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Physics 7C SS1, Lecture 5: Wave and Ray Models

Physics 7C SS1, Lecture 5: Wave and Ray Models. Standing waves Reflection/Refraction Optics. Agenda. Determine content students want lecture about Discuss any review material about interference (including beats and standing waves)

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Physics 7C SS1, Lecture 5: Wave and Ray Models

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  1. Physics 7C SS1, Lecture 5: Wave and Ray Models Standing waves Reflection/Refraction Optics

  2. Agenda • Determine content students want lecture about • Discuss any review material about interference (including beats and standing waves) • Develop any current material about the ray model (reflection and refraction) • Introduce any new material about the ray model (optics)

  3. What should lecture cover?(1st choice) Review of aspects of the wave model • Multiple-slit interference • Beats • Standing Waves Move forward with the ray model • Reflection and refraction • Next material: lenses

  4. What should lecture cover?(2nd choice) Review of aspects of the wave model • Multiple-slit interference • Beats • Standing Waves Move forward with the ray model • Reflection and refraction • Next material: lenses

  5. Interference Fundamentals • = -4, -2, 0, +2… is constructive • = -3, -, + … is destructive • =anything else…is partial

  6. Interference for differing frequencies: Beats 1 2 3 4 5 What type of interference occurs at each point?

  7. Interference for differing frequencies: Beats 1 2 3 4 5

  8. Interference for differing frequencies: Beats 1 2 3 4 5 The carrier frequency The beat frequency

  9. Standing Waves • Fundamental

  10. Standing Waves • 2nd Harmonic • 3rd Harmonic N = “node” A = “antinode”

  11. Standing Waves • 2nd Harmonic • 3rd Harmonic • What type of interference occurs at an antinode? • Constructive • Destructive • Time-dependent • Depends which antinode

  12. Testing Standing Waves • When the wave looks like this: the frequency is 45Hz. What is the lowest possible frequency? 15 Hz 30 Hz 135 Hz Other 9 Hz 12.5 Hz

  13. Testing Standing Waves • When the wave looks like this: the frequency is 45 Hz. What is the highest possible frequency? 15 Hz 30 Hz 135 Hz Other 9 Hz 12.5 Hz

  14. Testing Standing Waves • The figure below is for node-node waves. Draw the same harmonic for antinode-node waves. If this frequency is 45 Hz. What is the lowest possible frequency? (enter a numeric response rounded to the nearest whole number, no decimal places)

  15. Testing Standing Waves • The figure below is for node-node waves. Draw the same harmonic for antinode-node waves. If this frequency is 45 Hz. What is the lowest possible frequency? (enter a numeric response rounded to the nearest whole number, no decimal places) 15 Hz 30 Hz 135 Hz Other 9 Hz 12.5 Hz

  16. Wavefronts and Rays • A wavefront represent points of equal phase (e.g. the crest of the wave). • The ray shows the direction in which the wavefront is moving. • Rays are perpendicular to wavefronts.

  17. From spherical to planar wavefronts • Spherical wave front • Planar wave front

  18. Identify the representation: 1 2 Both wavefront Both ray 1 is wavefront, 2 is ray 1 is ray, 2 is wavefront

  19. What (typically) happens next? 1 2 There is a reflected wave There is a transmitted wave Both reflected and transmitted waves Neither reflected nor transmitted waves

  20. Transmitted Ray: • Drive a car from a well-paved road into rough grass. Which way will it go? (a) (b) (c)

  21. Transmitted Ray: • Transmitted ray is bent, or “refracted” Angle of Incidence Law of Refraction or “Snell’s Law:” n1sin1=n2sin2 n : “index or refraction” speed of light in vacuum speed of light in material Angle of Refraction

  22. Reflected Ray • Throw a ball at a wall, view from above. Which is the correct path, assuming a perfectly elastic collision? (a) (b) (c) (d) Depends

  23. Reflected Ray Law of Reflection: i=r Normal Line Angle of Incidence Angle of Reflection

  24. Reflected Wavefronts Law of Reflection: i=r Normal Line Angle of Incidence Angle of Reflection

  25. Observe the Water Compass In which medium does light travel faster? • Air • Water • Same in both What happens if the angle in the water gets increasingly large?

  26. Observe on the Optics Board • What are you seeing? • Reflection • Refraction • Total Internal Reflection • Convergence • Divergence

  27. Light at an Interface (Activity 8.6.3 & FNT 1) Incoming Ray Air n = 1 Plastic prism n = 1.5 On entering the prism (first boundary), which way will the light bend? a) Up b) Down c) No bend

  28. Light at an Interface Which Ray? Incoming Ray (a) (b) Air n = 1 Plastic prism n = 1.5 (c)

  29. Parallel rays are uncommon… how do lenses work on regular objects?

  30. What do we see?

  31. Every part of the tree reflects light, scattering it all directions. A ray diagram is used to simplify this picture.

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