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Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses

Physics 102: Lecture 18. Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses. Today’s Lecture will cover textbook sections 23.3-5, 9. Total Internal Reflection. q 2. “critical angle”. q r. q c. q i. q 1. Recall Snell’s Law: n 1 sin( q 1 )= n 2 sin( q 2 )

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Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses

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  1. Physics 102:Lecture 18 Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses • Today’s Lecture will cover textbook sections 23.3-5, 9 Physics 102: Lecture 18, Slide 1

  2. Total Internal Reflection q2 “critical angle” qr qc qi q1 Recall Snell’s Law: n1 sin(q1)= n2 sin(q2) (n1 > n2 q2 > q1 ) q1 = sin-1(n2/n1) then q2 = 90 Light incident at a larger angle will only have reflection (qi = qr) n2 n1 normal Physics 102: Lecture 18, Slide 2

  3. Preflight 18.1 Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ? 1) Yes 2) No Physics 102: Lecture 18, Slide 3

  4. Preflight 18.1 Java Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ? 1) Yes 2) No Physics 102: Lecture 18, Slide 4

  5. ACT: Refraction • As we pour more water into bucket, what will happen to the number of people who can see the ball? 1) Increase 2) Same 3) Decrease Physics 102: Lecture 18, Slide 5

  6. ACT: Refraction • As we pour more water into bucket, what will happen to the number of people who can see the ball? 1) Increase 2) Same 3) Decrease Physics 102: Lecture 18, Slide 6

  7. ACT: Refraction • As we pour more water into bucket, what will happen to the number of people who can see the ball? 1) Increase 2) Same 3) Decrease Physics 102: Lecture 18, Slide 7

  8. Fiber Optics At each contact w/ the glass air interface, if the light hits at greater than the critical angle, it undergoes total internal reflection and stays in the fiber. noutside ninside Telecommunications Arthroscopy Laser surgery Total Internal Reflection only works if noutside < ninside Physics 102: Lecture 18, Slide 8

  9. Fiber Optics At each contact w/ the glass air interface, if the light hits at greater than the critical angle, it undergoes total internal reflection and stays in the fiber. noutside ncladding ninside Add “cladding” so outside material doesn’t matter! We can be certain that ncladding < ninside Physics 102: Lecture 18, Slide 9

  10. Brewster’s angle horiz. polarized only! qB qB horiz. and vert. polarized 90º 90º-qB Reflected light is partially polarized (more horizontal than vertical). But… n1 n2 …when angle between reflected beam and refracted beam is exactly 90 degrees, reflected beam is 100% horizontally polarized ! tan θB = Physics 102: Lecture 18, Slide 10

  11. Brewster’s angle horiz. polarized only! qB qB horiz. and vert. polarized 90º 90º-qB Reflected light is partially polarized (more horizontal than vertical). But… n1 n2 …when angle between reflected beam and refracted beam is exactly 90 degrees, reflected beam is 100% horizontally polarized ! n1 sin qB = n2 sin (90-qB) n1 sin qB = n2 cos (qB) Physics 102: Lecture 18, Slide 11

  12. Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction? • horizontal • vertical Preflight 18.3, 18.4 Polarizing sunglasses are often considered to be better than tinted glasses because they… • block more light • block more glare • are safer for your eyes • are cheaper Physics 102: Lecture 18, Slide 12

  13. Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction? • horizontal • vertical Preflight 18.3, 18.4 Polarizing sunglasses are often considered to be better than tinted glasses because they… • block more light • block more glare • are safer for your eyes • are cheaper When glare is around qB, it’s mostly horiz. polarized! Physics 102: Lecture 18, Slide 13

  14. ACT: Brewster’s Angle When a polarizer is placed between the light source and the surface with transmission axis aligned as shown, the intensity of the reflected light: (1) Increases (2) Unchanged (3) Decreases T.A. Physics 102: Lecture 18, Slide 14

  15. ACT: Brewster’s Angle When a polarizer is placed between the light source and the surface with transmission axis aligned as shown, the intensity of the reflected light: (1) Increases (2) Unchanged (3) Decreases T.A. Physics 102: Lecture 18, Slide 15

