The dual nature of light

1
The dual nature of light

• wave theory of light explains most phenomena
  involving light
• propagation in straight line
• reflection
• refraction
• superposition, interference, diffraction
  
• polarization
• Doppler effect
• wave theory does not explain
• frequency dependence of thermal radiation
• photoelectric effect
• IS LIGHT A WAVE OR A PARTICLE?
• answer it is both, depending on what
  question you ask it has a wave' aspect and a
  particle aspect
• Note according to quantum theory,
  particles (e.g. electrons, protons,..) have
  also a wave aspect! (depends on what question
  you ask)

2
Thermal radiation

• experimental observations
• atoms of a hot solid emit radiation
• increase in temperature ? more radiation, and
  component of maximum intensity shifted towards
  higher frequency (shorter wavelength)
• classical explanation the hotter the
  solid, the more vibrational energy ? higher
  frequency of vibration of atoms/electrons ?
  higher frequency of radiation
• but frequency spectrum of this radiation (black
  body radiation calculated within framework of
  electromagnetism and thermodynamics did not agree
  with measured spectrum
• predicted ultraviolet catastrophe I ? f4
• Max Planck's hypothesis (1900) energy is
  quantized oscillators (oscillating atoms)
  can only have certain amounts of energy
• relation between energy and frequency of
  oscillator E h f, where h
  Plancks constant 6.63x10-34 Js
• calculation of black body spectrum using Planck's
  hypothesis gives formula (Planck formula) which
  describes measured spectra.
• first evidence that energy is quantized

3
Photoelectric effect

• (first observed by Heinrich Hertz in 1887)
• electrons are emitted when certain metallic
  materials exposed to light (now used in
  photocells in cameras, and solar energy cells)
  
• some aspects of photoelectric effect could not be
  explained by classical theory
• classical theory if light continuos flow of e.m.
  energy takes some (calculable) time for wave to
  supply sufficient energy for electron to be
  emitted
• find experimentally current flows almost
  immediately upon exposure to light
• classical theory light of any frequency could
  cause photoelectric effect - need only sufficient
  intensity
• find experimentally only light with frequency
  above certain minimum frequency causes electrons
  to be emitted
• classical theory energy of electrons depends on
  light intensity
• find experimentally energy of electrons depends
  on frequency
• Albert Einstein's explanation
• assume that not only energy in atoms is
  quantized, but also energy carried by light
• light comes in packets of energy called light
  quanta or photons
• energy of one photon h f, where f frequency
  of the light.
• with this assumption, all aspects of
  photoelectric effect could be explained
• photon energy vs color of light E hf hc/?
  ? blue
  light has more energy than red light