10/28/2015 1 Lecture 8: Reflection and Transmission of Waves 2 Normal incidence – propagating waves So far, we have considered plane waves in an infinite homogeneous medium. A natural question would arise: what happens if a plane wave hits some object? Such object can be either dielectric or conductor. Instructor: Dr. Gleb V. Tcheslavski To answer this question, we need to use boundary conditions. We study first normal incidence on the boundary. Contact: gleb@ee.lamar.edu We assume that a plane wave is generated in the region z < 0 in a lossless material with dielectric constant 1 and that a second lossless material is in the region z 0 with a dielectric constant 2. The permeabilities of both materials are 0. Office Hours: Room 2030 Class web site: www.ee.lamar.edu/gleb/e m/Index.htm A portion of the wave is transmitted to the medium 2, another portion is reflected back to medium 1. ELEN 3371 Electromagnetics ELEN 3371 Electromagnetics Fall 2008 Fall 2008 3 Normal incidence – propagating waves The direction of wave’s propagation can be defined using a right hand rule. This suggests that the polarization of the reflected field has to be altered after the incident wave strikes the interface. We assume that the electric component is unchanged, and the magnetic field will change its direction. (8.3.1) Therefore: Incident wave: E y ,i ( z , t ) Ai e j t k1z Reflected wave: Transmitted wave: E y ,r ( z , t ) Br e j t k1z E y ,t ( z , t ) At e j t k2 z ELEN 3371 Electromagnetics Fall 2008 Reflected wave: Transmitted wave: H x ,i ( z , t ) H x,r ( z , t ) Ai j t k1z e Z c ,1 Br j t k1z e Z c ,1 H x ,t ( z , t ) (8.3.3) Since the materials are assumed as lossless, the waves will not attenuate (i.e. equations or by using the characteristic impedances for two regions. Incident wave: (8.3.2) Here, k1 and k2 are the wave numbers for the regions (media) 1 and 2 respectively, constants A and B indicate the terms propagating in the +z and –z directions. = 0, = k). The magnetic field intensities can be found from the Maxwell’s 4 Normal incidence – propagating waves At j t k2 z e Z c ,2 (8.4.1) (8.4.2) (8.4.3) From the boundary conditions, the tangential components of the electric field must be continuous and the tangential components of the magnetic field intensity must differ by any surface current that is located at the interface. Usually, we assume that there are no surface currents, which implies that the tangential components of the magnetic field intensity are also continuous at the interface. Also, since we chose the interface between two media to be at z = 0, the exponent will be j t kz jt e ELEN 3371 Electromagnetics z 0 e (8.4.4) Fall 2008 1 10/28/2015 5 Normal incidence – propagating waves 6 Normal incidence – propagating waves Therefore, at the boundary z = 0, we can write: We specify the reflection and transmission coefficients as: E y ,i ( z 0, t ) E y ,r ( z 0, t ) E y ,t ( z 0, t ) (8.5.1) H x ,i ( z 0, t ) H x ,r ( z 0, t ) H x ,t ( z 0, t ) (8.5.2) Reflection: Br Z c ,2 Z c ,1 Ai Z c ,1 Z c ,2 (8.6.1) Transmission: 2 Z c ,2 At Ai Z c ,1 Z c ,2 (8.6.2) Substituting the last results to (8.3.x) and (8.4.x), we obtain: Ai Br At (8.5.3) Ai A B r t Z c ,1 Z c ,1 Z c ,2 (8.5.4) Knowing the characteristic impedances of the materials allows us to determine the propagation characteristics and amplitudes of both waves: transmitted to the second medium and reflected back. If the characteristic impedances on both sides are equal, all energy is transmitted into region 2 and none is reflected back. This is called matching the media (lenses, glasses etc.). Therefore, if one of three wave’s magnitudes is known, two other can be computed. ELEN 3371 Electromagnetics ELEN 3371 Electromagnetics Fall 2008 Fall 2008 7 Normal incidence – propagating waves Since the characteristic impedance is: Zc (8.7.1) For two dielectric materials: Reflection: Transmission: 1 2 1 2 1 2 1 1 2 8 Normal incidence – propagating waves (Example) (8.7.2) Example 7.1: Describe the expected reflectiontransmission characteristic of a time harmonic EM wave normally incident at a layered dielectric. Find the total reflection and transmission coefficients of a single layer with known