  16. White light Dispersion The index of refraction n depends on color! In glass: nblue = 1.53nred = 1.52 nblue > nred prism Blue light gets deflected more Physics 102: Lecture 18, Slide 16

  17. Preflight 18.5 (1) Wow look at the variation in index of refraction! (2) (1) (2) Which is red? Which is blue? Physics 102: Lecture 18, Slide 17

  18. Preflight 18.5 Wow look at the variation in index of refraction! Which is red? Which is blue? Skier sees blue coming up from the bottom (1), and red coming down from the top (2) of the rainbow. Blue light is deflected more! Physics 102: Lecture 18, Slide 18

  19. LIKE SO! In second rainbow pattern is reversed Physics 102: Lecture 18, Slide 19

  20. q1 q1 d Flat Lens (Window) Incident ray is displaced, but its direction is not changed. n2 n1 If q1 is not large, and if t is small, the displacement, d, will be quite small. t Physics 102: Lecture 18, Slide 20

  21. Preflight 18.6 A beacon in a lighthouse produces a parallel beam of light. The beacon consists of a bulb and a converging lens. Where should the bulb be placed? F F Converging Lens All rays parallel to principal axis pass through focal point F. Double Convex F P.A. nlens > noutside • At F • Inside F • Outside F P.A. Physics 102: Lecture 18, Slide 21

  22. Preflight 18.6 A beacon in a lighthouse produces a parallel beam of light. The beacon consists of a bulb and a converging lens. Where should the bulb be placed? F F F P.A. Converging Lens All rays parallel to principal axis pass through focal point F. Double Convex F P.A. nlens > noutside • At F • Inside F • Outside F Physics 102: Lecture 18, Slide 22

  23. Converging Lens Principal Rays Assumptions: • monochromatic light incident on a thin lens. • rays are all “near” the principal axis. Example F P.A. Object F 1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays through F emerge parallel to principal axis. Image is (in this case): Real or Imaginary Inverted or Upright Enlarged or Reduced Physics 102: Lecture 18, Slide 23

  24. Converging Lens Principal Rays Image Assumptions: • monochromatic light incident on a thin lens. • rays are all “near” the principal axis. Example F P.A. Object F 1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays through F emerge parallel to principal axis. Image is: real, inverted and enlarged (in this case). Physics 102: Lecture 18, Slide 24

  25. F P.A. Object F ACT: Converging Lens Which way should you move object so image is real and diminished? (1) Closer to lens (2) Further from lens (3) Converging lens can’t create real diminished image. Physics 102: Lecture 18, Slide 25

  26. F P.A. Object F ACT: Converging Lens Which way should you move object so image is real and diminished? (1) Closer to lens (2) Further from lens (3) Converging lens can’t create real diminished image. Demo Physics 102: Lecture 18, Slide 26

  27. Image Object Image Image Object Object 3 Cases for Converging Lenses Past 2F Inverted Reduced Real This could be used in a camera. Big object on small film Between F & 2F Inverted Enlarged Real This could be used as a projector. Small slide on big screen Inside F Upright Enlarged Virtual This is a magnifying glass Physics 102: Lecture 18, Slide 27

  28. Diverging Lens Principal Rays Example F P.A. Object F 1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays toward F emerge parallel to principal axis. Image is (always true): Real or Imaginary Upright or Inverted Reduced or Enlarged Physics 102: Lecture 18, Slide 28

  29. Diverging Lens Principal Rays Image Example F P.A. Object F 1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays toward F emerge parallel to principal axis. Image is virtual, upright and reduced. Physics 102: Lecture 18, Slide 29

  30. F P.A. Object F ACT: Diverging Lenses Which way should you move object so image is real? • Closer to lens • Further from lens • Diverging lens can’t create real image. Physics 102: Lecture 18, Slide 30

  31. F P.A. Object F ACT: Diverging Lenses Which way should you move object so image is real? • Closer to lens • Further from lens • Diverging lens can’t create real image. Demo Physics 102: Lecture 18, Slide 31

  32. See you next class! • Read Sections 23.9, 24.1, 3-4, 6 Physics 102: Lecture 18, Slide 32

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