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Wave-Particle Duality of Light

Wave-Particle Duality of Light. Light as Particle (part 1). Newton separated white light into ROYGBV using prism He believed that light was particle and that refraction was somehow due to the gravitational force between light and the prism. Light as wave.

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Wave-Particle Duality of Light

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  1. Wave-Particle Duality of Light

  2. Light as Particle (part 1) • Newton separated white light into ROYGBV using prism • He believed that light was particle and that refraction was somehow due to the gravitational force between light and the prism

  3. Light as wave • Young’s double-slit interference experiment • Explanation of thin-film interference • Description of light as electromagnetic wave (J. C. Maxwell)

  4. Diffraction • The longer the wavelength (l) compared to the gap or obstacle, the greater the diffraction • The less diffraction, the more “particle-like” the wave; the more diffraction, the more “wave-like” • X-rays and gamma-rays are relatively particle-like compared to longer l em waves

  5. What happens to the color of a bulb filament as it gets hotter?

  6. Max Planck realized that not all frequencies are represented equally… Cooler objects have more ROY… as objects get hotter, add more BV

  7. Planck’s “quantum” – Einstein’s “photon” • Planck proposed that light is composed of particles of em energy – the light is quantized into bundles called “quanta” • One bundle would be called a quantum and its energy would be given by h is now called Planck’s constant and is equal to 6.63  10-34 Js (4.14  10-15 eVs) Einstein called the quantum a photon

  8. Planck’s explanation of blackbody spectrum • Red photons are relatively small “buckets” of energy compared to violet photons (use E=hf) • As objects get hot, infrared photons are produced first, then red, then orange… An object is white hot when it is producing photons of all ROYGBV (peaked in the YG) • Violet and UV photons cannot be produced until the object is hot enough to fill those buckets

  9. Photoelectric Effect • When light is projected on a metal, certain wavelengths allowed electrons to escape, while others did not. The brightness (amplitude of E-field) did not matter, even though classical wave theory said it should. • If energy delivered in packets, then only those packets with enough energy could free electrons – increasing brightness of the wrong colors wouldn’t work • See Photoelectric Effect slides

  10. Energy and momentum of photon • Even though a photon has no mass, it carries momentum

  11. Compton Scattering When short l light (why not long l?) interacts with free charges (not bound to atoms), momentum and energy must be conserved. By giving some of its energy to the charge, the photon’s frequency ________, while its l ________. Momentum is conserved separately in x- and -y dimensions.

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