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 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 9 25.5 Polarizers and changing polarization / Demo 10 ¨ 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: �@=C@BA=: � 9 :;<=>?@::AB �:;<=>?@::AB = �@=C@BA=: 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 For fun: Polarization by scattering 13 ¨ 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, similar to the straight-line appearance of light with sharp shadows as light passes through large windows. ¨ 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.