Lecture%2005

Lecture%2005 - EEE 352: Lecture 05 Particle and Waves as...

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1 Particle and Waves as One and the Same? Last time we left off with the problem of photons as particles, rather than waves, and the confusion to which this led. * Light as a particle phenomenon * What is light? * Electrons as waves ! EEE 352: Lecture 05 In classical physics, all things are either particles or waves. Particles: atoms, electrons, cars, boats, houses, people…. . Waves: light, heat, water motion, radio signals, …. Particles and waves are distinct objects, and things just don’t have the properties of both. However, if we look a little closer, these facts and interpretations start to blur… COHERENT LIGHT SOURCE Last time, we talked about light as a wave phenomenon: INTERFERENCE and DIFFRACTION Light is diffracted by the aperatures Just as for X-ray diffraction, the light has constructive and destructive interference where the diffracted beams come together What is diffraction? Diffraction off a rough surface. Diffraction off water droplets in the air (fog).
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2 When we go through just one slit, we get some diffraction, but mainly just a single peak: But, going through two slits gives us this interference pattern: COHERENT LIGHT SOURCE Last time, we talked about light as a wave phenomenon: INTERFERENCE and DIFFRACTION Light is diffracted by the aperatures Just as for X-ray diffraction, the light has constructive and destructive interference where the diffracted beams come together Yet, the observation of black-body radiation and Planck’s new theory, including his new constant, opened the door to viewing light as a particle! The two limiting forms were known in the mid-1800s. Max Planck derived this form ca. 1900. This required him to postulate E = h ν h ~ 6.6248 × 10 -34 joule-sec ν The intensity of light falling on a surface was given by the number of photons per second that hit the surface. The photon WAS a particle! This was a strange view, and not really accepted until Einstein used it to explain the photoelectric effect. http://www.lon-capa.org/~mmp/kap28/PhotoEffect/photo.htm
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3 Light as a Particle E > E 0 (called the “work function”) ( mv 2 /2) electron = E E 0 SUITABLE METAL LIGHT ELECTRONS E = h ν This effect remained unexplained until Einstein used the particle properties of light inferred from Planck’s theory! IN THE PHOTOELECTRIC EFFECT A KEY OBSERVATION IS THAT ELECTRONS ARE ONLY EMITTED FROM THE METAL SURFACE IF THE FREQUENCY OF THE LIGHT EXCEEDS A CERTAIN VALUE The number of emitted electrons was proportional to the number of arriving photons! The kinetic energy of the photons emitted was given
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Lecture%2005 - EEE 352: Lecture 05 Particle and Waves as...

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