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Reflection and Refraction of Light

Physics 102: Lecture 17. Reflection and Refraction of Light. Today’s Lecture will cover textbook sections 23.3, 8. q i. q r. Last Time. Today. q 1. Next time. n 1. n 2. q 2. Overview. Reflection:. q i = q r. Flat Mirror: image equidistant behind. Spherical Mirrors:

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Reflection and Refraction of Light

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  1. Physics 102: Lecture 17 Reflection and Refraction of Light • Today’s Lecture will cover textbook sections 23.3, 8 Physics 102: Lecture 17, Slide 1

  2. qi qr Last Time Today q1 Next time n1 n2 q2 Overview Reflection: qi = qr Flat Mirror: image equidistant behind Spherical Mirrors: Concave or Convex Refraction: n1 sin(q1)= n2 sin(q2) Flat Lens: Window Spherical Lenses: Concave or Convex Absorption Physics 102: Lecture 17, Slide 2

  3. #1 #2 #3 Concave Mirror Principal Rays 1) Parallel to principal axis reflects through f. 2) Through f, reflects parallel to principal axis. 3) Through center. O f c Image is (in this case): Real or Imaginary Inverted or Upright Reduced or Enlarged **Every other ray from object tip which hits mirror will reflect through image tip Physics 102: Lecture 17, Slide 3

  4. Preflight 17.1 Which ray is NOT correct? p.a. 1) R f 2) 3) Physics 102: Lecture 17, Slide 4

  5. Preflight 17.1 Which ray is NOT correct? Ray through center should reflect back on self. p.a. 1) R f 2) 3) Physics 102: Lecture 17, Slide 5

  6. Mirror Equation do I di O f c • do = distance object is from mirror: • Positive: object _______ of mirror • Negative: object _______ mirror • di = distance image is from mirror: • Positive: _______ image (__________ of mirror) • Negative: _______ image (__________ mirror) • f = focal length mirror: • Positive: _________ mirror • Negative: _________ mirror (coming soon) Physics 102: Lecture 17, Slide 6

  7. Mirror Equation do I di O f c • do = distance object is from mirror: • Positive: object in front of mirror • Negative: object behind mirror • di = distance image is from mirror: • Positive: inverted image (in front of mirror) • Negative: upright image (behind mirror) • f = focal length mirror: • Positive: concave mirror • Negative: convex mirror (coming soon) Physics 102: Lecture 17, Slide 7

  8. Preflight 17.3 The image produced by a concave mirror of a real object is: • Always Real • Always Virtual • Sometimes Real, Sometimes Virtual ACT: Concave Mirror Where in front of a concave mirror should you place an object so that the image is virtual? 1) Close to mirror 3) Either close or far 2) Far from mirror 4) Not Possible Physics 102: Lecture 17, Slide 8

  9. Mirror Equation: Preflight 17.3 The image produced by a concave mirror of a real object is: • Always Real • Always Virtual • Sometimes Real, Sometimes Virtual Concave mirror: f > 0 Real Object means in front of mirror: do > 0 di can be negative or positive!

  10. ACT: Concave Mirror Where in front of a concave mirror should you place an object so that the image is virtual? Mirror Equation: • Close to mirror • Far from mirror • Either close or far • Not Possible • Concave mirror: f > 0 • Object in front of mirror: do > 0 • Virtual image means behind mirror: di < 0 • When do < f then di <0 : virtual image. Physics 102: Lecture 17, Slide 10

  11. Magnification Equation do do Angle of incidence I di ho di hi Angle of reflection O • ho = height of object: • Positive: • hi = height of image: • Positive: • Negative: • m = magnification: • Positive / Negative: same as for hi • < 1: • > 1: Physics 102: Lecture 17, Slide 11

  12. Magnification Equation q do do q Angle of incidence I di ho q di hi q Angle of reflection O • ho = height of object: • Positive: always • hi = height of image: • Positive: image is upright • Negative: image is inverted • m = magnification: • Positive / Negative: same as for hi • < 1: image is reduced • > 1: image is enlarged Physics 102: Lecture 17, Slide 12

  13. Preflight 17.2 Compared to the candle, the image will be: • Larger • Smaller • Same Size Example Solving Equations A candle is placed 6 cm in front of a concave mirror with focal length f=2 cm. Determine the image location. p.a. R f Physics 102: Lecture 17, Slide 13

  14. Preflight 17.2 Compared to the candle, the image will be: • Larger • Smaller • Same Size Example Solving Equations A candle is placed 6 cm in front of a concave mirror with focal length f=2 cm. Determine the image location. di = + 3 cm (in front of mirror) Real Image! p.a. R f Physics 102: Lecture 17, Slide 14

