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Notes 20 Nature of Light Reflection and refraction

Notes 20 Nature of Light Reflection and refraction. Honors Physics 2. Today’s summary. Nature of light and comparison of wave, ray, photon approaches. Ray optics and perception of images Electromagnetic waves at boundaries Reflection and refraction Fermat’s principle Basic imaging.

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Notes 20 Nature of Light Reflection and refraction

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  1. Notes 20Nature of Light Reflection and refraction Honors Physics 2 Honors Physics 2 - Persans

  2. Today’s summary • Nature of light and comparison of wave, ray, photon approaches. • Ray optics and perception of images • Electromagnetic waves at boundaries • Reflection and refraction • Fermat’s principle • Basic imaging Honors Physics 2 - Persans

  3. What is special about light? • the speed limit for information transfer • massless particles with energy and momentum • directly detected by our senses. • source of many beautiful and interesting effects in nature • the carrier for all of the energy we use from the sun • The historical source of our problems with wave/particle duality. Honors Physics 2 - Persans

  4. There are three important and useful ways to think about light: ·       rays- geometric optics ·       particles- quantum optics ·       waves- physical and nonlinear optics We will start with the properties of light and interpret them in the particle and wave pictures. ·       speed ·       energy transport ·       momentum transfer Honors Physics 2 - Persans

  5. Exercise 20.01 _____________ Estimate the following: a) speed of light in vacuum (m/s) b) wavelength of visible light (in meters) c) energy (in eV and joules) of a visible light photon d) frequency of visible light e) index of refraction of glass Honors Physics 2 - Persans

  6. Speed In the wave picture the wave front moves at: In the particle picture the energy is carried by particles called photons, which move at the speed of light. (Special relativity theory demands that photons be massless.) Honors Physics 2 - Persans

  7. Intensity in the wave picture Honors Physics 2 - Persans

  8. In the photon picture, each photon carries a discrete energy Ep=h where h=Plank’s constant= 6.6x10-34 J s = frequency of wave and intensity is due to addition of many photons. I=Fh where F=flux in photons/(s m2) ( where  is the particle density in space.) Honors Physics 2 - Persans

  9. Summary: Properties of Photons • move at the speed of light. • carry energy E=h=hc/. • carry momentum p=E/c=h/. • exhibit wave-like properties in interference experiments. • exhibit particle-like properties in collision experiments. • To be consistent with special relativity, photons must have zero mass. • Light exhibits “particle” or “wave” behavior depending on what experiment you do!!! Honors Physics 2 - Persans

  10. Electromagnetic waves at boundaries • Consideration of how electric fields must behave at a boundary leads to rules for reflection and refraction. • An example: • Electric fields parallel to conductor surfaces must be zero at the surface. (Charge moves to cancel the field.) • This means that the field switches sign and reflects. Honors Physics 2 - Persans

  11. Ray Optics • Ray propagation • Fermat’s Principle • Refraction • Reflection • Imaging • Surfaces and interfaces • Mirrors • Lenses Honors Physics 2 - Persans

  12. Geometric (Ray) Optics ·  In geometric (or ray) optics we treat light as a ray that takes the shortest time to travel from one point to another. The shortest time path is a straight line if the medium is uniform but may be a bent path if the speed depends on position. bundle of rays Honors Physics 2 - Persans

  13. Things that rays do • travel in straight line from point to point • specular reflection from a plane surface • diffuse reflection from a rough surface • refract when passing from one material (index of refraction) to another Honors Physics 2 - Persans

  14. Ray optics is a useful approximationfor imaging d  Honors Physics 2 - Persans

  15. A fundamental idea applied to ray tracing: Fermat’s principle • A light ray traveling from one point to another follows a path such that, compared with nearby paths, the time required is either a minimum or a maximum or remains unchanged. • A version of this idea is known as the Principle of Least Action in quantum mechanics and mechanics. Honors Physics 2 - Persans

  16. Fermat’s Principle and reflection Honors Physics 2 - Persans

  17. Specular Reflection • In specular reflection, the angle of incidence is equal to the angle of reflection. Don’t do this with laser beams. Honors Physics 2 - Persans

  18. Specular reflection and images:mirrors • Plane • Concave • Convex • Ray tracing • Sign conventions Honors Physics 2 - Persans

  19. Rays reflected from a plane mirror object image the image is where the various rays appear to come from i o Honors Physics 2 - Persans

  20. Perception of an image in a plane mirror two rays are sufficient to determine the image position rays from the real object real object the virtual image (where your head thinks the rays came from.) a head a mirror image distance=-object distance i=-o the virtual image is the same size as the object Honors Physics 2 - Persans

  21. iClicker question 20.2 How tall a mirror do you need to view your entire reflection? A) your full height B) ¾ height C) ½ height D) dependson distance Honors Physics 2 - Persans

  22. stick man Exercise 20.2 ____________ • How tall a mirror do you need to see your entire body? stick mirror • Where is the image and how large is it? Sketch it in. • Draw rays from the image to the eyes. Honors Physics 2 - Persans

  23. Refraction • Wave front directions • Ray pointing directions • Fermat’s Principle Honors Physics 2 - Persans

  24. Fermat’s Principle and refraction Honors Physics 2 - Persans

  25. Refraction: Snell’s Law n1 n2 2 1 iClicker: Is n1 A. >n2 B. <n2 C. equal Honors Physics 2 - Persans

  26. Refraction effect on apparent object position stick man stick goatee The stickhunter will miss if he doesn’t correct for refraction. a “fish” Honors Physics 2 - Persans

  27. iClicker question 19.3 If you view an object in a pool from directly above, does it appear to be • closer • further • the same distance compared to the measured distance? a shark Honors Physics 2 - Persans

  28. A cultural appendix How do we perceive objects? Honors Physics 2 - Persans

  29. How do you perceive an object? Only a small fraction of the rays from a point pass through the pupil of your eye. Rays from different points are focussed to different positions on the retina. A big eye: Note: You can determine the angular size, but you don't know how far away the object is. Honors Physics 2 - Persans

  30. Perception example http://www.psychologie.tu-dresden.de/i1/kaw/diverses%20Material/www.illusionworks.com/html/jump_page.html Honors Physics 2 - Persans

  31. Binocular vision a head • You determine distance both by the angle between your two eyes and by the relative position of things that you believe to be distant background objects. • You also determine relative distance by: • how near objects appear to move relative to distant objects • the apparent relative size of things • Your brain plays an big role in determining position and size. Honors Physics 2 - Persans

  32. Diffuse reflection is important for visibility of objects source “eye” scattering surface Honors Physics 2 - Persans

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