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Lecture 11 Electromagnetic Waves & Polarization

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Lecture 11 Electromagnetic Waves & Polarization LECTURE 11 ELECTROMAGNETIC WAVES & POLARIZATION Instructor: Kazumi Tolich Lecture 11 2 ¨ 25.5 Electromagnetic waves ¤ Induced fields ¤ Properties of electromagnetic waves ¤ Polarization ¤ Energy of electromagnetic waves ¤ Polarizers and changing polarization ¨ 17.1 What is light? ¤ Propagation of light waves ¤ Light is an electromagnetic wave ¤ The index of refraction 25.5 Induced fields ¨ A changing magnetic field creates an induced electric field, and a changing electric field creates an induced magnetic field. ¨ Electric and magnetic fields can sustain themselves free of charges and currents in the form of an electromagnetic wave. Quiz: 25.5-1 4 ¨ Suppose you charge a comb and a transparency. The comb exerts an electric force on the transparency, whose magnitude depends on the separation distance. Now suppose you shake the comb so that the separation distance is changing. Does the transparency reacts instantaneously? A. Yes. B. No. It takes some finite time. Quiz: 25.5-1 answer 5 ¨ No. It takes some finite time. ¨ The charged comb creates an electric field around it, and the charged transparency feels force due to this electric field. ¨ When you shake the comb, the electric field it creates is disturbed, and this disturbance travels. ¨ A traveling disturbance in an electromagnetic field is an electromagnetic wave. ¨ When the wave reaches the transparency, it reacts. 25.5 Properties of electromagnetic waves & 17.1 Light is an electromagnetic wave ¨ Electromagnetic waves of all frequencies, �, and wavelengths, �, including light, propagate through a vacuum at the same speed: 1 / � = �� = ) = 3.0 × 10 m⁄s �'�' ¨ The amplitudes of the fields in an electromagnetic wave are related: � = � � 25.5 Polarization ¨ The plane of polarization contains the electric field vectors of an electromagnetic wave. ¨ Radiation emitted from randomly moving atoms is unpolarized. 25.5 Energy of electromagnetic waves ¨ The energy of the electromagnetic wave depends on the amplitudes of the electric and magnetic fields. ¨ Intensity of electromagnetic wave is given by � 1 9 1 � 9 � = = ��'�' = �' � 2 2 �' Example: 25.5-1 (Knight P25.27) A microwave oven operates at 2.4 GHz with an intensity inside the oven of 2500W/m2. What are the amplitudes of the oscillating electric and magnetic fields? Quiz: 25.5-2 10 ¨ When unpolarized light passes through a polarizer, what happens? : A. The light emerges polarized in the polarizer axis direction with about the 9 intensity of the incident beam. B. The light emerges polarized perpendicular to the polarizer axis direction with : about the intensity of the incident beam. 9 : C. The light emerges polarized in the polarizer axis direction with about the ; intensity of the incident beam. D. The light emerges polarized perpendicular to the polarizer axis direction with : about the intensity of the incident beam. ; Quiz: 25.5-2 answer 11 ¨ The light emerges polarized in the polarizer axis direction with about : the intensity of the incident 9 beam. Polarizer axis ¨ Aligned conducting chains of �<=><?@=A molecules absorbs and blocks that component of the E field. �<=><?@=A ⁄2 ¨ The intensity of light is proportional to the square of the : field: � = �� �9 9 ' ' 25.5 Polarizers and changing polarization / Demo 12 ¨ A polarizer will transmit the component of light in the polarizer axis direction. ¨ The intensity of the transmitted beam is given by Malus’s law: �<=><?@=A � 9 ABC=DE<AA@? �ABC=DE<AA@? = �<=><?@=A cos � ¨ Demo: Polaroid sheets � 25.5 Polarizers and changing polarization - applications / Demo ¨ Different material change the polarization of light in different ways. ¨ LCDs use liquid crystals, whose polarization axis can be rotated depending on the voltage across them. ¨ Many organic compounds such as glucose rotate the polarization direction depending on its concentration. ¨ In polarizing sunglasses with vertical polarization axis cuts horizontally polarized glare from horizontal surfaces. ¨ 3D movie glasses. ¨ Stressed material acts like polarizers. ¤ Demo: Polarization by stress in plastic Quiz: 25.5-3 2 ¨ A vertically polarized light wave of intensity 1000 mW/m is coming toward you, out of the screen. After passing through this polarizing filter, what is the wave’s intensity in mW/m2? 35° Quiz: 25.5-3 answer 2 ¨ A vertically polarized light wave of intensity 1000 mW/m is coming toward you, out of the screen. After passing through this polarizing filter, what is the wave’s intensity in mW/m2? 9 9 ¨ �ABC=DE<AA@? = �<=><?@=A cos � = �<=><?@=A cos 90° − 35° = 1000 mW⁄m9 cos9 55° = 329 mW⁄m9 35° For fun: Polarization by scattering 16 ¨ Unpolarized light can be partially or completely polarized by scattering from atoms or molecules, which act as small antennas. 17.1 What is light? ¨ Each of the three models of light successfully explains the behavior of light within a certain domain. ¤ The wave model: Under many circumstances, light exhibits the same behavior as sound or water waves. Lasers and electro-optical devices are best understood in terms of the wave model of light. The study of light as a wave is called wave optics. ¤ The ray model: The properties of prisms, mirrors, lenses, and optical instruments such as telescopes and microscopes are best understood in terms of straight-line paths of light rays called ray optics. ¤ The photon model: In the quantum world, light consists of photons, the quanta of light, that have both wave-like and particle-like properties. Photons are. 17.1 The propagation of light waves / Demo ¨ The wave spreads out (diffracts) to fill the space behind the opening when the opening is comparable in size to the wavelength of the wave. ¨ When the opening is many times larger than the wavelength of the wave, the wave continues to move straight forward. ¤ The straight-line appearance of light with sharp shadows as light passes through large windows. ¤ We can hear around corners but not see around them. ¨ Demo: Ripple tank diffraction 17.1 The index of refraction ¨ The speed of light in a material is characterized by the material’s index of refraction �, defined by speed of light in vacuum � � = = speed of light in the material � Quiz: 17.1-1 ¨ A light wave travels, as a plane wave, from air (n = 1.0) into glass (n = 1.5). Which diagram shows the correct wave fronts? Quiz: 17.1-1 answer ¨ The frequency of a wave does not change as the wave moves from one medium to another. ¨ The wavelength in the transparent material is shorter than the wavelength in a vacuum. ^ ` bcde ¨ � = = = ECA _ a_ a ¨ � > 1 always C..
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