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Reflection and Refraction of Light

Physics 102: Lecture 17. Reflection and Refraction of Light. q i. q r. Last Time. Today. q 1. n 1. n 2. q 2. Recall from last time…. Reflection:. q i = q r. Flat Mirror: image equidistant behind. Spherical Mirrors: Concave or Convex. Refraction:.

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Reflection and Refraction of Light

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  1. Physics 102: Lecture 17 Reflection and Refraction of Light

  2. qi qr Last Time Today q1 n1 n2 q2 Recall from last time…. Reflection: qi = qr Flat Mirror: image equidistant behind Spherical Mirrors: Concave or Convex Refraction: n1 sin(q1)= n2 sin(q2)

  3. #1 #2 #3 I 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 (light rays actually cross) Inverted (Arrow points opposite direction) Reduced (smaller than object) **Every other ray from object tip which hits mirror will reflect through image tip

  4. Preflight 17.1 Which ray is NOT correct? p.a. 1) C f 2) 3)

  5. Mirror Equation do f I di Works for concave, convex, or flat O c • do = distance object is from mirror: • Positive: object in front of mirror • Negative: object behind mirror • di = distance image is from mirror: • Positive: real image (in front of mirror) • Negative: virtual image (behind mirror) • f = focal length mirror: • Positive: concave mirror +R/2 • Negative: convex mirror –R/2

  6. Preflight 17.3 The image produced by a concave mirror of a real object is: • Always Real • Always Virtual • Sometimes Real, Sometimes Virtual

  7. ACT: Concave Mirror Where in front of a concave mirror should you place an object so that the image is virtual? • Close to mirror • Far from mirror • Either close or far • Not Possible

  8. 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 25

  9. 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. C f

  10. 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

  11. 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

  12. f image object • Demo: • two identical spherical mirrors • each mirror is positioned at the focal point of the other

  13. #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!):

  14. 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?

  15. Preflight 17.4 The image produced by a convex mirror of a real object is • always real • always virtual • sometimes real and sometimes virtual

  16. 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

  17. 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!

  18. 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.

  19. 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 q2 = n2 normal

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

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