1. PHY138 – Waves, Lecture 7 Today’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 Mirror 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? 1. 90° 2. 45° 3. 180° A B

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