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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 speedsis reflected from mirrors in a predictable way. Newton observed that the reflection of light from a mirror resembles the rebound of a

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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

    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)

    4. Particles Position x Mass m Momentum p = mv

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

    6. 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 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

    9. Interference Fringes on a Screen

    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

    12. Ultraviolet Catastrophe

    13. Planck’s Quantum Postulate (1900) A blackbody can only emit radiation in discrete packets or quanta, i.e., in multiples of the minimum energy: E = hf where h is a constant and f is the frequency of the radiation

    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 be h = 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

    17. Photoelectric Effect (cont’d)

    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 electron if it has enough energy Each photon has energy: E = hf (same as Planck’s formula)

    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 colour because photons of different energies trigger different chemical reactions in retina cells

    22. Double-Slit Experiment to illustrate wave nature of light

    23. Double-Slit Experiment with a machine gun!

    24. Double-Slit Experiment with electron gun

    25. Interference Pattern of Electrons Determines the probability of an electron arriving at a certain spot on the screen After many electrons, resembles the inter- ference pattern of light

    26. Double-Slit Experiment with electron gun and detector

    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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