PHY122_L18_PostingAfterLecture

PHY122_L18_PostingAfterLecture - PHY122 – Physics for the...

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Unformatted text preview: PHY122 – Physics for the Life Sciences II Lecture 18 (Coincidence: Also Chapter 18 of textbook) Ray Optics HW 11 due Sunday, 13 Nov. at 8 pm Note: Clicker Channel 21 11/08/2011 Lecture 18 1 Grade distributions for midterm exams 11/08/2011 Lecture 18 2 The intensity of electromagnetic waves emitted from a point source in all directions, decreases like … 11/08/2011 Lecture 18 3 A. B. C. 9% 8% 83% A. 1/distance B. (1/distance) 2 C. (1/distance) 3 correct Lecture 18 4 Ray Optics and EM Waves Because the propagation direction v is ⊥ E and ⊥ B (via the right-hand rule), the propagation is in a straight line until a conductive, dielectric, or magnetic surface is encountered • at a conductive surface, the changing electric and magnetic fields cause the mobile charges to oscillate in a synchronous motion, and that motion in turn causes the re-emission of EM waves • REFLECTION is caused by the synchronously phased motions of mobile charges everywhere in the surface, which causes the emergence of an EM wave front which is an accurate reflection of the incident wave • similarly, at a dielectric surface REFRACTION also takes place: the refracted wave propagates at smaller speed inside the dielectric, and thus has a direction different from the incident wave 11/08/2011 Ray Optics Rays travel in straight lines … – A ray ravels on until it interacts with matter … – Rays can cross – Objects are SOURCES of light rays The eye sees by focusing bundles of rays onto the retina … 11/08/2011 Lecture 18 5 When an object like a tree is illuminated by the sun, and you are looking toward the tree, light rays leave the object 11/08/2011 Lecture 18 6 A. B. C. D. 2% 85% 4% 9% A. only from points at the top and base of the tree, but in every direction. B. from every point on the surface of the tree, but only toward your eyes. C. only from points at the top and base of the tree, but only toward your eyes. D. from every point on the surface of the tree and in every direction. correct Seeing Objects Directly or Indirectly … Light can reach our eye directly from a light source: or indirectly , after scattering off matter: 11/08/2011 Lecture 18 7 But we never “see an object”; we always see an “image of the object”. Shadows Shadows can be easily calculated with “Ray Tracing” when the sizes of the object and the light source, and their relative positions to the viewer (eye) are known – e.g., depending on the size of the light source, the shadow of the object may be total or partial....
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PHY122_L18_PostingAfterLecture - PHY122 – Physics for the...

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