Electromagnetism and Quantum Phenomena

• The electron beams are focused by magnets just like the lenses on a microscope.
• The photoelectric effect is a phenomenon where electrons are knocked off reactive metals when exposed to light of a certain frequency.
• The concept of wave-particle duality was the start of modern physics in the middle to late Nineteenth Century.
• Light shows wave properties such as reflection, refraction, diffraction, and polarisation.
• Light can also be shown to have particulate properties.
• The photoelectric effect can be demonstrated with apparatus like a charged electroscope and a reactive metal exposed to light of different wavelengths.
• The results of the photoelectric effect show that electrons are being knocked off reactive metals, with the amplitude of the light wave not being important.
• Red light only worked for caesium, a very reactive metal.
• There is a threshold frequency at which the phenomenon of the photoelectric effect starts to occur, with light waves with a frequency higher than this always showing the effect, whatever the brightness; light waves with a lower frequency never showing it.
• The more reactive the metal, the lower is the threshold frequency.
• The findings of the photoelectric effect led to the notion of light being tiny little packets of wave energy called photons.
• Max Planck's work in 1900 produced the Photon Model of Electromagnetic Radiation.
• Light and other electromagnetic radiation is emitted in bursts of energy, which are quantised and travel in straight lines.
• Energy of Photon = work done to remove electron + kinetic energy of the electron.
• Ek is the maximum kinetic energy, which is the charge × stopping voltage, and is the kinetic energy of the fastest electrons.
• The maximum kinetic energy is dependent only on the frequency, not the intensity.
• A more intense beam produces more photons per second, but each photon has the same energy.
• The work function of any metal can be worked out by plotting the maximum energy against the frequency.
• The gradient of the graph is constant, regardless of the metal, and is Planck’s constant, h.
• Atoms can interact with photons of lower energy than is required to remove electrons from them.
• The photons we looked at in the photoelectric effect could remove the electrons from very reactive metals like caesium.
• Photons can interact with other atoms to give them extra energy, which makes them excited.
• When a gas is ionised, one or more outer electrons are ripped off, turning the molecule positive.
• The molecule will recombine with an electron and lose energy, giving that energy in the form of a photon.
• Other atoms may not have been ionised, but are still in a very excited state.
• The electrons have moved to a higher energy level due to the interaction with the photon.
• About a microsecond later, the electrons lose their energy as a photon and return to the stable state, called the ground state.
• Electrons can only exist at permitted energy levels.
• If we look at a spectrum of hydrogen, we find lines at several discrete wavelengths, each representing the energy of a photon as the electron makes a transition from a higher energy level to a lower.
• The electron does a job of work in releasing a photon; it has lost potential energy.
• The highest energy level is where ionisation occurs.
• The lowest level is the ground state.
• Electrons can make transitions from any energy level to any other, giving us photons in the visible spectrum.
• The ground state of hydrogen is at –13.6 eV.
• When an atom emits a photon, its energy changes by an amount equal to the photon energy.
• The energy changes are discrete amounts or quanta.
• The frequency of the light and the energy are related by a simple equation: E = hf.
• The electron volt (eV) is the amount of energy used when a charge of electronic charge passes through a potential difference of 1 volt.
• The charge on an electron is 1.6 × 10-19 C, so 1 eV = 1.6 × 10-19 J.
• Albert Einstein developed the theory further to study how atoms interacted with photons.