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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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**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) • Develop any current material about the ray model (reflection and refraction) • Introduce any new material about the ray model (optics)**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**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**Interference Fundamentals**• = -4, -2, 0, +2… is constructive • = -3, -, + … is destructive • =anything else…is partial**Interference for differing frequencies: Beats**1 2 3 4 5 What type of interference occurs at each point?**Interference for differing frequencies: Beats**1 2 3 4 5 The carrier frequency The beat frequency**Standing Waves**• Fundamental**Standing Waves**• 2nd Harmonic • 3rd Harmonic N = “node” A = “antinode”**Standing Waves**• 2nd Harmonic • 3rd Harmonic • What type of interference occurs at an antinode? • Constructive • Destructive • Time-dependent • Depends which antinode**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**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**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)**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**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.**From spherical to planar wavefronts**• Spherical wave front • Planar wave front**Identify the representation:**1 2 Both wavefront Both ray 1 is wavefront, 2 is ray 1 is ray, 2 is wavefront**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**Transmitted Ray:**• Drive a car from a well-paved road into rough grass. Which way will it go? (a) (b) (c)**Transmitted Ray:**• Transmitted ray is bent, or “refracted” Angle of Incidence Law of Refraction or “Snell’s Law:” n1sin1=n2sin2 n : “index or refraction” speed of light in vacuum speed of light in material Angle of Refraction**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**Reflected Ray**Law of Reflection: i=r Normal Line Angle of Incidence Angle of Reflection**Reflected Wavefronts**Law of Reflection: i=r Normal Line Angle of Incidence Angle of Reflection**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?**Observe on the Optics Board**• What are you seeing? • Reflection • Refraction • Total Internal Reflection • Convergence • Divergence**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**Light at an Interface**Which Ray? Incoming Ray (a) (b) Air n = 1 Plastic prism n = 1.5 (c)**Parallel rays are uncommon…**how do lenses work on regular objects?**Every part of the tree reflects light, scattering it all**directions. A ray diagram is used to simplify this picture.

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