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13. Fresnel's Equations for Reflection and Transmission

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13. Fresnel's Equations for Reflection and Transmission 13. Fresnel's Equations for Reflection and Transmission Incident, transmitted, and reflected beams Boundary conditions: tangential fields are continuous Reflection and transmission coefficients The "Fresnel Equations" Brewster's Angle Total internal reflection Power reflectance and transmittance Augustin Fresnel 1788-1827 Posing the problem What happens when light, propagating in a uniform medium, encounters a smooth interface which is the boundary of another medium (with a different refractive index)? k-vector of the incident light nincident boundary First we need to define some ntransmitted terminology. Definitions: Plane of Incidence and plane of the interface Plane of incidence (in this illustration, the yz plane) is the y plane that contains the incident x and reflected k-vectors. z Plane of the interface (y=0, the xz plane) is the plane that defines the interface between the two materials Definitions: “S” and “P” polarizations A key question: which way is the E-field pointing? There are two distinct possibilities. 1. “S” polarization is the perpendicular polarization, and it sticks up out of the plane of incidence I R y Here, the plane of incidence (z=0) is the x plane of the diagram. z The plane of the interface (y=0) T is perpendicular to this page. 2. “P” polarization is the parallel polarization, and it lies parallel to the plane of incidence. Definitions: “S” and “P” polarizations Note that this is a different use of the word “polarization” from the way we’ve used it earlier in this class. reflecting medium reflected light The amount of reflected (and transmitted) light is different for the two different incident polarizations. Fresnel Equations—Perpendicular E field Augustin Fresnel was the first to do this calculation (1820’s). We treat the case of s-polarization first: ki kr Er Ei ni y Bi B i r r Interface z x Beam geometry for light with its electric field sticking up out of t E the xz plane (y = 0) the plane of incidence t nt (i.e., out of the page) Bt kt Boundary Condition for the Electric Field at an Interface: s polarization y The Tangential Electric Field is Continuous z x In other words, The component of k k the E-field that lies in i r Er the xz plane is Ei ni continuous as you B B i i r r move across the plane of the interface. Interface Here, all E-fields are t Et n in the z-direction, t which is in the plane Bt kt of the interface. (We’re not explicitly writing So: Ei(y= 0) + Er(y= 0) = Et(y= 0) the x, z, and t dependence, but it is still there.) Boundary Condition for the Magnetic y Field at an Interface: s polarization z x The Tangential Magnetic Field* is Continuous In other words, ki kr E Er n The total B-field in the i i plane of the interface is i B B i i r r continuous. i Interface Here, all B-fields are in t E the xy-plane, so we take t nt the x-components: Bt kt –Bi(y= 0) cosi + Br(y= 0) cosr = –Bt(y= 0) cost *It's really the tangential B/, but we're using i t 0 Reflection and Transmission for Perpendicularly Polarized Light Ignoring the rapidly varying parts of the light wave and keeping only the complex amplitudes: EE00ir E 0 t BB00iirrcos( ) cos( ) B 0 tt cos( ) But BEcn/(00 / ) nEc / and ir . Substituting into the second equation: nEir()cos()cos()00 E i i nE tt 0 t Substituting for EEEE0000tirt using : nEir(00 E i )cos( i ) nE tr ( 00 E i )cos( t ) Reflection & Transmission Coefficients for Perpendicularly Polarized Light RearrangingnEiriitrit()cos()()cos()00 E nE 00 E yields: En00ricos( i ) n t cos( t ) En ii cos( i ) n t cos( t ) Solving for EE00ri/ yields the reflection coefficient : rEE 00riiittiitt/ n cos( ) n cos( ) / n cos( ) n cos( ) Analogously, the transmission coefficient, EE00ti/, is tEE 00tiiiiitt/ 2 n cos( ) / n cos( ) n cos( ) These equations are called the Fresnel Equations for perpendicularly polarized (s-polarized) light. Fresnel Equations—Parallel electric field Now, the case of P polarization: y ki kr Ei B r× ni z x Bi E i r r Note that Hecht Interface uses a different notation for the Beam geometry reflected field, for light with its which is confusing! electric field t E Ours is better! parallel to the t nt plane of incidence Bt This leads to a (i.e., in the page) difference