Chapter 33 The Nature and Propagation of Light PowerPoint Lectures for University Physics, 14th Edition Hugh D. Young and Roger A. Freedman 2016 Pearson Education Inc. Lectures by Jason Harlow Learning Goals for Chapter 33 Looking forward at what light rays are, and how they are related to wave fronts.
the laws that govern the reflection and refraction of light. the circumstances under which light is totally reflected at an interface. how to make polarized light out of ordinary light. how the scattering of light explains the blue color of the sky. how Huygenss principle helps us analyze reflection and refraction. 2016 Pearson Education Inc. Introduction When a cut diamond is illuminated with white light,
it sparkles brilliantly with a spectrum of vivid colors. These distinctive visual features are a result of light traveling much slower in diamond than in air, and that light of different colors travels at different speeds in diamond. But by studying the branch of physics called optics, we can understand the blue color of the sky and the design of optical devices such as telescopes, microscopes, cameras, eyeglasses, and the human eye. 2016 Pearson Education Inc. Waves and wave fronts
A wave front is the locus of all adjacent points at which the phase of a wave is the same. Spherical wave fronts of sound spread out uniformly in all directions from a point source. Electromagnetic waves in vacuum also spread out as shown here. 2016 Pearson Education Inc.
Wave fronts and rays Its often convenient to represent a light wave by rays rather than by wave fronts. A ray is an imaginary line along the direction of travel of the wave. When waves travel in a homogeneous isotropic material, the rays are always straight lines normal to the wave fronts. 2016 Pearson Education Inc.
Wave fronts and rays Far away from a source, where the radii of the spheres have become very large, a section of a spherical surface can be considered as a plane, and we have a plane wave. 2016 Pearson Education Inc. Reflection and refraction 2016 Pearson Education Inc. Reflection and refraction
When a light wave strikes a smooth interface separating two transparent materials (such as air and glass or water and glass), the wave is in general partly reflected and partly refracted (transmitted) into the second material. The segments of plane waves can be represented by bundles of rays forming beams of light. For simplicity we often draw only one ray in each beam. 2016 Pearson Education Inc. Diffuse and specular reflection
Our primary concern in this chapter will be with specular reflection from a very smooth surface such as highly polished glass or metal (a). Scattered reflection from a rough surface is called diffuse reflection (b). The vast majority of objects in your environment are visible to you because they reflect light in a diffuse manner. 2016 Pearson Education Inc.
The law of reflection The angle of reflection is equal to the angle of incidence for all wavelengths and for any pair of materials. Note that all angles are measured from the normal. 2016 Pearson Education Inc. Index of refraction The index of refraction of an optical material (also called the refractive index), denoted by n, is defined as:
For the case shown here, material b has a larger index of refraction than material a (nb > na) and the angle b is smaller than a. 2016 Pearson Education Inc. The law of refraction This result is also called Snells law, after the Dutch scientist Willebrord Snell (15911626). 2016 Pearson Education Inc.
Reflection and refraction: Case 1 of 3 When a ray passes from one material into another material having a larger index of refraction and hence a slower wave speed, the angle b with the normal is smaller in the second material than the angle a in the first. 2016 Pearson Education Inc. Reflection and refraction: Case 2 of 3 When a ray passes from one material into another material having a smaller index of refraction and hence a faster wave speed, the angle b with the normal is larger in the second material than the angle a in the first.
2016 Pearson Education Inc. Reflection and refraction: Case 3 of 3 In the case of normal incidence, the transmitted ray is not bent at all. In this case a = 0 and sin a = 0, so b is also equal to zero; the transmitted ray is also normal to the interface. r is also equal to zero, so the reflected ray travels back along the same path as the incident ray. 2016 Pearson Education Inc.
Why does the ruler appear to be bent? The law of refraction explains why a partially submerged straight ruler appears bent. Light rays coming from below the surface change in direction at the airwater interface, so the rays appear to be coming from a position above their actual point of origin. 2016 Pearson Education Inc. Why does the ruler appear to be bent? 2016 Pearson Education Inc.
