EM radiation

• The photoelectric effect is the emission of electrons from a metal surface when light above a certain frequency, known as the threshold frequency, is shone on it.
• The photon model of light suggests that EM waves travel in discrete packets called photons which have an energy equal to planks constant multiplied by frequency.
• The wave theory of light could not explain the photoelectric effect as it suggests that any frequency of light should be able to cause photoelectric emission as the energy absorbed by each electron will increase with each oncoming wave. However, it can be explained by the particle (photon) model as that suggests that energy is proportional to frequency and that each electron can only absorb a single photon.
• Above the threshold frequency, increasing the intensity of light on a metal surface will increase the rate of photoelectron emission.
• The work function of a metal is the minimum energy required for electrons to be emitted from the surface of a metal.
• The stopping potential is the potential difference needed to be applied across the metal to stop the photoelectrons with the maximum kinetic energy. The maximum kinetic energy equals the charge of an electron multiplied by the stopping potential as this can be derived from E= Q x V.
• The photoelectric equation is E = work function + maximum kinetic energy.
• The energy of a photon can be related to its frequency and wavelength by E= h f = (h c)/wavelength.
• Electrons in atoms can only exist in discrete energy levels and can gain energy via collisions with free electrons or the absorption of a photon and be excited (move up to a higher energy level) or be ionised (removed from the atom entirely).
• If an electron becomes excited, it will quickly return to its original energy level (the ground state) and will release the energy it gained in form of a photon.
• In a fluorescent tube, a high voltage is applied which accelerates free electrons to collide with mercury vapour with enough energy to excite some of its electrons. When they de-excite, they release photons mainly in the UV range, The phosphorus fluorescent coating on the inside of the tube absorbs the UV photons, causing electrons to be exited. When these de-excite, they release visible light photons.
• An electron volt is the energy gained by one electron when accelerated through a potential difference of one volt. One eV is 1.6 x10^-19 joules.
• Spectra can be used to find the difference in energy between energy levels in an atom by finding the wavelengths of the photons absorbed and emitted.
• Emission spectra are produced by hot gas by photons released through de-excitation and only shows specific wavelengths of light while absorption spectra are produced when when white light is shone through cool gas by absorption of specific wavelengths and shows a whole spectrum but with black lines for absorbed wavelengths.
• Light can be shown to have both wave properties: diffraction and interference and particle properties: photoelectric effect.
• Wave - particle duality is when something exhibits both wave and particle properties under the same conditions.
• De Broglie hypothesised that if light was shown to have particle properties, then particles should also have wave properties. The De Broglie wavelength of a particle is h/m v.