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Copyright © 2012 Pearson Education Inc. PowerPoint ® Lectures for University Physics, Thirteenth Edition – Hugh D. Young and Roger A. Freedman Lectures by Wayne Anderson Chapter 33 The Nature and Propagation of Light
Copyright © 2012 Pearson Education Inc. Goals for Chapter 33 To understand light rays and wavefronts To analyze reflection and refraction of light To understand total internal reflection To analyze the polarization of light To use Huygens’s principle to analyze reflection and refraction
Copyright © 2012 Pearson Education Inc. Introduction Why does a rainbow of colors appear when these tools are placed between polarizing filters? Our study of light will help us understand why the sky is blue and why we sometimes see a mirage in the desert. Huygens’s principle will connect the ray and wave models of light.
Copyright © 2012 Pearson Education Inc. The nature of light Light has properties of both waves and particles. The wave model is easier for explaining propagation, but some other behavior requires the particle model. The rays are perpendicular to the wave fronts. See Figure 33.4 at the right.
Copyright © 2012 Pearson Education Inc. Reflection and refraction In Figure 33.5 the light is both reflected and refracted by the window.
Copyright © 2012 Pearson Education Inc. Specular and diffuse reflection Specular reflection occurs at a very smooth surface (left figure). Diffuse reflection occurs at a rough surface (right figure). Our primary concern is with specular reflection.
Copyright © 2012 Pearson Education Inc. Laws of reflection and refraction The frequency does not change on passing through a surface, but velocity does, and so wavelength. f = f 0 => v/ = v 0 / 0 => v/c = v 0 /c 0 The index of refraction is n = c/v >1. Angles are measured with respect to the normal. Reflection: The angle of reflection is equal to the angle of incidence. Refraction: Snell’s law applies. In a material = 0 /n. Figure 33.7 (right) illustrates the laws of reflection and refraction.
Copyright © 2012 Pearson Education Inc. Reflection and refraction in three cases Figure 33.8 below shows three important cases: If n b > n a, the refracted ray is bent toward the normal. If n b < n a, the refracted ray is bent away from the normal. A ray oriented along the normal never bends.
Copyright © 2012 Pearson Education Inc. Why does the ruler appear to be bent? The straight ruler in Figure 33.9(a) appears to bend at the surface of the water. Figure 33.9(b) shows why.
Copyright © 2012 Pearson Education Inc. Some indexes of refraction Air
Copyright © 2012 Pearson Education Inc. An example of reflection and refraction Read Problem-Solving Strategy Example 33.1, find the angles of reflection ( r ) and refraction ( b ). Use Figure below.
Copyright © 2012 Pearson Education Inc. The eye and two mirrors Example 33.3 reflection from two mirrors. Use Figure below.
Copyright © 2012 Pearson Education Inc. Total internal reflection Light striking at the critical angle emerges tangent to the surface. (See Figure below.) If a > crit, the light is undergoes total internal reflection.
Copyright © 2012 Pearson Education Inc. Some applications of total internal reflection A binocular using Porro prisms (below) and a “light pipe” (right) make use of total internal reflection in their design. Can the above work if the outside of the rod is air? How do optical fibers work?
Copyright © 2012 Pearson Education Inc. A diamond and a periscope Diamonds sparkle because they are cut so that total internal reflection occurs on their back surfaces. See Figure below. Example 33.4, leaky, crown-glass periscope. Why will periscope no longer work if one of the prisms is in water?
Copyright © 2012 Pearson Education Inc. Dispersion Dispersion: The index of refraction depends on the wavelength of the light. See Figure (right). Figure (below) shows dispersion by a prism.
Copyright © 2012 Pearson Education Inc. Rainbows—I The formation of a rainbow is due to the combined effects of dispersion, refraction, and reflection. (See Figure below and on the next slide.)
Copyright © 2012 Pearson Education Inc. Rainbows—II
Copyright © 2012 Pearson Education Inc. Polarization An electromagnetic wave is linearly polarized if the electric field has only one component. Figure at the right shows a Polaroid polarizing filter.
Copyright © 2012 Pearson Education Inc. Malus’s law Figure below shows a polarizer and an analyzer. A polarizer reduces the intensity of unpolarized light (I 0 ) by a factor of 2, so the intensity of transmitted light is I 0 /2. A second polarizer (the analyzer) at angle relative to the first further reduces the intensity according to: Malus’s law: I = I max cos 2 . Example polarizer and analyzer with = 30°.
Copyright © 2012 Pearson Education Inc. Polarization by reflection When light is reflected at the polarizing angle p (Brewster’s angle), the reflected light is linearly polarized. See Figure below. The polarizing angle p is when the reflected and refracted rays are 90° from each other, i.e. when p + b = 90 °.
Copyright © 2012 Pearson Education Inc. Reflection from a swimming pool Follow Example 33.6 using Figure below. fully polarized partially polarized fully polarized partially polarized
Copyright © 2012 Pearson Education Inc. Circular polarization Circular polarization results from the superposition of two perpendicularly polarized electromagnetic waves having equal amplitude but a quarter-cycle phase difference. The result is that the electric field vector has constant amplitude but rotates about the direction of propagation. (Figure below.)
Copyright © 2012 Pearson Education Inc. Scattering of light Scattering occurs when light has been absorbed by molecules and reradiated. Figure below shows the effect of scattering for two observers.
Copyright © 2012 Pearson Education Inc. Why are clouds white? Clouds are white because they scatter all wavelengths efficiently. See Figure below.
Copyright © 2012 Pearson Education Inc. Huygens’s principle Huygens’s principle: Every point of a wave front can be considered to be a source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the wave. See Figure at the right.
Copyright © 2012 Pearson Education Inc. Reflection and Huygens’s principle Figure at the right shows how Huygens’s principle can be used to derive the law of reflection.
Copyright © 2012 Pearson Education Inc. Refraction and Huygens’s principle Huygens’s principle can be used to derive the law of refraction. Follow the text analysis using Figure below.
Copyright © 2012 Pearson Education Inc. A mirage Huygens’s principle can also explain the formation of a mirage. See Figure below.
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