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Dr. Quantum

Dr. Quantum. The Nature of Light. When studying geometric optics, we used a ray model to describe the behavior of light. A wave model of light is necessary to describe phenomena such as: interference diffraction

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Dr. Quantum

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  1. Dr. Quantum Light as a Wave

  2. The Nature of Light • When studying geometric optics, we used a ray model to describe the behavior of light. • A wave model of light is necessary to describe phenomena such as: • interference • diffraction • A particle model of light is necessary to describe phenomena observed in modern physics, for example, the interaction between light and atoms. We’ll get back to this later... Light as a Wave

  3. Wave Nature of Light • Christian Huygens (1629-1695) • contemporary of Newton • developed wave theory of light • Huygen’s Principle • Every point on a wave front can be considered as a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself. • The new wave front is the envelope of all the wavelets - tangent to all of them Light as a Wave

  4. Huygen’s Principle Light as a Wave

  5. Diffraction • Huygen’s Principle is useful for understanding diffraction - the bending of waves behind obstacles into the shadow region Light as a Wave

  6. Interference • Thomas Young (1773-1829) • definitively (at least temporarily) demonstrates wave nature of light • Young’s Double-Slit Experiment • coherent light passes through 2 slits, S1 and S2 • light from S1 and S2 then interferes and pattern of dark and light spots is observed on the screen Light as a Wave

  7. Interference • Constructive interference occurs when • d sin = m  , m = 0,1,2,... • m = order • Destructive interference occurs when • d sin = (m + 1/2)  , m = 0,1,2,... • Source must be coherent • waves at S1 and S2 are in-phase Light as a Wave

  8. what you see on the screen: Light as a Wave

  9. Think-Pair-Share • Monochromatic light falling on two slits 0.016 mm apart produces the fifth-order fringe at an 8.8 degree angle. What is the wavelength of the light used? Light as a Wave

  10. Pair Problem • Light of wavelength 680 nm falls on two slits and produces an interference pattern in which the fourth-order fringe is 38 mm from the central fringe on a screen 2.0 m away. What is the separation of the two slits?(Hint: tan =  for small angles, and angles must be in radians!) Light as a Wave

  11. Visible Spectrum Light as a Wave

  12. Dispersion • Index of refraction varies with wavelength of light • As a result, white light is separated into component colors by a prism or by water (rainbow) Light as a Wave

  13. Dispersion & Rainbow • red is bent the least • red light reaches observer’s eye from higher water droplets • violet is bent the most • violet light reaches observer’s eye from lower water droplets Light as a Wave

  14. Diffraction by a Disk • Diffracted light interferes constructively at center of shadow • requires a point source of monochromatic light (e.g. laser) Light as a Wave

  15. Diffraction by a Single Slit • D sin  = m  • m = 1, 2, 3, ... position of minima for m=1, theta gives 1/2 width of central maximum Motivation for making large diameter telescopes Light as a Wave

  16. Diffraction Grating • a large number of equally spaced parallel slits • same relation as double-slit • d sin  = m  • m = 0, 1, 2, ... • produces sharper and narrower interference patterns that double slit Light as a Wave

  17. Diffraction Grating • double slit versus diffraction grating • for multi-wavelength light Light as a Wave

  18. Interference by Thin Films Produces rings of constructive/destructive interference Light as a Wave

  19. Thin-Film Interference: Examples Oil on water Soap bubbles Beetles Butterflies Light as a Wave

  20. Find the diffraction grating spacing • Align the laser and the diffraction grating • Measure the length and height of peaks to determine the angle • Use the diffraction equation to calculate the diffraction grating spacing. • The laser wavelength is 630 nm Light as a Wave

  21. PhET Conceptual Question • Use PhET simulation to answer the following questions • http://phet.colorado.edu/en/simulation/wave-interference • What happens to the interference pattern if the wavelength of light is increased from 500 nm to 700 nm? • What happens instead if the wavelength stays at 500 nm but the slits are moved farther apart? Light as a Wave

  22. Emission Tubes – real and PhET • Look at several emission tubes using diffraction gratings & sketch spectrum • Foundation of spectroscopy, a technique used in numerous scientific applications • Compare with PhET simulation • http://phet.colorado.edu/en/simulation/discharge-lamps Element Light as a Wave

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