Physics Edexcel Unit 2 - The Nature of Light

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What is Light?


It is an electromagnetic wave.

The waves oscillate perpendicuar to the propagation of the wave (direction of travel).

Light can be polarised, reflected, refracted, TIR (Total Internal Reflection), or diffracted.

Wave or Photon?

Light produces interference and diffraction patterns - alternating bands of dark and light. These patterns are only explained by using the wave theory as caused by 2 sources of light with the same wavelength.

- Bright bands = constructive interference (2 waves overlap in phase)

- Dark bands = destructive interference (out of phase and cancel out/anti-phase)

- But photoelectric effect changed this as it can only be explained by light acting as a particle. 

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Wave As A Particle

A photon is a quantum of electromagnetic radiation.

Planck investigated black body radiation and suggested that EM waves can only be released by discrete packets (quanta).

A single dinscrete packet of EM radiaion (light energy) = a quantum

E = hf = hc/wavelengh           (f=speed(c)/wavelength)     

So higher the frequency of EM radiation means mre energy in the packets (proportional)

Einstein went on to say that EM waves can only exist in discrete packets (photons)

Acting as a particle means that all or more of its energy is transferred when it interacts with another particle (like an electron)

Photon Energies

KE = eV;  KE (J) & eV (e=1.6x10-19C)(V=volts --> W=QV, so V=W/Q = J/C) so eV = CxJ/C = J and 1 Joule = 1.6x10-19eV (J->eV = /1.6x10-19 & eV->J = x1.6x10-19)

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Wave As A Particle Continued...

1 eV= kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt. (Accelerates when between 2 electrodes, transfers electrical potential energy to KE.)

Photons and Energy Levels

PHOTONS are released from electrons in atoms, which exist in discrete energy levels.

n=1 represents gound state (most stable are as low level as possible).

When electrons release a photon (emit energy) they move down an energy level.

When atoms are heated, electrons absorb energy (photon) and become excited, and move up an energy level.

Transitions are between definite energy levels (energy of each photon emitted can only take a certain allowed value. Energy carried by each photon is equal to difference in energies between 2 levels.

dE = E2 - E1 = hf = hc/wavelength

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Emission Spectra

  • When atoms are heated and the electrons are excited to higher energy levels by absorbing a photon (a packet of light energy/quantum of EM radiation)
  • Electrons immediately then fall back down to fill gap to get to lowest energy level possible (most stable)
  • When they fall back down they release a photon
  • Energy emitted = difference in energy levels
  • You can split light from a hot gas through a prism and get a line spectrum
    • Bright lines on black background
    • The lines correspond to specific wavelengths of light emitted by source
    • Only photon energies are allowed, so only see corresponding wavelengths
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Photoelectric Effect

  • Shine a light of a known frequeny onto a surface of metal (electrode) - cathode (-ve)
  • If it is above the threshold frequency it will emit electrons, usually in the UV range
  • Free electrons just under surface of metal absorb energy from light and vibrate 
  • So, a photon of light strikes surface of metal, hits electron, energy is transferred to it
  • If electron absorbs enough energy, bonds holding it to metal break, electron is released
  • Photoelectric effect - electrons are emitted as photoelectrons.

Einstein's Conclusions

  • Energy from incident EM radiation is transferred to free electrons in material, which liberates electrons. But no photoelectrons are emitted if frequency is below a certain value (0 or fo, threshold frequency)
  • AMOUNT of photoelectrons released is proportional to INTENSITY of incident radiation
  • Once fo is reached there is no delay in emission of photoelectrons
  • P.electrons emitted with varying kinetic energies from zero to maximum:- value of maximum KINETIC ENERGY increases with FREQUENCY of radiation (not intensity)
  • Therefore due to KE=1/2mv^2, SPEED of p.electrons = FREQUENCY of incident radiation
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Threshold Frequency

                                                    fo= 0/h AS 0=hfo

  • Where h is Planck's constant; fo is the threshold frequency; 0 is the work function (J) - minimum energy required to liberate an electron
  • If not, the electron will just vibrate then release the gained energy as a photon
    • hf >or= 0 is the amount of energy to be liberated from the surface of metal