in the kt signs of some equations... Note that the reflected magnetic field must point into the screen to achieve EB k for the reflected wave. The x with a circle around it means “into the screen.” Reflection & Transmission Coefficients for Parallel Polarized Light For parallel polarized light, B0i B0r = B0t and E0icos(i) + E0rcos(r) = E0tcos(t) Solving for E0r / E0i yields the reflection coefficient, r||: rEE|| 0riittiitti/ 0 n cos( ) n cos( ) / n cos( ) n cos( ) Analogously, the transmission coefficient, t|| = E0t / E0i, is tEE|| 0tiiiitti/ 0 2 n cos( ) / n cos( ) n cos( ) These equations are called the Fresnel Equations for parallel polarized (p-polarized) light. To summarize… plan plan e of e of incid incid incident ence incident ence wave wave interface interface E-field vectors are red. k vectors are black. transmitted wave transmitted wave s-polarized light: p-polarized light: nniittcos( ) cos( ) nnitticos( ) cos( ) r r|| nniittcos( ) cos( ) nnitticos( ) cos( ) 2cos()nii 2cos()nii t t|| nniittcos( ) cos( ) nnitticos( ) cos( ) And, for both polarizations: nniittsin( ) sin( ) Reflection Coefficients for an Air-to-Glass Interface The two polarizations are indistinguishable at = 0° 1.0 nair 1 < nglass 1.5 Total reflection at = 90° r for both polarizations. Brewster’s angle .5 r =0! Zero reflection for parallel || r|| polarization at: “Brewster's angle” 0 The value of this angle depends on the value of the ratio ni/nt: -1 Reflection coefficient, -.5 r Brewster = tan (nt/ni) ┴ For air to glass (nglass = 1.5), this is 56.3°. -1.0 0° 30° 60° 90° Sir David Brewster Incidence angle, i 1781 - 1868 Reflection Coefficients for a Glass-to-Air Interface nglass > nair 1.0 Critical r angle Total internal reflection .5 r above the "critical angle" ┴ Total internal reflection sin-1(n /n ) crit t i 0 41.8° for glass-to-air Brewster’s angle (The sine in Snell's Law Reflection coefficient, -.5 can't be greater than one!) Critical r|| angle -1.0 0° 30° 60° 90° Incidence angle, i The obligatory java applet. http://www.ub.edu/javaoptics/docs_applets/Doc_PolarEn.html Reflectance (R) 00c 2 In E0 2 IA R Reflected Power / Incident Power rr A = Area IAii w i r i wi ni nt Because the angle of incidence = the angle of reflection, the beam’s area doesn’t change on reflection. Also, n is the same for both incident and reflected beams. E 2 2 0r 2 So: R r since 2 r E0i 00c 2 In E0 Transmittance (T) 2 IA T Transmitted Power / Incident Power tt A = Area IAii i If the beam wi ni Awcos( ) has width w : tt t i nt Awiicos( i ) wt t The beam expands (or contracts) in one dimension on refraction. 00c 2 nEtt0 2 2 IttAwnw2 tnEtt0 w t tt2 E0t 2 Tt 2 since 2 t I Awnw c 2 ii00 inEii0 w i ii E0i nEii0 2 nttcos Tt 2 niicos Reflectance and Transmittance for an Air-to-Glass Interface Perpendicular polarization Parallel polarization 1.0 1.0 T T Brewster’s .5 .5 angle R R 0 0 0° 30° 60° 90° 0° 30° 60° 90° Incidence angle, i Incidence angle, i Note that it is NOT true that: r + t = 1. But, it is ALWAYS true that: R + T = 1 Reflectance and Transmittance for a Glass-to-Air Interface Perpendicular polarization Parallel polarization 1.0 1.0 T T .5 .5 R R 0 0 0° 30° 60° 90° 0° 30° 60° 90° Incidence angle, i Incidence angle, i Note that the critical angle is the same for both polarizations. And still, R + T = 1 Reflection at normal incidence, i = 0 2 When = 0, the Fresnel i nnti 4nnti equations reduce to: R T nn 2 ti nnti For an air-glass interface (ni = 1 and nt = 1.5), R = 4% and T = 96% The values are the same, whichever direction the light travels, from air to glass or from glass to air. This 4% value has big implications for photography. “lens flare” Where you’ve seen Fresnel’s Equations in action Windows look like mirrors at night (when you’re in a brightly lit room). One-way mirrors (used by police to interrogate bad guys) are just partial reflectors (actually, with a very thin aluminum coating). Disneyland puts ghouls next to you in the haunted house using partial reflectors (also aluminum-coated one- way mirrors). Smooth surfaces can produce pretty good mirror-like reflections, even though they are not made of metal. Fresnel’s Equations in optics Optical fibers only work because of total internal reflection. Many lasers use Brewster’s angle components to avoid reflective losses: R = 100% 0% reflection! Laser medium R = 90% 0% reflection!.
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