thickness d and dielectric constants are 2 = r0; 1 = 3 = 0. Plot the frequency dependence of these coefficients assuming d = 0.1 m and r = 4. z (8.7.3) The amplitudes of the total transmitted and reflected fields are Et ( z ) E ( z )n3 n 1 (8.8.1) wave index Er ( z ) E ( z ) n1 (8.8.2) n 1 -z direction region number ELEN 3371 Electromagnetics Fall 2008 ELEN 3371 Electromagnetics Fall 2008 2 10/28/2015 10 9 Normal incidence – propagating waves (Example) The wave incident to the boundary with the second medium is partially reflected back and partially transmitted to medium 2. At the interface between materials 2 and 3, the transmitted to material 2 field is partially reflected back and partially transmitted to the material 3, etc. Normal incidence – propagating waves (Example) The total reflected electric field is: Ei Ei 1 Ei 1 2 1 2 Er ( z ) Ei 1 1 2 1 e j 2 k2 d 1 2 1 2 1 e j 4 k2 d 1 2 1 2 1 e j 6 k2 d ... 2 j 2 k2 d 2 2 j 2 k2 d 2 j 2 k2 d Ei 1 1 1 1e 1 1 e (8.10.1) 1 e ... Ei 1 Ei 1 2 The total reflection coefficient is: The phase of individual terms in summations is different: an additional phase difference of k2d appears after each crossing the slab. 1 12 e j 2 k2 d 1 1 e j 2 k2 d Er ( z ) 1 1 Ei 1 12 e j 2 k2 d 1 12 e j 2 k2 d (8.10.2) Similarly, the total transmitted electric field is: 2 Et ( z ) Ei 1 1 e jk2 d 1 2 1 2 e j 3k2 d 1 2 1 2 e j 5 k2 d ... 2 Ei 1 12 e jk2 d 1 12 e j 2 k2 d 12 e j 2 k2 d ... We assume next that the reflection coefficient from the first interface is 1 in the +z direction and -1 in the –z direction. Since the medium 3 is identical to medium 1, 2 = -1, and -2 = 1. Similarly, the transmission coefficient through the first interface will be 1+ = 1 + 1 in the +z direction and 1- = 1 - 1 in the -z direction. ELEN 3371 Electromagnetics ELEN 3371 Electromagnetics Fall 2008 (8.10.1) Fall 2008 12 11 Normal incidence – propagating waves Normal incidence – propagating waves (Example) What if the medium where the wave is propagating is lossy, i.e. 0? The total transmission coefficient is: jk2 d 2 E ( z ) 1 1 e t 2 j 2 k2 d Ei 1 1 e The reflection coefficient of the first boundary is: 1 r 1 2 1 1 3 1 r 1 2 (8.11.1) The magnetic field must be: H (r ) j E (r ) E (r ) The Helmholtz equation is: 2 E (r ) 2 E (r ) 0 (8.11.2) j j 1 j (8.11.3) Fall 2008 (8.12.3) If the electric field is linearly polarized in the x direction, wave equation reduces to: 2 Ex ( z ) 2 Ex ( z ) 0 x 2 The frequency dependence of the total transmission and reflection coeffs: ELEN 3371 Electromagnetics (8.12.2) where the complex propagation constant is: The wave number in the slab: k 2 r k 2k (8.12.1) ELEN 3371 Electromagnetics (8.12.4) Fall 2008 3 10/28/2015 13 Normal incidence – propagating waves The solution will be in form: z Ex ( z ) E e z E e 14 Normal incidence – propagating waves (8.13.1) If the material 2 is a good conductor, the characteristic impedance will be very small and it would approach zero as the conductivity approaches infinity. Therefore, it will be NO transmission of EM energy into the conductor, it all will be reflected. In this case: Traveling exponentially decaying waves 0 The first term – a wave propagating in +z direction: 1 z E e z j z E e e z E e cos t z (8.13.2) The magnetic field will be: H y ( z) E e z E e z Zc (8.13.3) Alternatively, loss can be incorporated by: 0; ' j " (8.13.4) ELEN 3371 Electromagnetics (8.14.2) Fall 2008 15 Normal incidence – propagating waves Since the Poynting’s vector has no real part, NO average power is delivered to the region 2 (perfect conductor)! Fields exist only in the region z < 0 (medium 1 – dielectric): H ( z) Hi ( z) H r ( z) E0 E e jkc z e jkc z u y 2 0 coskc z Zc Zc 16 Normal incidence – propagating waves At the interface, the amplitudes of both pulses add up to equal zero in order to satisfy the requirement that the tangential component of the electric field must be zero at a perfect conductor. E ( z ) Ei ( z ) Er ( z ) E0 e jkc z e jkc z u x j 2 E0 sinkc zu x (8.14.1) Considering the most general solution (7.12.3) of the wave equation and a traveling pulse instead of a time-harmonic wave, we may assume that a virtual pulse with a negative amplitude was launched at z = + in