

1. Electromagnetic force is present in attraction and repulsion of charges, its exchange particle is a photon.

2. Weak nuclear force is invilved in beta decay exchange particles are W and Z bosons.

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