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Flashcards in Mod 7 Nature of Light Deck (32)
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What does a red shift mean

Star is moving away from us


What does a blue shift mean

Star is moving toward us


How did Thomas Young's double slit experiment support one of the proposed models of light

  • Experiment demonstrated an interference pattern of bright and dark bands formed by constructive and destructive interference
  • This is a wave phenomenon
  • Concluded light had a wave nature and supported wave model of light


Summarise Fizeau time of flight measurements of speed of light

  • A beam of light is passed through the gaps of a rotating cog wheel to reach a mirror 8km away
  • As the beam travels to the mirror and back the wheel continues to rotate
  • Angular velocity of the wheel is adjusted until the returning light is completely blocked (by cog 2 below) and does not reach the target
  • Using known angular velocity of the cog wheel the time it took to rotate half a turn is calculated as shown by equation
  • dt = dtheta/w
  • Time it takes for cog wheel to rotate half the distance between cogs is caclulated 
  • v = 16000m/dt


Two classical models of light

  • Huygens wave
  • Newtons particle


What does youngs double slit experiment show

  • Diffraction and interference
  • Support wave model
  • Constructive light bands
  • Destructive light bands


What does polaristaion show

  • Light is a wave


What are the two proposals of Maxwells theory

  1. Light is an electromagnetic oscillation
  2. Light is a self sustaining mutual generation of electric and magnetic fields


What is condition for production of em wave 

  • Acceleration of a charged particle


What did Hertz do

  • Supported Maxwell's predictions 
  • Showed that EM waves were reflected, interefered, polarised and their speed


Hertz experiment

  • To generate and detect EM waves
  • use of transmitter (induction coil), parabolic reflectors and receiver (electrodes)
  • Refelction shown as sparks detected at receiver upon reflecting off zinc plates
  • Intereference shown as standing waves were created
  • Speed calculated using wavelength from standing waves and frequency from paramters of circuit Nodes and anti nodes
  • Polarisation shown as sparks only detected when receiver is parallel to plane of transmitter not perpendicular


What did classical theory predict of black body's

  • Intensity of peak radiation should be higher for higher frequencies of radiation
  • Curver would continue to increase with frequency (no peak)


What was Plank's hypothesis

  1. The energy levels of oscillating atoms that emit EMR is dicrete or quantised --> amount of energy atoms can lose or gain is discrete
  2. frequency of EMR is emitted proportional to the energy released E = hf --> to emit higher frequencies an atom must release a larger amount of energy


What did Max Planck's hypothesis lead to

  • Chnage in energy of atoms determines the frequency of the quantum 
  • Changes in energy levels corresponding to high frequency radiation (UV, X-ray and gamma) do not occur in atoms 
  • No high frequency radiation is emitted (UV catastrophe)
  • Certain changes in quantised states are more probable at given temperatures 
  • Most energy is emitted at this frequency peak radiation


What are two charcteristsics of a black body

  • Perfect emitter 
  • Perfect absorber


How to spectra of discharge tubes relate to Planck's theory

  • Planck proposed that the energy of radiation could only increase by fixed amounts that depended on the frequency, given by E = hf.
  • This meant that atoms emitting radiation at a specific frequency had to lose fixed amounts of energy, and hence occupied discrete energy levels.
  • The emission spectrum of hydrogen shows that only specific wavelengths are emitted. This means every time a hydrogen atom loses energy by emitting radiation at a wavelength
  • it loses a fixed amount of energy E = hc/lambda
  • Hence spectra indicate that the energy of the atoms emitting radiation changes by discrete amounts, which is consistent with Planck’s hypothesis


How is max KE of photolectrons determined

  • Kmax = hf - phi
  • qVs = hf - phi
  • Where Vs is stopping potential


What are significant features of photoelectric graph

  • x intercept is threshold frequency
  • y intercept is work function
  • gradient is h
  • hence all metals are parallel since gradient is h


What was Einsteins proposal for light:

  • Light or any em wave is a stream of particles known as photons
  • Each photon has energy E given by E = hf


Using photon theory how do we interpret frequency and intensity

  • Frequency is related to amount of energy each photon carries
  • Intensity is related to the number of photons


How does Einstein explain instantaneous emission time of photo electrons

  • They absorb entire photon in single 1:1 interaction
  • NOT gradually abosorbed as predicted by wave model


What is nature of light

  • Dual wave and particle nature


What us doppler broadening

  • Spectral lines are broadened due to a stars rotation, simulatenous blue and red shifts


What is diference between absorption and emission spectra

  • Absorption light is shone on a cold gas and it wavelengths that is not transmitted are absorbed by element
  • Emission spectra is heating hot gas and wavelengths that are transmitted have been emitted by element


What are Einsteins two postulates

  1. The laws of physics are the same in all inertial frames of reference
  2. The speed of light in vacuum has same value in all inertial frames of reference


What's Einsteins first thought experiment

  • Mirror on train moving at speed of light
  • Would he see reflection
  • If NO he would know that he was in inertial frame of reference. Violating principle of relativity
  • If YES, light would travel at its normal speed relative to train, however light inside the train would be travelling at twice its usual speed relative to staionary observer
  • Concluded YES, since principle of relativity can never be violated, stated that speed of light is same for any observer and upheld this as basic law of physics
  • Hence decided that speed = distance/time stationary and moving observer must percirve space and time differently


What is evdidence of constant speed of light

  • Michelson Morley experiment 
  • De sitter double star experiment 


What is evdidence for time dilation and length contraction

  • Muons 
  • Hafele Keating experiment


Michelson Morley experiment summary

  • Deisgned to determine the relative velocity of the Earth through the Aether (Aether wind)
  • Used Michelson interferometer
  • Expected that if Aether wind did exist the two split beams of light would recombine and interfere because of differences in phase
  • Rotating apparatus interference pattern was expected to change since light would be travelling at different angles relative to aether
  • No change in pattern 
  • Proved Earth was not in relative motion to aether
  • Einstein proposed light did not need medium to propagate


How do Muons provdie evidence for time dilation and length contraction

  • Unstable elementary particles
  • Charge equal to electron mass 207 times greater
  • Produced by absorption of cosmic radiation high in the atmosphere and have a lifetime of 2 microseconds
  • Assume speed is close to v = 0.998c particles could only travel distance of 600m before they decayed
  • Hence could not reach Earth from upper atmosphere where they are produced but experiments show a large number of muons reach the Earth
  • Time dilation explains this 
  • Muons experience contracted length
  • Observers experience dilated time