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p2020chap11

# p2020chap11 - PHYS-2020 General Physics II Course Lecture...

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PHYS-2020: General Physics II Course Lecture Notes Section XI Dr. Donald G. Luttermoser East Tennessee State University Edition 3.3

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Abstract These class notes are designed for use of the instructor and students of the course PHYS-2020: General Physics II taught by Dr. Donald Luttermoser at East Tennessee State University. These notes make reference to the College Physics, 9th Edition (2012) textbook by Serway and Vuille.
XI. Reflection and Refraction of Light A. Huygens’ Principle. 1. In 1678, Huygens proposed the wave theory of light. a) At that same time, Newton maintained that light acted like a particle. b) As we saw in the last section, Planck derived in 1900 that light has the characterists of both a wave and a particle = a wavicle , which he called a photon . c) In the last sections of the notes, we discussed the parti- cle characteristics of a photon. In this section, we will concentrate on the wave-like characteristics. 2. All points on a wavefront can be considered point sources for the production of spherical secondary wavelets. After time t , the new position of the wavefront will be the surface of tangency to these secondary wavelets. This geometrical argument is called Huygens’ Principle . t = 0 old wavefront r = ct new wavefront XI–1

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XI–2 PHYS-2020: General Physics II B. Reflection of Light. 1. When light travels from one medium to another, part of the light can be reflected at the media interface. a) Reflection off of a smooth surface is called specular re- flection (which we will assume from this point forward). b) Reflection off of a rough surface is called diffuse reflec- tion . 2. Law of Reflection : The angle of incidence with respect to the normal of the reflecting surface, θ i , equals the angle of reflection, θ r : θ i = θ r . (XI-1) Reflecting Surface normal line incident ray reflected ray θ i θ r 3. The amount of energy that is reflected compared to the amount incident is called the reflectivity of the surface. a) This also is called albedo . b) The reflectivity of a mirror is about 96% (albedo = 0.96).
Donald G. Luttermoser, ETSU XI–3 C. Refraction of Light. 1. When light travels from one medium to another, part of the light can be transmitted across the media surface and refracted . a) Refraction means that the light beam bends. b) This bending takes place because the light beam’s ( i.e. , photon’s) velocity changes as it goes from one medium to the next, following the relation: sin θ r sin θ i = v r v i = constant . (XI-2) i) v r and θ r are the velocity and the angle of the refracted beam with respect to the normal line of the surface. ii) v i and θ i are the velocity and the angle of the incident beam with respect to the normal line of the surface. normal line incident ray refracted ray θ i θ r v i v r n i = n 1 n r = n 2 2. The index of refraction of a material is n speed of light in vacuum speed of light in medium = c v . (XI-3)

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XI–4 PHYS-2020: General Physics II a) n for some common substances: i) Vacuum: 1.000000 ii) Air (0 C, 1 atm): 1.000293 iii) Ice (H 2 O, 0 C): 1.309 iv) Glass (crown): 1.52 b) n also is a function of wavelength. For the index of re-
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