Phys13

# Phys13 - 1 Unit 13 Physical Properties of Waves II 13.1...

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Unformatted text preview: 1 Unit 13 Physical Properties of Waves II 13.1 Electromagnetic waves 13.2 Doppler effect of light 13.3 The electromagnetic spectrum 13.4 Polarization 13.5 Polarization by reflection 13.6 Polarization by scattering 13.1 Electromagnetic waves Electromagnetic waves are traveling waves of oscillating electric and magnetic fields. The electric and magnetic fields in an electromagnetic wave are perpendicular to each other, and to the direction of propagation. They are also in phase. To find the direction of propagation of an electromagnetic wave, point the fingers of your right hand in the direction of E , then curl them toward B . Your thumb will be pointing in the direction of propagation. Electromagnetic waves can be generated by an antenna, which is connected to an AC generator. 2 In fact, another way to generate electromagnetic waves is to accelerate electric charge. As accelerated charges radiate electromagnetic waves. The intensity of radiated electromagnetic waves depends on the orientation of the acceleration relative to the viewer. For example, viewing the antenna perpendicular to its length, so that the charges accelerate at right angles to the line of sight, results in maximum intensity. Conversely, viewing the antenna straight down from above, in the same direction as the acceleration, results in zero intensity. According to Maxwell’s work in electric and magnetic fields, he proposed the light is an electromagnetic wave. Visible light is of course an electromagnetic wave. Unlike sound or a wave on a string requires a medium through which it can propagate, light can propagate through a vacuum. In fact, electromagnetic waves travel through a vacuum with the maximum speed, c , that any form of energy can have. It is also the speed of light. s m c / 10 00 . 3 8 × = A beam of light can travel around the world about seven times in a single second. Light travels slower in a denser medium, e.g. glass and water. Example An electromagnetic wave propagates in the positive y direction, as shown in the figure. If the electric field at the origin is in the positive z direction, is the magnetic field at the origin in (a) the positive x direction, (b) the negative x direction, or (c) the negative y direction? 3 Answer: According to the right hand, which has your thumb pointing the propagation direction, the direction of magnetic field should be along positive x . 13.2 Doppler effect of light The Doppler effect for light is different from that of sound by two ways. (a) Sound waves require a medium through which to travel, whereas light can propagate across a vacuum. Therefore, the Doppler effect for sound depends on whether the medium – that is, the air – is moving or is still....
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Phys13 - 1 Unit 13 Physical Properties of Waves II 13.1...

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