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Lecture 12: Waves versus particles

Lecture 12: Waves versus particles. 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.

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Lecture 12: Waves versus particles

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  1. Lecture 12: Waves versus particles

  2. 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

  3. Wave Theory of Light (1802) • Thomas Young showed that light is a wave, because it • undergoes diffraction and interference (Young’s double-slit experiment) Thomas Young (1773-1829)

  4. Particles • Position x • Mass m • Momentum p = mv

  5. Waves • Wavelength l • Amplitude A • Frequency f • number of cycles per second (Hertz) f = c /l

  6. Waves versus Particles • A particle is localized 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

  7. Diffraction

  8. Interference applet

  9. Interference Fringes on a Screen applet

  10. 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

  11. Spectrum of Blackbody Radiation Plot of intensity of the blackbody radiation versus wavelength for various temperatures applet

  12. Ultraviolet Catastrophe Classical theory predicts a graph that deviates from experimental data, especially at short wavelengths

  13. Planck’s 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

  14. Planck’s Quantum Postulate (cont’d) • 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 Planck’s formula • This explains the origin of the ultraviolet catastrophe

  15. Planck’s Constant • Experimentally determined to beh = 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’

  16. Photoelectric Effect When blue light is shone on the emitter plate,a current flows in the circuit

  17. Photoelectric Effect (cont’d) But for red light, no current flows in the circuit video clip

  18. 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

  19. 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

  20. Einstein’s 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 Planck’s formula) Albert Einstein won a Nobel Prize for his work on the photoelectric effect and not his theory of relativity!

  21. 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 color because photons of different energies trigger different chemical reactions in retina cells

  22. Double-Slit Experimentto illustrate wave nature of light

  23. Double-Slit Experiment with a machine gun!

  24. Double-Slit Experiment with electron gun Electrons behave like waves!

  25. 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

  26. Double-Slit Experiment with electron gun and detector Trying to detect which slit the electrons pass through causes them to behave like particles

  27. 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

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