Index of refraction for yellow light Substance Index of Refraction, n Ice (H2O) 1.309 Water (H2O) at 20C 1.333
Glycerine at 20C 1.473 Crown glass (typical value) 1.52 Rock salt (NaCl) 1.544
Quartz (SiO2) 1.544 Diamond (C) 2.417 2016 Pearson Education Inc. Index of refraction and the wave aspects of light The frequency f of a wave does not change when passing
from one material to another. In any material, v = ff ; since f is the same in any material as in vacuum and v is always less than the wave speed c in vacuum, f is also correspondingly reduced. When a wave passes from one material into a second material the waves get squeezed (the wavelength gets shorter) if the wave speed decreases and get stretched (the wavelength gets longer) if the wave speed increases. 2016 Pearson Education Inc. Total internal reflection Under certain circumstances, all of the light can be reflected
back from an interface, even though the second material is transparent. This is true for rays 3 and 4. The reflected portions of rays 1, 2, and 3 are omitted for clarity. 2016 Pearson Education Inc. Total internal reflection If the angle of incidence is larger than a critical angle,
the ray cannot pass into the upper material; it is completely reflected at the boundary surface. This situation occurs only when nb < na. 2016 Pearson Education Inc. Fiber optics When a beam of light enters at one end of a transparent rod, the
light can be totally reflected internally if the index of refraction of the rod is greater than that of the surrounding material. The light is trapped within even a curved rod, provided that the curvature is not too great. 2016 Pearson Education Inc.
Dispersion The speed of light in vacuum is the same for all wavelengths, but the speed in a material substance is different for different wavelengths. The dependence of wave speed and index of refraction on wavelength is called dispersion. In most materials the value of n decreases with increasing wavelength and decreasing
frequency. 2016 Pearson Education Inc. Dispersion Ordinary white light is a superposition of waves with all visible wavelengths. The band of dispersed colors is called a spectrum. 2016 Pearson Education Inc. How rainbows form: Slide 1 of 3 When sunlight enters a spherical water droplet
suspended in the air, it is (partially) reflected from the back surface of the droplet, and is refracted again upon exiting the droplet. A light ray that enters the middle of the raindrop is reflected straight back. All other rays exit the raindrop within an angle of that middle ray, with many rays piling up at the angle . 2016 Pearson Education Inc. How rainbows form: Slide 2 of 3
2016 Pearson Education Inc. How rainbows form: Slide 3 of 3 In many cases you can see a second, larger rainbow. It is the result of two reflections from the back surface of the droplet. Just as a mirror held up to a book reverses the printed letters, so the second reflection reverses the sequence of colors in the
secondary rainbow. 2016 Pearson Education Inc. Polarization An electromagnetic wave is linearly polarized if the electric field has only one component. Light from most sources such as light bulbs is a random mixture of waves linearly polarized in all possible transverse directions; such light
is called unpolarized light or natural light. A Polaroid polarizing filter can convert unpolarized light to linearly polarized light. 2016 Pearson Education Inc. Maluss law When polarized light of intensity Imax is incident on a polarizing filter used as an analyzer, the intensity I of the light transmitted through the
analyzer depends on the angle between the polarization direction of the incident light and the polarizing axis of the analyzer. 2016 Pearson Education Inc. Polarization by reflection Unpolarized light can be polarized, either partially or totally, by reflection. At one particular angle of
incidence, called the polarizing angle, the light for which lies in the plane of incidence is not reflected at all but is completely refracted. 2016 Pearson Education Inc. Circular polarization Circular polarization occurs when the vector has a constant magnitude but rotates around the direction of
propagation. When the wave is propagating toward you and the vector appears to be rotating clockwise, it is called a right circularly polarized electromagnetic wave. If instead the vector of a wave coming toward you appears to be rotating counterclockwise, it is called a left circularly polarized electromagnetic wave. The lenses of the special glasses you wear to see a 3-D movie are circular polarizing filters.
2016 Pearson Education Inc. Scattering of light When you look at the daytime sky, the light that you see is sunlight that has been absorbed and then re-radiated in a variety of directions. This process is called scattering. Light scattered by air molecules contains 15 times as much blue light as red, and thats why the sky is blue. Clouds contain a high concentration of suspended water droplets or ice crystals, which scatter light of all
wavelengths equally, so the cloud looks white. 2016 Pearson Education Inc. Huygenss principle Huygenss principle states that every point of a wave front may be considered the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the wave. The new wave front at a later time is then found by constructing a surface
tangent to the secondary wavelets or, as it is called, the envelope of the wavelets. The figure shows the application of Huygenss principle to wave front to construct a new wave front 2016 Pearson Education Inc. Reflection and Huygenss principle To derive the law of reflection from Huygenss principle, we consider a plane wave approaching a plane reflecting surface. The effect of the reflecting surface is to change the direction of travel of those wavelets that strike it.
The angle a therefore equals the angle r, and we have the law of reflection. 2016 Pearson Education Inc. Refraction and Huygenss principle Huygenss principle can be used to explain the law of refraction. Consider wave fronts traveling
across the boundary surface SS between two transparent materials a and b, with wave speeds vb < va. We can apply Huygenss principle to find the relation of the angle b to a. 2016 Pearson Education Inc. A mirage Mirages are an example of Huygenss principle.
A thirsty traveler can interpret the apparent reflecting surface as a sheet of water. 2016 Pearson Education Inc.
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