12 Mar 3 Rainbows Etc Actual Presented

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Discussion of dispersion and rainbows. Also some cool photos of blackbody and fluorescent spectra from Tom Decaro and Analisa Goodman as part of the homework question.

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12 Mar 3 Rainbows Etc Actual Presented

  1. 1. Today: Total Internal Reflection, Refraction, Dispersion, Rainbows <ul><li>Next exam one week from Thursday! </li></ul><ul><li>Quiz posted, due Thursday </li></ul>Spectacular rainbow, late afternoon Sweden, 1999. http://www.atoptics.co.uk/index.htm
  2. 2. HW Question Tom Decaro—Sunlight reflections through a diffraction grating
  3. 3. Tom Decaro—Sunlight reflections through a diffraction grating
  4. 4. Tom Decaro -- Fluorescent lights through a diffraction grating
  5. 5. Analisa Goodman—Fluorescent lights through a diffraction grating
  6. 6. Homework problem <ul><li>Incandescent – Blackbody radiation Color is yellow (cooler than the sun) Most photons are infrared </li></ul><ul><li>Fluorescent – Electrons excite mercury, which emits UV photons. UV photons absorbed by phosphors, which fluoresce in visible </li></ul><ul><li>Incandescent “wastes” lots of photons in the IR. </li></ul>Visible range Images:wikipedia
  7. 7. Conservation of energy in lighting Electrical Energy Heat flow Invisible photons Visible photons In winter, these are beneficial for heating the room Lighting Efficiency Visible photons Electrical Energy
  8. 8. First off, let’s experiment with refraction Key Concept Compared with speed in a vacuum (300 million meters per second) Light travels SLOWER inside matter
  9. 9. Clicker Question--Refraction <ul><li>Which of the following diagrams best represents what happens when a red laser hits a glass / air interface? </li></ul>A B C Air Glass
  10. 10. Clicker Question -- Refraction <ul><li>Which of the following diagrams best represents what happens when a red light wave encounters an air / diamond interface? </li></ul>Reflection: Angle of reflection = angle of incidence Refraction: Imagine the wave as a two wheeled cart encountering a boundary A B C Air Glass FAST SLOW
  11. 11. Total internal reflection (TIR) <ul><li>Imagine what happens when a ray of light is in a higher index of refraction material… </li></ul>Air glass “ Evanescent” Wave Total internal reflection Let’s try a laser demo again
  12. 12. TIR is a key for fiber optic communication Air glass http://www.fiberoptic-connectors.com/images/reflection.gif
  13. 13. TIR enables a special surface microscopy technique Air glass “ Evanescent” Wave Excite fluorescence in cell with evanescent wave www.olympusamerica.com Red = TIRF Green = regular
  14. 14. <ul><ul><li>Yes </li></ul></ul><ul><ul><li>No </li></ul></ul>Is it possible to have a total internal reflection prevent someone from seeing a coin at the bottom of a pool?
  15. 15. <ul><ul><li>Yes </li></ul></ul><ul><ul><li>No </li></ul></ul>Is it possible to have a total internal reflection prevent someone from seeing a coin at the bottom of a pool?
  16. 16. Clicker Question—Speed of light <ul><li>Do all colors of light travel at the same speed inside of glass? </li></ul><ul><li>Yes </li></ul><ul><li>No </li></ul>
  17. 17. Clicker Question—Speed of light <ul><li>Do all colors of light travel at the same speed inside of glass? </li></ul><ul><li>Yes </li></ul><ul><li>No </li></ul>No! This is called dispersion and because of this, different colors have different angles of refraction! Let’s see if we can do this with sunlight…
  18. 18. Demos Prisms Acrylic sphere rainbow http://www.youtube.com/watch?v=rQukmSPctks&feature=related
  19. 19. Rainbows are the result of dispersion inside spherical raindrops (close to 1 millimeter in size) NOT THIS SHAPE! Higher frequencies Refract MORE Each raindrop emits it’s own “fan” For a given raindrop, your eye will only see rays of particular angle. http://www.atoptics.co.uk/ http://www.tpwd.state.tx.us
  20. 20. These special angles form a 3-D cone… which you see as a portion of a CIRCLE “ Primary rainbow” = one reflection http://www.atoptics.co.uk/rainbows/primrays.htm
  21. 21. Two reflections allow light to go in other directions And produce the secondary rainbow Two reflections http://www.atoptics.co.uk/rainbows/orders.htm
  22. 23. Rainbow facts <ul><li>Rainbows are perceived as circular </li></ul><ul><li>The center of the circle is on a line continuing the sun through your eyes (antisolar point) </li></ul><ul><li>Rainbows result from large, but spherical raindrops </li></ul><ul><li>Refraction, dispersion, and reflection are the keys to raindrops. </li></ul><ul><li>Rainbows are not quite the same colors as light from a prism </li></ul><ul><li>Bright inside primary bow, dark between </li></ul>
  23. 24. Rainbow colors are not exactly the same as through a prism (or diffraction grating)…what differences do you notice? Rainbow Prism Diffraction Grating
  24. 25. Rainbows differ from regular prism spectra, because colors overlap Website demo http://www.sundog.clara.co.uk/rainbows/primrays.htm
  25. 26. Why is it dark between the primary and secondary rainbow? http://www.atoptics.co.uk/rainbows/adband.htm Alexander’s dark band Spectacular rainbow, late afternoon Sweden, 1999. http://www.atoptics.co.uk/index.htm
  26. 27. Reflection Bows
  27. 28. Clicker…which bow is formed by the reflection? A B
  28. 29. Go to this website for many fascinating atmospheric phenomena! http://www.atoptics.co.uk/

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