Experimental History Leading to Quantum Theory - PowerPoint PPT Presentation

Loading...

PPT – Experimental History Leading to Quantum Theory PowerPoint presentation | free to download - id: 77b348-NGIxN



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Experimental History Leading to Quantum Theory

Description:

Experimental History Leading to Quantum Theory Particles versus Waves – PowerPoint PPT presentation

Number of Views:26
Avg rating:3.0/5.0
Slides: 19
Provided by: hpo54
Learn more at: http://www2.sjs.org
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Experimental History Leading to Quantum Theory


1
Experimental History Leading to Quantum Theory
  • Particles versus Waves

2
Properties of Light1650
  • Travels in straight lines in a given material
  • Casts sharp shadows
  • Comes in colors
  • Reflects from smooth surfaces
  • Refracts when traveling from one material to
    another
  • Research limited to descriptions of how light
    behaved
  • In 1660s, question changes to why light
    behaves as it does and to what is the fundamental
    nature of light

3
Model 1Isaac Newton Particle Model
  • Light is a stream of small particles
  • Model adequately explains reflection refraction
  • Colors produced by particles of different shapes
  • Behavior of particles governed by law of gravity
  • Predicts that light speeds up when going from air
    into a refracting medium

4
Model 2Christian Huygens Wave Model
  • Light is a wave and each point on a wavefront
    serves as point sources of subsequent wavelets
  • Adequately explains reflection refraction
  • Colors correspond to different wavelengths
  • Predicts that light slows down when going from
    air into refracting medium

5
Setting up the Problem
  • Is light a particle or a wave?

6
Resolving the DilemmaWhat is the speed of light?
  • Galileo 1600
  • Speed of light is very fast too fast to measure
  • Roemer - 1676
  • Used occultation of Io by Jupiter
  • Speed of light is finite, but very fast
  • Fizeau and Foucault 1840s
  • Measured speed of light in air and water
  • Found it to slow down in water

7
New Experimental ResultsYoungs Double Slit
Experiment
  • Interference fringes can only be explained with
    the wave model
  • Provided first technique for measuring wavelength
    of visible light

8
Merger of Optics and EM
  • Maxwell unifies experiment work of Faraday,
    Ampere and Oersted into four laws expressed in
    vector calculus notation known today as Maxwells
    equations
  • Simultaneous solution results in an equation
    known to describe wave behavior, with the speed
    of the wave in vacuum being 3 x 108 m/s

9
Conclusion?
  • Newton was wrong
  • Light is a wave of electromagnetic origin
  • Wavelengths extend beyond the visible into
    ultraviolet and infrared regions
  • Waves carry energy
  • In 1880s, physics is declared a dead science
    the physical world is totally known
  • All thats left are details and applications

10
One of the Details
  • What determines the colors emitted by an ideal
    radiator as it heats up?
  • Max Planck and blackbody radiation
  • A solid can be modeled as a matrix of small
    masses connected to nearest neighbors by springs
  • Vibrational modes of the springs dictate what
    wavelengths are emitted

11
Problems with Classical Explanation
  • Classically, vibrational modes form a continuum
  • Classical explanation leads to ultraviolet
    catastrophe
  • Plancks resolution only discrete, quantized
    vibrational modes are possible
  • E nhf, where h is an empirical constant and n
    0, 1, 2

12
Another DetailThe Photoelectric Effect
  • When a polished metal surface is exposed to
    ultraviolet light, electrons (a.k.a.
    photoelectrons) are emitted from the surface with
    measurable kinetic energy

13
Another Problem with Classical Physics
  • Experimental results
  • Increased UV intensity results in larger number
    of emitted photoelectrons
  • Increased UV frequency results in larger kinetic
    energy of photoelectrons
  • Classical predictions
  • Increased UV intensity results in larger kinetic
    energy of photoelectrons
  • Increased UV frequency results in larger number
    of emitted photoelectrons

14
Einsteins Explanation
  • Light comes in packets, called photons
  • Intensity is proportional to the number of
    photons
  • Energy of each photon is proportional to the
    frequency E hf
  • Result 1921 Nobel Prize

15
Further Evidence
  • The Compton Effect
  • Photons of light carry momentum as well as energy
  • Result 1927 Nobel Prize

16
The Real Problem
  • How can light be both a wave and a particle?
  • deBroglies response If a wave can behave like
    a particle, then a particle can behave like a
    wave
  • Result 1929 Nobel Prize

17
Experimental Confirmation
  • Davisson-Germer Experiment
  • When electrons pass through a crystalline
    lattice, they interfere to form an interference
    pattern similar to Youngs double slit experiment
    with light
  • Result 1937 Nobel Prize

18
Whats Going On?
  • Are photons like particles or like waves?
  • Are particles like waves or like particles?
About PowerShow.com