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PHY138 – Waves, Lecture 7 Today’s overview

This lecture covers the nature of light, ray tracing, the law of reflection, and Snell's law of refraction. Students are required to read assigned chapters and complete a quiz.

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PHY138 – Waves, Lecture 7 Today’s overview

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  1. PHY138 – Waves, Lecture 7Today’s overview • The Nature of Light • Ray Tracing in Optics • Reflection of Light: Law of Reflection • Refraction of Light: Snell’s Law

  2. Reading Assignment • Please read the following from Serway and Jewett before class on Wednesday: Chapter 25, Sections 25.7, 25.8 and Chapter 26, Section 26.1. • A Web-CT quiz is due Wednesday morning, which tests your basic familiarity with Chapter 26 assigned reading. It’s the last Web-CT quiz of this quarter.

  3. Announcements: • The assignment due on Monday, December 6 at 5:00 PM is now available online. It includes problems from Chapter 25 and one from Chapter 26. It’s the last assignment of the Waves Quarter. • Test 2 is at 9:00 AM sharp on Friday Dec. 10. One double-sided aid-sheet on letter-sized paper per student will be allowed. Test 2 covers material from the Waves Quarter.

  4. Wave Fronts and Rays • Wave fronts connect points of equal phase in neighbouring points on an extended wave. • If the wave source is a point, wave fronts are concentric expanding spheres, all centred on the source. • A very large or distant source can create plane wave fronts. • Rays show the propagation direction of waves, and are always perpendicular to wave fronts.

  5. The Law of Reflection

  6. Quiz A dentist uses a mirror to look at the back of a second molar (A). Next, she wishes to look at the back of a lateral incisor (B), which is 90° away. By what angle should she rotate her mirror? Mirror A B 1. 90° 2. 45° 3. 180°

  7. Specular vs. Diffuse Reflection • Specular Reflection • The surface is flat at distance scales near or above the wavelength of light • It looks “shiny”, like a mirror.

  8. Specular vs. Diffuse Reflection • Diffuse Reflection • The surface is rough at distance scales near or above the wavelength of light • Almost all surfaces reflect in this way.

  9. Index of Refraction • v is the speed of light in a transparent medium. • c is the speed of light in a vacuum (c=3.00×108 m/s) • n is a new dimensionless constant: n≥1

  10. Snell’s Law of Refraction

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