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Yr12 Physics > Mod 7 Nature of Light > Flashcards

Mod 7 Nature of Light Flashcards

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1
Q

What does a red shift mean

A

Star is moving away from us

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2
Q

What does a blue shift mean

A

Star is moving toward us

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3
Q

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

A
  • 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
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4
Q

Summarise Fizeau time of flight measurements of speed of light

A
  • 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
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5
Q

Two classical models of light

A
  • Huygens wave
  • Newtons particle
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6
Q

What does youngs double slit experiment show

A
  • Diffraction and interference
  • Support wave model
  • Constructive light bands
  • Destructive light bands
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7
Q

What does polaristaion show

A
  • Light is a wave
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8
Q

What are the two proposals of Maxwells theory

A
  1. Light is an electromagnetic oscillation
  2. Light is a self sustaining mutual generation of electric and magnetic fields
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9
Q

What is condition for production of em wave

A
  • Acceleration of a charged particle
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10
Q

What did Hertz do

A
  • Supported Maxwell’s predictions
  • Showed that EM waves were reflected, interefered, polarised and their speed
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11
Q

Hertz experiment

A
  • 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
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12
Q

What did classical theory predict of black body’s

A
  • Intensity of peak radiation should be higher for higher frequencies of radiation
  • Curver would continue to increase with frequency (no peak)
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13
Q

What was Plank’s hypothesis

A
  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
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14
Q

What did Max Planck’s hypothesis lead to

A
  • 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
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15
Q

What are two charcteristsics of a black body

A
  • Perfect emitter
  • Perfect absorber
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16
Q

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

A
  • 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
17
Q

How is max KE of photolectrons determined

A
  • Kmax = hf - phi
  • qVs = hf - phi
  • Where Vs is stopping potential
18
Q

What are significant features of photoelectric graph

A
  • x intercept is threshold frequency
  • y intercept is work function
  • gradient is h
  • hence all metals are parallel since gradient is h
19
Q

What was Einsteins proposal for light:

A
  • Light or any em wave is a stream of particles known as photons
  • Each photon has energy E given by E = hf
20
Q

Using photon theory how do we interpret frequency and intensity

A
  • Frequency is related to amount of energy each photon carries
  • Intensity is related to the number of photons
21
Q

How does Einstein explain instantaneous emission time of photo electrons

A
  • They absorb entire photon in single 1:1 interaction
  • NOT gradually abosorbed as predicted by wave model
22
Q

What is nature of light

A
  • Dual wave and particle nature
23
Q

What us doppler broadening

A
  • Spectral lines are broadened due to a stars rotation, simulatenous blue and red shifts
24
Q

What is diference between absorption and emission spectra

A
  • 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
25
Q

What are Einsteins two postulates

A
  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
26
Q

What’s Einsteins first thought experiment

A
  • 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
27
Q

What is evdidence of constant speed of light

A
  • Michelson Morley experiment
  • De sitter double star experiment
28
Q

What is evdidence for time dilation and length contraction

A
  • Muons
  • Hafele Keating experiment
29
Q

Michelson Morley experiment summary

A
  • 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
30
Q

How do Muons provdie evidence for time dilation and length contraction

A
  • 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
31
Q

Outline how HERTZ calculated the speed of EM waves

A
  • In order to calculate the speed of the waves you require the speed and frequency of the waves, v = f(lamda)
  • Hertz calculated the frequency of oscillation of the sparks (and the EM wave) from the known paramters of the circuit
  • Hertz then formed a standing wave of electromagnetic radiation by reflecting the radio waves from a large flat zinc plate
  • Hertz moved his reciever coil along the wave. A spark in the gap was produced at the anti nodes (contructive interference) but not at the nodes (destructive interference)
  • The distance between consecutive anti-nodes (or nodes) is half a wavelength, doubling this distance gave a full wavelength
  • Using the wave equation, he calculated the speed of the waves as 3 x 108ms-1 as predicted by Maxwell
  • He varied the frequency (and thus wavelength) of the waves and repeated teh experiment, he obtained the same value for the speed of the waves. (reliability/precision)
32
Q

Describe HERTZ main experiental set-up

A
  • High voltage source connected to a transmitter in the shape of a circle
  • Transmitter either produces waves off the transmitter which bounce off parabolic reflectors onto reciever
  • Transmitter produces a sine wave which then bounces off a zinc plate and returns to transmitter

Yr12 Physics (5 decks)

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