  15. ACT: Magnification A 4 inch arrow pointing down is placed in front of a mirror that creates an image with a magnification of –2. • What is the size of the image? • 2 inches • 4 inches • 8 inches 4 inches • What direction will the image arrow point? • Up 2) Down Physics 102: Lecture 17, Slide 15

  16. ACT: Magnification A 4 inch arrow pointing down is placed in front of a mirror that creates an image with a magnification of –2. • What is the size of the image? • 2 inches • 4 inches • 8 inches 4 inches Magnitude gives us size. • What direction will the image arrow point? • Up 2) Down (-) sign tells us it’s inverted from object Physics 102: Lecture 17, Slide 16

  17. C C C F F F Object Image Image Object Object Image 3 Cases for Concave Mirrors Upright Enlarged Virtual Inside F Inverted Enlarged Real Between C&F Inverted Reduced Real Past C Physics 102: Lecture 17, Slide 17

  18. #1 #2 #3 Convex Mirror Rays 1) Parallel to principal axis reflects ______________. 2) Through f, reflects ______________________. 3) Through center. Complete the rays! O c f Image is: Virtual or Real Upright or Inverted Reduced or Enlarged (always true for convex mirrors!) Physics 102: Lecture 17, Slide 18

  19. #1 I #2 #3 Convex Mirror Rays 1) Parallel to principal axis reflects through f. 2) Through f, reflects parallel to principal axis. 3) Through center. O f c Image is: Virtual (light rays don’t really cross) Upright (same direction as object) Reduced (smaller than object) (always true for convex mirrors!): Physics 102: Lecture 17, Slide 19

  20. Example Solving Equations A candle is placed 6 cm in front of a convex mirror with focal length f=-3 cm. Determine the image location. Determine the magnification of the candle. If the candle is 9 cm tall, how tall does the image candle appear to be? Physics 102: Lecture 17, Slide 20

  21. Virtual Image! Image is Upright! Example Solving Equations A candle is placed 6 cm in front of a convex mirror with focal length f=-3 cm. Determine the image location. Determine the magnification of the candle. If the candle is 9 cm tall, how tall does the image candle appear to be? di = - 2 cm (behind mirror) m = + 1/3 hi = + 3 cm Physics 102: Lecture 17, Slide 21

  22. Preflight 17.4 Where should you place an object in front of a convex mirror to produce a real image? • Object close to mirror • Object far from mirror • Either close or far • You can’t Physics 102: Lecture 17, Slide 22

  23. di is negative! do is positive f is negative Preflight 17.4 Where should you place an object in front of a convex mirror to produce a real image? Mirror Equation: • Object close to mirror • Object far from mirror • Either close or far • You can’t • Convex mirror: f < 0 • Object in front of mirror: do > 0 • Real image means di > 0 Physics 102: Lecture 17, Slide 23

  24. Mirror Summary • Angle of incidence = Angle of Reflection • Principal Rays • Parallel to P.A.: Reflects through focus • Through focus: Reflects parallel to P.A. • Through center: Reflects back on self • |f| = R/2 Physics 102: Lecture 17, Slide 24

  25. qi qr q1 n1 n2 q2 Light Doesn’t Just Bounce It Also Refracts! Reflected: Bounces (Mirrors!) qi = qr Refracted: Bends (Lenses!) n1 sin(q1)= n2 sin(q2) Physics 102: Lecture 17, Slide 25

  26. Speed of light in vacuum Speed of light in medium Index of refraction Index of Refraction 186,000 miles/second: it’s not just a good idea, it’s the law! so always! Physics 102: Lecture 17, Slide 26

  27. Snell’s Law 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) Preflight 17.6 n1 q1 1) n1 > n2 2) n1 = n2 3) n1 < n2 q2 n2 Compare n1 to n2. Physics 102: Lecture 17, Slide 27

  28. Snell’s Law 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) Preflight 17.6 n1 q1 1) n1 > n2 2) n1 = n2 3) n1 < n2 q1 < q2 q2 n2 sinq1< sinq2 n1> n2 Compare n1 to n2. Physics 102: Lecture 17, Slide 28

  29. 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 = 60. The other part of the beam is refracted. What is q2? n1 n2 normal Physics 102: Lecture 17, Slide 29

  30. 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 = 60. The other part of the beam is refracted. What is q2? q1 =qr =60 sin(60) = 1.33 sin(q2) n1 q2 = 40.6 degrees n2 normal Physics 102: Lecture 17, Slide 30

  31. Apparent depth: d apparent fish d actual fish Apparent Depth n2 n1 50

  32. See you later! • Read Sections 23.3 - 23.5, 23.9 Physics 102: Lecture 17, Slide 32

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