Lecture 12: Waves versus particles

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Lecture 12 Waves versus particles
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Corpuscular Theory of Light (1704)

• Isaac Newton proposed that light consists of a
  stream of small particles, because it
• travels in straight lines at great speeds
• is reflected from mirrors in a predictable way

Newton observed that the reflection of light from
a mirror resembles the rebound of a steel ball
from a steel plate
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Wave Theory of Light (1802)

• Thomas Young showed that light is a wave, because
  it
• undergoes diffraction and interference (Youngs
  double-slit experiment)

Thomas Young (1773-1829)
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Particles

• Position x
• Mass m
• Momentum p mv

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Waves

• Wavelength l
• Amplitude A
• Frequency f
• number of cycles per second (Hertz)

f c / l
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Waves versus Particles

• A particle is localised in space, and has
  discrete physical properties such as mass
• A wave is inherently spread out over many
  wave-lengths in space, and could have amplitudes
  in a continuous range
• Waves superpose and pass through each other,
  while particles collide and bounce off each other

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Diffraction
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Interference
applet
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Interference Fringes on a Screen
applet
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Blackbody Radiation

• A blackbody is an object which totally absorbs
  all radiation that falls on it
• Any hot body (blackbodies included) radiates
  light over the whole spectrum of frequencies
• The spectrum depends on both frequency and
  temperature

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Spectrum of Blackbody Radiation
Plot of intensity of the blackbody radiation
versus wavelength for various temperatures
applet
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Ultraviolet Catastrophe
Classical theory predicts a graph that deviates
from experimental data, especially at short
wavelengths
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Plancks Quantum Postulate (1900)

• A blackbody can only emit radiation indiscrete
  packets or quanta, i.e., in multiples of the
  minimum energy E
  hfwhere h is a constant and f is the frequency
  of the radiation

Max Planck (1858-1947) is generally regarded as
the father of quantum theory
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Plancks Quantum Postulate (contd)

• Thus, it is harder for a blackbody to emit
  radiation at short wavelengths (high frequency)
  
• since higher energies are required to produce
  each quanta of radiation, by Plancks formula
• This explains the origin of the ultraviolet
  catastrophe

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Plancks Constant

• Experimentally determined to be
  h 6.63 x 10-34 Joule sec(Joule kg m2 /
  sec2)
• A new constant of nature, which turns out to be
  of fundamental importance in the new quantum
  theory

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Photoelectric Effect
When blue light is shone on the emitter plate,a
current flows in the circuit
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Photoelectric Effect (contd)
But for red light, no current flows in the circuit
video clip
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Experimental Observations

• Only light with a frequency greater than a
  certain threshold will produce a current
• Current begins almost instantaneously, even for
  light of very low intensity
• Current is proportional to the intensity of the
  incident light

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Problems with Wave Theory of Light

• The wave theory of light is unable to explain
  these observations
• For waves, energy depends on amplitude and not
  frequency
• This implies that a current should be produced
  when say, high-intensity red light is used

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Einsteins Explanation (1905)

• Light consists of particles, now known as
  photons
• A photon hitting the emitter plate will eject an
  electronif it has enough energy
• Each photon has energy E
  hf(same as Plancks formula)

Albert Einstein won a Nobel Prize for his work
on the photoelectric effect and not his theory
of relativity!
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Everyday Evidence for Photons

• Red light is used in photographic darkrooms
  because it is not energetic enough to break the
  halogen-silver bond in black and white films
• Ultraviolet light causes sunburn but visible
  light does not because UV photons are more
  energetic
• Our eyes detect colour because photons of
  different energies trigger different chemical
  reactions in retina cells

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Double-Slit Experimentto illustrate wave nature
of light
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Double-Slit Experiment with a machine gun!
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Double-Slit Experiment with electron gun
Electrons behave like waves!
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Interference Pattern of Electrons

• Determines the probability of an electron
  arriving at acertain spot on the screen
• After many electrons, resembles the
  inter-ference pattern of light

applet
Electron interference pattern after (a) 8
electrons, (b) 270 electrons, (c) 2000electrons,
and (d) 6000 electrons
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Double-Slit Experiment with electron gun and
detector
Trying to detect which slit the electrons pass
through causes them to behave like particles
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Summary

• Waves and particles exhibit very different
  behaviour
• Yet, light sometimes behaves like particles
• spectrum of blackbody radiation
• photoelectric effect
• And electrons sometimes behave like waves
• interference pattern of electrons
• In quantum theory, the distinction between waves
  and particles is blurred