the –z direction simultaneously with the real pulse. Both pulses meet at z = 0 at the time t = t + 3t after that moment, they keep propagating but the virtual pulse becomes real and the real one becomes virtual. ELEN 3371 Electromagnetics Fall 2008 Polarization change (8.15.1) The surface current on the conductor can be found from the boundary conditions on the tangential magnetic field: J s u z H ( z 0) (8.15.2) 2 E0 ux Zc (8.16.1) Note that at z =0: E(z) = 0 and H(z) = 2E0/Zc uy. This is where “no metal plates in a microwave oven” comes from! The Poynting vector for the first region (z < 0) will be: S0 ( z ) ELEN 3371 Electromagnetics 2E 2 1 E ( z ) H ( z )* j 0 cos k0 z sin k0 z u z 2 Zc Fall 2008 (8.15.3) ELEN 3371 Electromagnetics Fall 2008 4 10/28/2015 18 17 Normal incidence – propagating waves (Example) Normal incidence – propagating waves (Example) During t, the car travels a distance z; therefore, the velocity of the car can be estimated. Example 7.2: Pulse radars can be used to determine the velocity of cars. Show how such radars could work. A repetitive EM pulse from the radar is incident on the car. Because of the high conductivity of the car, the pulse is reflected back to the radar where the total time of travel t for the given pulse can be estimated. The pulse repetition time (the time between two consecutive pulses) is t. The difference in arrival time for two pulses ti ti 1 The velocity is: 2 L 2( L z ) 2z 2Vcar t c c c c c t i t i 1 vcar 2 t (8.18.1) (8.18.2) The actual distance between the car and the radar L is not important to determine the car’s speed; although it can be computed as well. ELEN 3371 Electromagnetics ELEN 3371 Electromagnetics Fall 2008 Fall 2008 19 Oblique incidence – propagating waves When a plane EM wave incident at an oblique angle on a dielectric interface, there are two cases to be considered: incident electric field has polarization parallel to the plane of incidence, and incident electric field has polarization that is perpendicular to the plane of incidence. 20 Oblique incidence – propagating waves Incident wave: Ei E0 cos i u x sin i u z e Hi Reflected wave: 1. Parallel polarization: Transmitted wave: ELEN 3371 Electromagnetics Fall 2008 ELEN 3371 Electromagnetics E0 jk x sin z cost uye 2 t Zc 2 (8.20.1) (8.20.2) E0 jk x sin z cos r u ye 1 r Z c1 Et E0 cos t u x sin t u z e Ht Note: the angles are measured with respect to normal. E0 jk x sin z cosi u y e 1 i Z c1 Er E0 cos r u x sin r u z e jk1 x sinr z cosr Hr The incident, reflected, and transmitted electric field vectors lie in the plane of incidence: the x-z plane. jk1 x sin i z cosi (8.20.3) (8.20.4) jk2 x sin t z cost (8.20.5) (8.20.6) Fall 2008 5 10/28/2015 21 Oblique incidence – propagating waves From the boundary conditions: i.e., continuity of tangential electric and magnetic fields at the interface z = 0, we derive: cos i e jk1x sini cos r e jk1x sinr cos t e jk2 x sint e jk1 x sin i Z c1 e jk1 x sin r Z c1 e 22 Oblique incidence – propagating waves (8.21.1) jk2 x sin t For parallel polarization, a special angle of incidence exists, known as the Brewster’s angle, or polarizing angle, i = B, for which the reflection coefficient is zero: = 0. It happens when Z c 2 cos t Z c1 cos i (8.21.2) Zc2 To satisfy these conditions, the Snell’s laws of reflection and refraction must hold: i r ;k1 sin i k2 sin t Therefore: (8.21.3) Z c 2 cos t Z c1 cos i Z c 2 cos t Z c1 cos i (8.21.4) 2 Z c 2 cos i Z c 2 cos t Z c1 cos i (8.21.5) (8.22.2) 1 1 2 If the incidence angle equals to Brewster’s angle, the reflected field will be polarized perpendicularly to the plane of incidence. ELEN 3371 Electromagnetics Fall 2008 1 sin B These simplifications lead to: ELEN 3371 Electromagnetics (8.22.1) Fall 2008 23 Oblique incidence – propagating waves 1. Perpendicular polarization: The incident, reflected, and transmitted electric field vectors are perpendicular to the plane of incidence: the x-z plane. jk1 x sin i z cosi Incident wave: Ei E0u y e Hi Reflected wave: E0 cos i ux sin i uz e jk1 x sini z cosi Z c1 Er E0u y e jk1 x sinr z cosr Et E0u y e Ht ELEN 3371 Electromagnetics From the continuity of tangential electric and magnetic fields at the interface