13 March 5 Microwaves, Polarization

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Microwaves and polarization

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13 March 5 Microwaves, Polarization

  1. 1. Today: Quiz Results, Light as a Wave: Microwave Radiation, Polarization <ul><li>Exam #2 One week from today, BRING A PENCIL!!! </li></ul>
  2. 2. Quiz #5
  3. 4. A blue photon has a higher energy than a green photon
  4. 6. Total internal reflection (TIR) only happens for a wave inside a medium with a slower speed Air glass Air glass
  5. 7. Total internal reflection with microwaves Introductory explanation of microwave source and detector
  6. 8. OK, Now a couple clicker questions to lead into next topic <ul><li>Imagine a slinky stretched across the room in the z-direction, and a longitudinal wave traveling down it. </li></ul>Z X y <ul><li>Along which direction are the displacements of the slinky? </li></ul><ul><li>X-direction </li></ul><ul><li>Y-direction </li></ul><ul><li>Z-direction </li></ul>
  7. 9. OK, Now a couple clicker questions to lead into next topic <ul><li>Imagine a slinky stretched across the room in the z-direction, and a longitudinal wave traveling down it. </li></ul>Z X y <ul><li>Along which direction are the displacements of the slinky? </li></ul><ul><li>X-direction </li></ul><ul><li>Y-direction </li></ul><ul><li>Z-direction </li></ul>Compression / rarefaction wave
  8. 10. OK, Now a couple clicker questions to lead into next topic <ul><li>Imagine the rubber tube stretched across the room, and a transverse wave traveling down it. It is stretched in the z-direction </li></ul>Z X y <ul><li>Along which direction are the displacements of the rubber tube? </li></ul><ul><li>X-direction </li></ul><ul><li>Y-direction </li></ul><ul><li>Z-direction </li></ul>Rubber rope
  9. 11. OK, Now a couple clicker questions to lead into next topic <ul><li>Imagine the rubber tube stretched across the room, and a transverse wave traveling down it. It is stretched in the z-direction </li></ul>Z X y <ul><li>Along which direction are the displacements of the rubber tube? </li></ul><ul><li>X-direction </li></ul><ul><li>Y-direction </li></ul><ul><li>Z-direction </li></ul>Rubber rope Either X, Y, or any angle in between! Transverse waves have polarization
  10. 12. Remember: Light is an electromagnetic wave <ul><ul><li>Carries energy </li></ul></ul><ul><ul><li>Has momentum (oomph) (but does NOT have mass) </li></ul></ul><ul><ul><li>In a vacuum travels at “light speed” </li></ul></ul><ul><ul><li>Behaves like particle AND wave </li></ul></ul><ul><ul><li>Oscillating Electric and Magnetic Field: TRANSVERSE WAVE </li></ul></ul>So if EM radiation is a transverse wave, can it be polarized?
  11. 13. EM Waves can be “linearly polarized” Similarly, a transverse wave on a rubber tube has a polarization hyperphysics.phy-astr.gsu.edu
  12. 14. Special materials can interact preferentially with one polarization of light vertical polarization is let through horizontal polarization is absorbed <ul><li>Stretched rubber tube with student barriers </li></ul><ul><li>Polarized visible light </li></ul><ul><li>Polarized microwaves </li></ul>You can find this demo from MIT on youtube: http://www.youtube.com/watch?v=nCAKQQjfOvk
  13. 15. Some applications of polarized light <ul><li>3-D Movie Projection </li></ul><ul><li>Polarized Light Microscopy </li></ul><ul><li>Photography (blue sky polarizer) </li></ul><ul><li>Polarized sunglasses </li></ul>
  14. 16. How do polarized sunglasses work? The amount of light reflecting off the surface of water depends on the polarization One component reflects more than the other, So, reflected sunlight is polarized Polarized sunglasses can specifically absorb this polarization You tube: http://www.youtube.com/watch?v=jNXeNdmz92o&feature=related http://commons.wikimedia.org/wiki/File:Mudflats-polariser.jpg
  15. 17. So far, we have talked about linearly polarized light A trickier concept is circularly polarized light This occurs when the X and Y components are out of phase. http://www.ecs.umass.edu/ece/ece334/ece334/JavaApplets/Polarization/Polarization%20Applet.htm Circular polarization demo with laser? Specific rotation with sugar
  16. 18. A final application of polarized light Nerdy physics demo More cool demo: http://www.austine.com/
  17. 19. OK, so EM Radiation Can be Polarized <ul><li>Remembering back to sound… What other phenomena can happen with waves? </li></ul><ul><li>Interference …Let’s talk a little about microwave EM radiation… </li></ul>
  18. 20. Clicker Question—Microwave Wavelength <ul><li>Do you remember what the speed of light is? A wireless internet router typically operates at a frequency of 2.4 GHz (2.4 billion Hz, 2.4 * 10 9 Hz). Which is closest to the wavelength of these microwaves ? </li></ul><ul><li>100 nanometers </li></ul><ul><li>100 microns </li></ul><ul><li>100 millimeters </li></ul><ul><li>100 meters </li></ul><ul><li>100 kilometers </li></ul>
  19. 21. Clicker Question—Microwave Wavelength <ul><li>Do you remember what the speed of light is? A wireless internet router typically operates at a frequency of 2.4 GHz (2.4 billion Hz, 2.4 * 10 9 Hz). Which is closest to the wavelength of these microwaves ? </li></ul><ul><li>100 nanometers </li></ul><ul><li>100 microns </li></ul><ul><li>100 millimeters (about 12 centimeters actually) </li></ul><ul><li>100 meters </li></ul><ul><li>100 kilometers </li></ul>
  20. 22. Microwave Radiation <ul><li>The boundaries between microwave and radio / infrared are somewhat arbitrary. </li></ul><ul><li>Microwaves are in the gigahertz range, about centimer wavelengths…This is convenient size for observing interference </li></ul>Wavelength (meters) Frequency (Hz) Temperature required to glow this color
  21. 23. A microwave oven can produce standing waves <ul><li>Microwave commonly operates at similar frequency to wireless routers (2.45 GHz) </li></ul><ul><li>Why does a microwave oven heat food? </li></ul><ul><li>What would size-scale of a standing wave pattern be inside a microwave? (Distance between nodes?) </li></ul><ul><li>Can we observe microwave interference in class room? </li></ul>Marshmallows: http://www.colorado.edu/physics/2000/applets/oven.html

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