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

Physics 102: Lecture 18. Snell’s Law, Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses. Snell’s Law: A Quick Review. When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends:. n 1 sin( q 1 )= n 2 sin( q 2 ).

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

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  1. Physics 102:Lecture 18 Snell’s Law, Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses

  2. Snell’s Law: A Quick Review When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends: n1 sin(q1)= n2 sin(q2) ACT: n1 q1 q1 < q2 sinq1< sinq2 n1> n2 1) n1 > n2 2) n1 = n2 3) n1 < n2 q2 n2 Compare n1 to n2. 44

  3. 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 06

  4. Example Snell’s Law Practice 1 r Usually, there is both reflection and refraction! A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60o. The other part of the beam is refracted. What is q2? n1 n2 normal 48

  5. Example Snell’s Law Practice 1 r Usually, there is both reflection and refraction! A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60o. The other part of the beam is refracted. What is q2? q1 =qr =60o sin(60o) = 1.33 sin(q2) n1 q2 = 40.6o n2 normal 48

  6. Preflight 18.1 Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ? “There are millions of light ’rays’ coming from the light. Some of the rays will be totally reflected back into the water, but most of them will not.” 1) Yes 2) No 51% 49% 10 8

  7. Green Flash • See http://mintaka.sdsu.edu/GF/ for all you could ever want to know about green flashes • For students looking for an honors project, this would be an interesting topic • see me if you are interested

  8. 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 11

  9. 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 11

  10. 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 Arthoscopy Laser surgery Total Internal Reflection only works if noutside < ninside 13

  11. 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 13

  12. Brewster’s angle horiz. polarized only! qB qB horiz. and vert. polarized 90º 90º-qB Reflected light is usually unpolarized (mixture of horizontally and vertically polarized). 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) 15

  13. 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. 19

  14. Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction? 60% 40% • 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 27% 62% 9% 1% When glare is around qB, it’s mostly horiz. polarized! 21

  15. 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 23

  16. Rainbow: 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! 25

  17. LIKE SO! In second rainbow pattern is reversed 25

  18. 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 27

  19. 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). 35

  20. 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 53% 20% 27% 30

  21. 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 40

  22. 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. 40

  23. 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. Only 1 case dir dirverging lens: Image is always virtual, upright, and reduced. 45

  24. 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 50

  25. See You Wednesday

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