Maximum Kinetic Energy

  • hf = 1/2mv(max)^2 + 0; as hf = an energy
  • OR Ek = Ep - 0 & Ep = Ek + 0
  • Energy transferred to electron = hf = Ep
  • Ke carried by electron when it leaves metal is... hf (Ep) - energy lost on way out (0)
    • '- (0)' Because it uses the energy of the work function to liberate itself from the atom.
  • 0 is the minimum amount of energy lost
  • Electrons closer to surface of metal at start are ejected with ^velocities than lower e-'s
  • Energy used up by lower e-'s to get to surface before emission = less energy for speed
  • E=hf (^f = ^E = faster). Energy after liberation remains as the p.electron's Ke for velocity
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Photoelectric Effect - Further

  • Light of known frequency shone onto metal electrode (usually -ve cathode)
  • P.electrons travel (attracted/repelled) to collecting electrode - flow around circuit = current
  • Max Ke of ejected electrons can be found be appling pd (reverse pd) acress electrodes
    • Makes collecting electrode negative wth respect to the emitting electrode (due to flow of electrons which is -ve --> +ve)
  • As pd is increased (potential divider) there is a decrease in the number of electrons reaching the collecting electrode from emitting electrode as not many electrons have sufficient energy to overcome the potential barrier
  • Eventually a pd is reached that even most energeic electrons fail to reach collecting electrode - this is the STOPPING POTENTIAL - Vs or Vstop
  • This measured value of Vstop gives the max. Ke of electrons (value at which the fastest electrons are stopped)
    • 1/2 mv(max)^2 = eVstop
  • Could Use: 1. varying frequency of incident radiation
  •                    2. use spectrometer as source of incident light
  •                    3. Filters admiting narrow range of frequencies
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  • Spectrometer separate wavelengths in a beam of radiation to show wavelength present
  • 2 types of spectra produced: Line or continuous
  • These are way in which electrons in matter are able to radiate energy = emission spectra
  • Gases = convenient sources of line spectra
  • -- low pressures
  • -- in electric discharge tubes
  • -- and substances heated in blue bunsen flame
  • Line spectrum from a sample of an element is characteristic of that element - identifies

Continuous Spectra

  • Can be observed using light from tungsten filament lamp
  • First produced by Newton using prism
  • Cannot identify source
  • Wavelength of maximum intensity of spectrum is linked to the temperature of the source
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Spectra Continued...

Absorption Spectra

  • Light from tungsten filament lamp passes through vapourised sodium atoms
  • This produces 2 black lines on light spectrum NOT 2 yellow lies on black background
  • Due to absorption of the light by sodium atoms at exactly same wavelength
  • Atoms move up energy levels (transition of electron within atom)

Solar Spectrum

  • emission of light from the sun/absorption
  • Absorption: dark lines in spectrum of sunlight (visible) = solar spectrum
  • Strong absorption at wavelength corresponding to emission spectra of hydrogen and helium
  • --- present in mantle of hot gas around sun
  • --- led to discovery of helium 50 years later
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Energy Level Diagrams

  • Photons with particular wavelength represents a fixed amount of energy
  • Each line in emission spectra corresponds to an atom losing a fixed amount of energy given out as a photon of light
  • Largest energy change in spectrum = electron moving from (n=infinity) most energetic state  -> least energetic state (ground state, where n=1)
  • An atom is ionised when it absorbs enough energy to reach n = infinity --- ionised from ground state (n=1) to n= infinity, is the energy change
  • Photon with highest energy in a spectrum (shortest wavelength) corresponds to free electrons moving to LOWEST energy level (emission of energy)
  • Remember: E=hf = hc/wavelength
  •  Lowest wavelength = smallest energy change in transitions
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Energy Level Diagrams Continued...

  • These are transitions between intermediate energy levelss
  • E.g. spectral line with longest wavelength (least energy) from transition involving ground state would be between n=1 and n=2
  • Other transitions: e.g. n=4 to n=2, wavelength produced will still correspond to Edifference between 2 levels.
  • Absorption onlly occurs for fixed wavelength of light.
  • -- Atom absorbs energy + electrons move up (excited) energy levels.
  • -- Amount absorbes corresponds to energy different between levels. 

