Chapter 23 Electromagnetic Waves Lecture 14 23.1 The Discovery of Electromagnetic Waves 23.2 Properties of Electromagnetic Waves 23.3 Electromagnetic Waves Carry Energy and Momentum 23.4 Types of Electromagnetic Radiation: The Electromagnetic Spectrum 23.5 Generation and Propagation of Electromagnetic Wave 23.6 Polarization 23.7 Doppler Effect
Generation of EM Waves AC Source with Antennas A radio wave can be generated by using an AC voltage source connected to two wires The two wires act as an antenna At any particular moment, the two wires are oppositely charged Section 23.5
Generation of EM Waves As the voltage of the AC source oscillates, the electric potential of the two wires also oscillate Electric charges are also flowing onto and off the wires as the voltage alternates Section 23.5
Generation of EM Waves, cont. The electric field continues to oscillate in size and direction The wave propagates away from the antenna The charges are accelerated The charges undergo simple harmonic motion with a given frequency which is also the frequency of the AC voltage source and the frequency of the wave Section 23.5
Generation of EM Waves, cont.
Antennas EM Wave Propagation At any particular moment, the two wires are oppositely charged The waves propagate perpendicular to the antenna s axis Section 23.5
Antennas, cont. Electromagnetic waves also propagate inside the antenna wires For a very long antenna, these tend to cancel Therefore, most dipole antennas have a total length of λ/2 (one half of the antenna has a length of λ/4) More complicated antennas also have the same cancellation effect, so the length of the antenna is usually comparable to the wavelength of the radiation
Antenna to Detect Radiation The same antenna that generates an em wave can also be used to detect the wave The electric field associated with the wave exerts a force on the electrons in the antenna This produces a current and an induced voltage across the antenna wires This is the voltage source of the circuit in the receiver Section 23.5
Point Source and Spherical Wave There are cases where the charges are not confined to one direction In these cases, the radiation can propagate outward in all directions The ideal case of a very small source producing spherical wave fronts is called a point source The intensity of a spherical wave decreases with distance: I 1/r 2 The intensity decreases as the constant amount of energy spreads out over greater areas Section 23.5
Polarization There are many directions of the electric field of an em wave that are perpendicular to the direction of propagation Knowing the actual direction of the electric field is important to determining how the wave interacts with matter The previous wave (from the dipole antenna) was linearly polarized The electric field was directed parallel to the z-axis Most light is unpolarized Section 23.6
Polarizers Polarized light can be created using a polarizer The type of polarizer shown consists of a thin, plastic film that allows an em wave to pass through it only if the electric field of the wave is parallel to a particular direction called the axis of the polarizer Section 23.6
Polarizers, cont. The polarizer absorbs radiation with electric fields that are not along the axis When the unpolarized light strikes a polarizer, the light that come out is linearly polarized Assume linearly polarized light strikes a polarizer (1) If the incident light is polarized parallel to the axis of the polarizer and also (2) the outgoing electric field is equal in amplitude to the incoming field, then all the incident energy is transmitted through the polarizer Section 23.6
Polarizers, final If the incident light is polarized perpendicular to the axis of the polarizer, no light is transmitted If the incident light is polarized at an angle θ relative to the axis of the polarizer, only a component of electric field is transmitted E E cos out in
Polarizers and Malus Law If the electric field is parallel to the polarizer s axis: E out = E in If the electric field is perpendicular to the polarizer s axis, E out = 0 If the electric field makes some angle θ relative to the polarizer s axis, E out = E in cos θ This relationship can be expressed in terms of intensity and is then called Malus Law: I out = I in cos 2 θ SI unit is W/m 2 Section 23.6
Malus Law and Unpolarized Light Unpolarized light can be thought of as a collection of many separate light waves, each linearly polarized in different and random directions Each separate wave is transmitted through the polarizer according to Malus Law The average outgoing intensity is the average of all the incident waves: I out = (I in cos 2 θ) ave = ½ I in Since the average value of the cos 2 θ is ½ Section 23.6
Polarization Examples In figure A, the unpolarized light passes through polarizers oriented at 90 The intensity is reduced to ½ by the first polarizer and to 0 by the second In figure B, three polarizers are used and a non-zero intensity results Section 23.6
Polarizers, Summary When analyzing light as it passes through several polarizers in succession, always analyze the effect of one polarizer at a time The light transmitted by a polarizer is always linearly polarized The polarization direction is determined solely by the polarizer axis The transmitted wave has no memory of its original polarization Section 23.6
Operation of a Polarizer Most applications use a sandwich structure with certain types of long molecules placed between thin sheets of plastic When the molecules are aligned parallel to each other, the sheets act as a polarizer with the axis perpendicular to the direction of the molecules Section 23.6
Operation, cont. Electrons in the polarizer molecules respond to electric fields When the electric field is parallel to the molecules light is absorbed When the electric field is perpendicular to the molecular direction the light is transmitted The polarization axis is always perpendicular to the molecular direction Section 23.6
Polarization by Reflection Light can be polarized by scattering Air molecules act as antennas Charged particles respond to sunlight by oscillating in the direction of the electric field These particles produce new outgoing waves that are polarized The outgoing waves are called scattered waves The light is said to be polarized by reflection Section 23.6
Optical Activity When linearly polarized light passes through certain materials, the polarization direction is rotated This effect is called optical activity These materials generally contain molecules with a screwlike or helical structure Section 23.6
Applications of Polarized Light Many objects use LCD s Liquid Crystal Displays Incident light is linearly polarized by a polarizing sheet The light encounters an optically active material called a liquid crystal Section 23.6
LCDs, cont. The molecules in the liquid crystal rotate the light by 90 so that it can pass through an output polarizer Voltages can be applied to rotate the light with respect to the output polarizer and thus make the display appear dark By applying different voltages to different areas of the liquid crystal, a pattern of light and dark regions can be formed Corresponding to letters and numbers you see in the display Section 23.6
Doppler Effects Dopplar Shifts of Spectral Lines Astronomers use spectral lines to determine properties of stars Each dark line in the spectrum corresponds to a color absorbed by the atoms in the object The location of each line corresponds to a particular wavelength of light Some spectra are observed to be shifted Section 23.7
Doppler Shift for Light The Doppler Shift relationships for light are different than for sound For light, the frequency is shifted as: ƒ obs ƒ source 1 1 v c v c rel rel v rel is the velocity of the source relative to the observer A positive value of v rel corresponds to a source moving away from the observer Section 23.7
Red Shifts Observations by Edwin Hubble showed that distant galaxies were shifted to longer wavelengths relative to the wavelength of the same spectral line on Earth This is called a red shift Hubble proposed that those galaxies must be moving away from us This would cause the frequency to appear lower This is similar to the Doppler Effect seen for sound The size of the frequency shift can be used to determine the velocity of the galaxy emitting the light Section 23.7
Expanding Universe Most galaxies in the observable universe were found to be moving away from us The farther the galaxy is from the Earth, the faster it is receding From any viewpoint, the galaxies would appear to be moving away from you Section 23.7
Demo Polarizer Effects