z = 0, we again derive: e jk1x sin i e jk1x sin r e jk2 x sin t (8.23.1) E0 Hr cos r ux sin r uz e jk1 x sinr z cosr Z c1 Transmitted wave: 24 Oblique incidence – propagating waves jk2 x sin t z cost E0 cos t ux sin t uz e jk2 x sint z cost Zc 2 Fall 2008 (8.23.2) jk1 x sin i cos i e Z c1 jk1 x sin r cos r e Z c1 (8.24.1) jk2 x sin t cos t e Zc 2 (8.24.2) As before, the Snell’s laws of reflection and refraction must hold: (8.23.3) (8.23.4) i r ;k1 sin i k2 sin t (8.24.3) These simplifications lead to very similar expressions: (8.23.5) (8.23.6) ELEN 3371 Electromagnetics Z c 2 cos i Z c1 cos t Z c 2 cos i Z c1 cos t (8.24.4) 2 Z c 2 cos i Z c 2 cos i Z c1 cos t (8.24.5) Fall 2008 6 10/28/2015 26 25 Oblique incidence – propagating waves Oblique incidence – propagating waves Total internal reflection and surface waves A magnitude of the reflection coefficient for the parallel and perpendicular polarization: Assume that the uniform plane wave incident on an interface between two perfect dielectrics with k1 > k2, for instance, water to air. From the Snell’s law: 2 k cos t 1 1 sin 2 i k2 (8.26.1) For a particular angle of incidence, the quantity under the square root becomes zero. This angle is called critical angle: c sin 1 k2 k1 (8.26.2) At the incidence angles exceeding the critical angle, the phenomenon of total internal reflection occurs. ELEN 3371 Electromagnetics ELEN 3371 Electromagnetics Fall 2008 Fall 2008 27 Oblique incidence – propagating waves Let us denote: cos t j ,wheni c 28 Fabry-Perot resonator – standing waves Let us consider again the electric component of a linearly polarized EM field normally incident on a perfect conductor with a reflection coefficient = -1. (8.27.1) In both cases of parallel and perpendicular polarization, the reflection coefficient will be: j j E y ( z , t ) Re B e j (t kz ) e j (t kz ) Re Be jt e jkz e jkz Re j 2 Be (8.27.2) jt sin kz E y ( z , t ) A sin tsin kz Here, both and are real. As a consequence, the magnitude of the reflection coefficient || = 1, and all incident power is reflected off the interface. (8.28.1) (8.28.2) Where A = 2B. As a result, for instance for the perpendicular polarization, the transmitted electric field is: Et TE0u y e z e jk2 x sin i (8.27.3) The tangential electric field Ey(z,t) = 0 at the interface z = 0. In this case, the signal consisting of two oppositely propagating waves appears to be stationary in space and oscillating in time. This is a standing wave. If the incidence angle exceeds the critical angle, the field in region 2 propagates in the x direction but rapidly exponentially decays in the z direction – away from the interface. This is a surface wave. ELEN 3371 Electromagnetics Fall 2008 ELEN 3371 Electromagnetics Fall 2008 7 10/28/2015 29 Fabry-Perot resonator – standing waves 30 Fabry-Perot resonator – standing waves (Example) Example 7.3: An EM wave propagating in a vacuum in the region z < 0 is normally incident upon a perfect conductor at z = 0. The frequency of the wave is 3 GHz, the amplitude of incident electric field is 10 V/m, and it is polarized in the uy direction. Find the phasor and the instantaneous expressions for the incident and the reflected field components. The standing wave results from the constructive and destructive interference of the two counter propagating waves. k Observe that the separation distance between two successive null points (nodes) equals to the separation distance between two successive maxima (antinodes) and equals to one half of the wavelength. The incident field in the phasor form: 2 f 2 3 109 20 m 1 c 3 108 Ei ( z ) Ae jkz u y 10e j 20 z u y V m Hi ( z) Or: u z Ei ( z ) 10 j 20 z e u x A m 120 Zc (8.30.3) (8.30.4) 10 cos 6 109 20 z u x A m H i ( z , t ) Re H i ( z )e 120 (8.30.5) ELEN 3371 Electromagnetics Fall 2008 (8.30.2) Ei ( z , t ) Re Ei ( z )e jt 10 cos 6 109 20 z u y V m jt ELEN 3371 Electromagnetics (8.30.1) Fall 2008 31 Fabry-Perot resonator – standing waves (Example) The reflected field in the phasor form: Er ( z ) Ei ( z ) 10e j 20 z u y V m Hi ( z) u z Er ( z ) 10 j 20 z e u x A m Zc 120 32 Fabry-Perot resonator – standing waves (8.31.1) (8.31.2) Examination of a standing wave suggests that it