Waves versus particles
Waves versus particles
https://images.pond5.com/abstract-animated-background-spacefutiristic-lines-footage-000795912_prevstill.jpeg
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
Wave Theory of Light (1802) Thomas Young showed that light is a wave, because it – undergoes diffraction and interference (Young s double-slit experiment)
Particles Position x Mass m Momentum p (Mass x velocity)
Waves Wavelength λ Amplitude A Frequency f – number of cycles per second (Hertz)
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 wavelengths 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
Diffraction – bending of waves
Diffraction in everyday life
Diffraction in everyday life In photo cameras, circular apertures tend towards a polygon when set to high f-numbers (small apertures). Diffraction around edges causes diffraction spikes on photographs
Comparison of diffraction spikes for apertures of different shapes and blade count
Diffraction in everyday life
7 blades giving 14 spikes
Interference – superposition of waves
Interference in everyday life
Interference Fringes on a Screen
pattern of light & dark bands
Photoelectric Effect
When blue light is shone on the emitter plate, it knocks out electrons and a current flows in the circuit
Photoelectric Effect (cont d)
But for red light, no current flows in the circuit, even for high intensity light
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
Einstein s Explanation (1905)
Light consists of packets of energy (quanta), now known as photons Photon energy depends on frequency A photon hitting the emitter plate will eject an electron if it has enough energy Blue photons have higher energy than red ones, because blue light has higher frequency than red.
Albert Einstein won a Nobel Prize for his work on the photoelectric effect and not his theory of relativity!
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
Wave-particle duality Light has a dual nature. Sometimes it behaves like a wave, sometimes like particles. Do other quantum-scale (very small) objects exhibit wave-particle duality? E.g., may electrons behave like waves?
Double-Slit Experiment
to illustrate wave nature of light
Double-Slit Experiment with a machine gun!
Double-Slit Experiment with electron gun
Electrons behave like waves!
Interference Pattern of Electrons After many electrons shot, the pattern resembles the interference pattern of light
Electron interference pattern after (a) 8 electrons, (b) 270 electrons, (c) 2000 electrons, and (d) 6000 electrons
Double-Slit Experiment
with electron gun and detector
Trying to detect which slit the electrons pass through causes them to behave like particles
Double-Slit Experiment
with electron gun and detector
https://www.youtube.com/watch?v=DfPeprQ7oGc
Summary Waves and particles exhibit very different behaviour Yet, light sometimes behaves like particles (photoelectric effect)
And electrons sometimes behave like waves (interference pattern of electrons)
In quantum theory, the distinction between waves and particles is blurred Quantum objects appear to behave differently when observed than when unobserved.
https://images.pond5.com/abstract-animated-background-spacefutiristic-lines-footage-000795912_prevstill.jpeg
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
Wave Theory of Light (1802) Thomas Young showed that light is a wave, because it – undergoes diffraction and interference (Young s double-slit experiment)
Particles Position x Mass m Momentum p (Mass x velocity)
Waves Wavelength λ Amplitude A Frequency f – number of cycles per second (Hertz)
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 wavelengths 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
Diffraction – bending of waves
Diffraction in everyday life
Diffraction in everyday life In photo cameras, circular apertures tend towards a polygon when set to high f-numbers (small apertures). Diffraction around edges causes diffraction spikes on photographs
Comparison of diffraction spikes for apertures of different shapes and blade count
Diffraction in everyday life
7 blades giving 14 spikes
Interference – superposition of waves
Interference in everyday life
Interference Fringes on a Screen
pattern of light & dark bands
Photoelectric Effect
When blue light is shone on the emitter plate, it knocks out electrons and a current flows in the circuit
Photoelectric Effect (cont d)
But for red light, no current flows in the circuit, even for high intensity light
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
Einstein s Explanation (1905)
Light consists of packets of energy (quanta), now known as photons Photon energy depends on frequency A photon hitting the emitter plate will eject an electron if it has enough energy Blue photons have higher energy than red ones, because blue light has higher frequency than red.
Albert Einstein won a Nobel Prize for his work on the photoelectric effect and not his theory of relativity!
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
Wave-particle duality Light has a dual nature. Sometimes it behaves like a wave, sometimes like particles. Do other quantum-scale (very small) objects exhibit wave-particle duality? E.g., may electrons behave like waves?
Double-Slit Experiment
to illustrate wave nature of light
Double-Slit Experiment with a machine gun!
Double-Slit Experiment with electron gun
Electrons behave like waves!
Interference Pattern of Electrons After many electrons shot, the pattern resembles the interference pattern of light
Electron interference pattern after (a) 8 electrons, (b) 270 electrons, (c) 2000 electrons, and (d) 6000 electrons
Double-Slit Experiment
with electron gun and detector
Trying to detect which slit the electrons pass through causes them to behave like particles
Double-Slit Experiment
with electron gun and detector
https://www.youtube.com/watch?v=DfPeprQ7oGc
Summary Waves and particles exhibit very different behaviour Yet, light sometimes behaves like particles (photoelectric effect)
And electrons sometimes behave like waves (interference pattern of electrons)
In quantum theory, the distinction between waves and particles is blurred Quantum objects appear to behave differently when observed than when unobserved.
