WAVE-PARTICLE DUALITY
Katrina & Emily
History of Light Newton’s Particle Model: • Newton proposed that light is made up of extremely small particles that travel extremely fast • Proposed that light must have mass in order to explain some of its properties • Theory grew support because it can be used to explain various properties of light Ex. Reflection, Refraction, Dispersion
Newton’s Particle Model Reflection: The angle of incidence is equal to angle of reflection. • Observed when a particle collided with a surface. Ex. Ball thrown against wall
∠Incidence = ∠Reflection ϴi = ϴr
Newton’s Particle Model Refraction: Light bends when travelling from one medium to the next. • Theorized this was due to the attraction of particles to the molecules of the medium • Implied that light travels faster in water than in air, which was disproved in 1850 when the speed of light in water was first measured
Newton’s Particle Model Dispersion: The separation of light into its different colours (ROYGBV). • Theorized different colours of light were different sized particles, smaller ones deflected the farther than larger ones
However, Newton’s particle theory was unable to accurately explain all properties of light.
History of Light Huygens’ Wave Model: • Huygens proposed that light is actually comprised of waves • Theorized that space is filled with an ether to provide the medium for light waves • Theory gained support because it’s able to explain various properties of light, some which Newton’s theory cannot Ex. Refraction, Diffraction, Interference
Huygens’ Wave Model Refraction: • Observed that water waves bend toward the normal when traveling from deep water to shallow water, likewise with light as it travels from air to water • Theory implied that light travels slower in water than in air Snell’s Law: nisinϴi = nrsinϴr n = index of refraction ϴi = angle of incidence ϴr = angle of refraction
Huygens’ Wave Model Diffraction: The bending of waves as a result of encountering an obstacle or passing through a narrow opening. • Can result in an interference pattern Interference: The interaction of waves within a medium. • The crest of one wave meets the trough of another they will cancel out (destructive) • Two crests or two troughs coincide, they will build a stronger wave (constructive)
Electromagnetic Theory • James Maxwell improved Huygens’ theory of waves • Predicted that accelerating electric charges will emit electric and magnetic waves (electromagnetic) that require no medium in order to interact • Calculated that electromagnetic waves must be travelling at the speed of light (3.0 x 108m/s) in order to interact with one another • Concluded that light is an electromagnetic wave.
Electromagnetic Theory The existence of electromagnetic waves was later proven by Heinrich Hertz, who theorized that light waves are of a narrow band of frequencies in the electromagnetic wave spectrum.
Property
Example
Waves
Particles
Transmission through vacuum
Star Light
The concept of the “ether” was developed as a medium to propagate light waves through a vacuum.
Speed
c = 3.0 x 108m/s
No particle having mass has been accelerated to the speed of light.
Reflection
Both particles and waves obey the Law of Reflection.
Refraction
Particles and waves both obey Snell’s Law.
Difficult to explain with particles.
Waves naturally do this, particles do not.
Waves naturally do this, particles do not.
Waves naturally do this, particles do not.
Particles naturally do this, waves do not.
Colour
ROYGBV
Dispersion
R O Y G B V
Diffraction
Interference +
Photo-Electric Effect
=
Notes
Wave-Particle Duality Though they are very different, both particle and wave models of light can be used to describe the varying properties of light. Due to this fact, scientists have accepted that neither theory can be correct in describing light on its own, but instead concluded the wave-particle duality of light. Either the wave or particle model of light must be used to understand any experiment of light, though to understand light as a whole, both theories must be used. • Physical quanta of light is called a photon
Wave-Particle Duality Principle of Complementarity: • Neil Bohr proposed this principle to summarize wave-particle duality • Fundamental principle of quantum mechanics • States that objects have complementary properties that cannot simultaneously be measured with accuracy. The Planck-Einstein Relation: • The equation for the energy of a photon shows the relationship between the two theories E= hv
Where: E= Energy of the Photon (quanta) h= Planck’s Constant (6.63 x 10-34Js) v= Frequency
Wave-Particle Duality • Proof of wave-particle duality was determined by
the Double Slit Experiment
Wave-Particle Duality Video
Heisenberg’s Uncertainty Principle • Werner Heisenberg proposed this principle when he tried to observe particles v = c_ λ
Where: v = Frequency c = Speed of Light (3.0 x 108m/s) λ = Wavelength
• To accurately measure a particle’s position a shorter wavelength must be used, though the high frequency would disturb the particle more • To accurately measure a particle’s speed, a low frequency wavelength must be used, though its position will be less accurately known
Heisenberg’s Uncertainty Principle The position and velocity of a particle can never be simultaneously known, the more accurately one knows the one, the less accurately one can know the other. Uncertainty Equation: Uncertainty of x Uncertainty of x Mass of ≥ Planck’s Constant Position Velocity Particle (6.63 x 10-34Js)
• Position of the particle cannot be known more accurately than the wavelength (distance between crests) of the radiation used
Modern Physics • Newton had a deterministic view of the universe with all matter being composed of particles • With the uncertainty principle and wave-particle duality it has been determined that the universe is governed by probability, not determinism • Some of these probabilities are so high that they give the appearance of determinism • If a particle cannot be measured with certainty, then it cannot be accurately predicted where it will go next • Generally light moves like a wave and interacts with matter like a particle • There is not distinction between waves and particles, particles may behave like a wave and waves like particles
