Presentation on theme: "About these slides These slides are used as part of my lessons and shouldn’t be considered comprehensive There’s no excuse for not turning up to lessons!"— Presentation transcript:
1 About these slidesThese slides are used as part of my lessons and shouldn’t be considered comprehensiveThere’s no excuse for not turning up to lessons!These slides use material from elsewhere on the assumption of fair/educational useIf you own the copyright to any of this material and want it credited/removed please contact meThese slides may contain errorsUse at your own risk!
3 Wave-particle duality You should be familiar with the idea that light can be thought of as a wave or a particleThe particle is called a __________The energy of this particle is given by the equationE=hfWhere h is ________ constant and f is the frequency of the lightRewrite the equation in terms of wavelength, l
5 Evidence for wave-particle duality One piece of evidence that light can behave as a wave or a particle is the photoelectric effectThis was discovered by Hertz as a side effect of his work on radio wavesHe noticed that metals emit electrons when electromagnetic radiation of a certain frequency is directed at a metal
6 The photoelectric effect in action Incident Radiationelectrodegold leafzinc plateImage from tap.iop.org
7 Photoelectric effectThe photoelectric effect gives rise to the following observations:Emission of electrons only takes place above a certain threshold frequency of incident electromagnetic radiation. This minimum frequency is dependent on the metal.If light was a wave then electrons would be emitted at any frequency, as electrons would gradually gather energy from the waves regardless of their frequency
8 Photoelectric effect continued The photoelectric effect gives rise to the following observations:Electrons are emitted as soon as the source of electromagnetic radiation is switched on, no matter what the intensity of the radiation.If light was a wave then electrons would gather energy more quickly, and be emitted sooner, by high intensity light sources.
9 Photoelectric effect continued The photoelectric effect gives rise to the following observations:The number of electrons emitted per second is proportional to the intensity of the incident radiation, unless its frequency is less than the threshold frequency, in which case emission isn’t observed
12 Electron diffractionIf electrons were acting as particles we would see them either pass straight through the crystal, or bounce offInstead we see an interference patternExactly the same as if electrons are acting as wavesIt is possible to do the Young’s slits experiment and see an interference pattern even if the electrons are fired one at a time
13 de BroglieIn 1923 de Broglie was the first to suggest that matter could be a waveHe said that the de Broglie wavelength, l, of a particle is related to its momentum, p, by the equationWe know that momentum = mass x velocity so
14 Question Calculate the de Broglie wavelength of An electron moving at 2.0 x 107 ms-1A proton moving at the same speedI weigh 85kg and can run at 6 ms-1 what would my de Broglie wavelength be?
15 Question Calculate the momentum and speed of An electron that has a de Broglie wavelength of 500nmA proton that has the same de Broglie wavelength
17 The photoelectric effect We saw this last lessonAs long as light is above a certain frequency it can eject electrons from a metal surfaceThis can be explained as the photon having a certain energy, E = hfIt therefore follows that there is a minimum energy needed to eject an electron from a metal surface at zero potentialThis is the work function, f
18 Explaining the photoelectric effect In order to explain photoelectricity Einstein said that:Electrons must be absorbing a single photon in order to escape from the metalThis photon transfers energy given by E=hfIf the photon has less energy than the work function, f, of the metal the electron cannot escapeThe work function is the minimum energy required for an electron to escape the metal surface
19 Photoelectric effectWe can calculate the maximum kinetic energy of an electron ejected by a photon with energy hfEkmax = hf – fSo emission will take place as long as hf > fWhich means there is a threshold frequency above which an electron is emitted:
20 Plotting Ekmax against frequency Since EKmax = hf – f we can plot a graph of the form y = mx + cSo if we plot EKmax against f we get a straight lineEKmax /JWhat is the gradient?What is the intercept on the x axis?What would the intercept on the y axis be?f / Hz
21 QuestionsWhy does photoelectric emission only take place above a certain frequency of incident radiation?Calculate the frequency and energy of a photon of wavelength450nm1500nm
22 Questions The work function of a metal plate is 1.1 x 10-19J Calculate:The threshold frequencyThe maximum kinetic energy of electrons emitted from this plate when light of wavelength 520nm is directed at the metal surface
23 Electron volts We’ve already encountered the electron volt It is defined as the unit of energy equivalent to the work done when an electron is moved through a potential difference of 1 volt and is equal to 1.6 x 10-19J
24 Ionisation An ion is a charged atom Ionisation is the process of making an ion by adding or removing electronsThe energy to do this can be provided by electricity, heat, light or by collisions between alpha, beta or gamma radiation and an atom
25 Energy levels in atomsElectrons surrounding atoms are like standing wavesThey have certain allowed energy levelsIt is the movement between these levels that happens during excitationThe lowest energy state of an atom is its ground stateIf an electron moves into a higher energy level the atom is in an excited state
26 ExcitationIn excitation an electron can be moved from an inner shell to an outer shellThis only happens at specific excitation energies which are dependent on the atom and correspond to its excited statesThis is excitation by collision – one electron hits another and promotes it to a higher energy level
27 Excitation continuedThe excitation energy will always be less than the ionisation energyThere are a number of excitation energies for each atomThese correspond to the difference between different shellsElectrons can also ‘jump’ more than one shell at a time
28 Energy Levels Example: Mercury Ionised 0.00 eV-1.59 ev-1.60 eV-2.51 eV-2.71 eV-3.74 eV-4.98 eV-5.55 eV-5.77 eVeVThis is excitation by absorption of a photon. If it has exactly the right frequency it promotes the electron to a higher energy level
29 De-excitation The excited states of an atom are unstable They have a gap in a low energy electron shellElectrons from a higher shell can drop into this lower level, emitting a photon in the process
30 De-excitation Example: Mercury Ionised 0.00 eV-1.59 ev-1.60 eV-2.51 eV-2.71 eV-3.74 eV-4.98 eV-5.55 eV-5.77 eVeVThis is fluorescenceAn atom in an excited state can de-excite by emitting photons of the same or lesser energy than the one that excited itIn this case the photons have energies of 0.79 eV and 4.67 eV
31 Calculating the frequency of photons Since the photon emitted comes from the transition between two energy levels it will have an energy, E given by:E = E1 – E2Where E1 is the energy of the higher level and E2 the energy of the lowerWe also know that E = hf, so we can calculate the frequency of the emitted photon
32 Fluorescent TubesFluorescent tubes contain mercury vapour and have a fluorescent coating on the inner surfaceFilament ElectrodesMercury Vapour
33 Fluorescent Tubes When a tube is turned on: Ionisation and excitation of mercury atoms occurs as they collide with each other and with electronsPhotons are emitted at UV and visible frequenciesUV photons are absorbed by atoms in the fluorescent coatingCoating atoms de-excite and emit visible photonsA range of coatings is used to give a ‘white’ light
34 QuestionsIonised 0.00 eV-1.59 ev-1.60 eV-2.51 eV-2.71 eV-3.74 eV-4.98 eV-5.55 eV-5.77 eVeVHow much energy is needed, in Joules, to excite the atom to its highest excitation level?How many different energies could photons released by a mercury atom in the 5.46 eV excited state have?
35 QuestionAn atom absorbs a photon of energy 3.8 eV and subsequently emits photons of energy 0.6 eV and 3.2 eVSketch an energy level diagram to represent these changesDescribe what is happening to the electrons in the atom during this processWhat is the frequency of the photons emitted?
37 Spectral LinesSince each element has different energy levels, and therefore transitions between levels, each element will emit different frequencies (wavelengths) of electromagnetic radiationThis means the photons emitted are characteristic of that element