USE: determine the chemical composition of stars.

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Franck & Hertz

  • Tube containing low-pressure mercury vapour = triode valve
  • Found in radio sets until the transistor was invented
  • Electrons from cathode accelerated towards grid
  • Continue towards anode (+ve) - flow back to power supply (via galvanometer)
  • Small stopping potential (2V) applied to grid and anode (i.e. anode has potential of about 2V less than grid)
  • As accelerating potential (cathode and grid) increases, the current increases through tube, reaching max. of 4.9V
  • Then decreases sharply
  • Then increase accelerating potential = 9.8V --> falls
  • Then increase accelerating potential = 14.7V --> falls
  • As peaks occured, mercury glowed
  • Energy levels - below 4.9V electrons collided elastically with mercury atoms in path
  • Above 4.9V of accelerating potential - electrons underwent inelastic collisions with mercury atoms - showing just enough energy to escite an electron to higher energy level in a mercury atom. In process lose all of own energy.
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Franck & Hertz Continued...

  • As accelerating potential increased above 4.9V, electrons had some energy remaining after an inelastic collision that could overcome stopping potential between grid and anode.
  • Peaks at 9.8V and 14.7V corespond to electrons having sufficient energy to excite 2 and 3 mercury atoms respectively
  • Similar arrangement can be used to measure ionisation energy of atom
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Laser Light

  • Laser light: is emitted when atoms undergo similar energy transitions at same time
  • Achieved: Promote large number of atoms to an energy level above ground state
  • As electron in one of excited atoms jumps down from higher energy level it emits photon
  • As photon passes another excited atom, it causes electron in atom to jump down to a lower level as well
  • Passage of light encourages (stimulates) the emission of radiation from other atoms - producing intense, coherent beam of light

In tungsten filament lamp:

  • Atoms emit light as they move from higher to lower energy levels independently = jumbled combination of waves

Solar Cells (Photovoltaic Cells):

  • Convert solar (light) --> electrical energy (But are they too expensive/inefficient?!)
  • Unlimited and free from sun without emitting greenhouse gases - fossil fuels = CO2
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Radiation Flux

  • Radiation flux is power per unit area
  • 2 ways to increase amount of of sunlight on solar cell:
  • 1) Increase power of light (put on stronger sunlight)
  • 2) Increase surface area of cell - could do this by physically increasing or angling the cell so that larger amount of surface faces sun
  • Amount of light falling on an area facing (at right angle to) the sun is radiation flux
  • So rad. flux is the power of light / area:
  • Equation:
  • Rad.Flux. (Wm^-2) = Power (W)
  •                                    Area (m^2)

Future Photovoltaic Technology:

  • Electrolysis (at lower temps - solid silica = silicon for solar cells --> reduces production costs
  • Thin film = 99% less silicon
  • Replace silicon with inexpensive plastics (conductive polymers)
  • Use chemicals that absorb other wavelengths of light - only 23% of atmosphere.
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  • How well a device converts energy you put in into energy you want out.
  • E.g. Solar (light) into electrical energy
  • Efficiency (%) = Useful energy/power out  x 100
  •                              Total energy/power in
  • Can use power or energy as they are proportional due to Power=Energy/time
  • You can use this with radiation flux - working out power from a given rad.flux. and area.
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Solar Cell Effieiency

Solar Cell Efficiency

  •                     FOR                                                               AGAINST                               
  • Energy input is free                                        Expensive manufacture:- power stations
  •                                                                        give out much more power for amount
  •                                                                        money
  • Energy is unlimited - fossil fuels are not        Inefficient:- 14%-19% little energy
  •                                                                        generated
  • Non-polluting - apart from manufacture         Output (efficiency) depends on weather
  •                                                                        so are quite unreliable

Solar Cells Can Power Remote Sensors:

  • Such as space probes that can't easily refuel but sunlight is always available
  • Allows the studying of difficult/dangerous places
  • Faster/cheaper than humans
  • Doesn't disturb area (so not influence data)
  • Satellite can also record data for long time.
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Really helpful notes for revision :)

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