should be possible to insert another conductor (a conductive wall) at any of the nodes where the tangential electric field is zero without changing a structure of electric field! The applicable boundary condition, therefore, is that the tangential electric field must be zero at a conductive surface. Or in the instantaneous form: Er ( z , t ) Re Er ( z )e jt 10 cos 6 109 20 z u y V m (8.31.3) H r ( z , t ) Re H r ( z )e jt (8.31.4) 10 cos 6 109 20 z u x A m 120 Let us assume that the plates were inserted instantaneously and the EM energy was “trapped” between the plates. Constructive and destructive interference will lead to appearance of a standing wave. ELEN 3371 Electromagnetics Fall 2008 ELEN 3371 Electromagnetics Fall 2008 8 10/28/2015 33 Fabry-Perot resonator – standing waves For the 1D Helmholtz equation d 2 Ey ( z) dz 2 k Ey ( z) 0 2 (8.33.1) and a time-harmonic signal, the solution will be in a form: E y ( z ) A sin kz B cos kz (8.33.2) The integration constants A and B can be found from the boundary condition that the tangential electric field must be zero at a metal wall. Therefore, B = 0, k n (8.33.3) L where n is an integer (resonator mode) and L is the distance between the metal walls. If the maximum magnitude of electric field is Ey0, the electric field is n z E y ( z ) E y 0 sin L ELEN 3371 Electromagnetics 34 Fabry-Perot resonator – standing waves The structure consisting of a parallel plate cavity is called a Fabry-Perot resonator. If the frequency of a wave “matches” the dimensions of the resonator (a resonant frequency) – the length of cavity equals an integer number of half-wavelengths – a standing wave will be formed. All other frequency components will be canceled out by a destructive interference. The Q-factor (2 ratio of stored energy to the power dissipated per cycle) of this resonator may be very high (approaches a million). Fabry-Perot resonators are widely used in EM and optics: a He-Ne laser is basically a Fabry-Perot resonator. (8.33.4) ELEN 3371 Electromagnetics Fall 2008 Fall 2008 35 Fabry-Perot resonator – standing waves Recall that the wave number is a function of frequency and the velocity of light between plates. k r c n L The resonant frequency for a free space (or air) can be computed or measured for known resonator’s dimensions. The relative frequency difference is 1 1 r2 1 r1 r1 r (8.35.1) Therefore, we can find the resonant frequency as r n L 0 (8.35.2) ELEN 3371 Electromagnetics n L 1 1 r 0 0 0 0 Fall 2008 (8.36.1) Therefore, if we can measure this frequency difference, we can estimate the permittivity of unknown material placed in the resonator and, thus, identify the material. Considering two resonators of the same length L but one of the filled with air (left) and the other filled with a dielectric , we can find that they will resonate at two different frequencies. The frequency difference will be r 1 r 2 36 Fabry-Perot resonator – standing waves (8.35.3) Example 7.4: An empty Fabry-Perot resonator has a resonant frequency of 35 GHz. Determine the thickness of a sheet of paper that is inserted later between the plates if the resonant frequency changes to 34.99 GHz. The separation between plates is 50 cm. Assume that the integer n specifying the mode doesn’t change and that there is no reflection from the paper. 3 paper ELEN 3371 Electromagnetics Fall 2008 9 10/28/2015 37 Fabry-Perot resonator – standing waves (Example) Example 7.5: A helium-neon laser emits light at a wavelength of 6328 Å in air. Calculate the frequency of oscillation of the laser, the period of oscillation, and the wave number. 1 Å (Åmgstrom) = 10-10 m. The relative dielectric constant separating the plates with paper inserted can be approximated as r L L L L paper 1 paper 1 L L L paperadded 1 vacuum 1 1 vacuum r 1 Therefore: 1 paper L 1 L 2 L paper 1 L 35 34.99 L (3 1) 35 2 0.5 38 Fabry-Perot resonator – standing waves (Example) (8.37.1) The frequency: f (8.37.2) c The period: T (8.37.3) 2.998 108 4.738 1014 Hz473.8THz 6.328 107 1 1 2.11 1015 s 2.11 fs f 4.738 1014 The wave number: k 2 2 9.93 106 m 1 6.328 107 Finally: L 1.4 104 m ELEN 3371 Electromagnetics Fall 2008 ??QUESTIONS?? ELEN 3371 Electromagnetics Fall 2008 10