Feynman Physics Lectures V2 Ch33 1963-02-21 Surface Reflection

Feynman Physics Lectures V2 Ch33 1963-02-21 Surface Reflection

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Unformatted text preview: 33 Iloflmitian from Surfaces 33-1 Reflection and refraction of light The subject of this chapter is the reflection and refraction of light—or electro- magnetic waves in general—at surfaces. We have already discussed the laws of reflection and refraction in Chapter 35 of Volume 1. Here’s what we found out there: 1. The angle of reflection is equal to the angle of incidence. With the angles defined as shown in Fig. 33—1, (33.1) 2. The product n sm 0 is the same for the incident and transmitted beams (Snell’s law). n1 sin 0, = 112 sin 0,. (33.2) 3. The intensity of the reflected light depends on the angle of incidence and also on the direction of polarization. For E perpendicular to the plane of inCidence, the reflection coefficient R i is - 2 RL 2 {1 _ Sln (6, 0;) I, H Sin2 (6, + Ht) (333) For E parallel to the plane of inCidence, the reflection coei‘fiCient RH is I, _ tan2 (0, — 0t) 4. For normal inCidence (any polarization, of course!), IT n2 — n1 2 _, :2 _ . 3 I, (’12 + 711) (3‘ (Earlier, we used 1 for the inCident angle and r for the refracted angle Since we can’t use r for both “refracted” and “reflected” angles, we are now using 6, = incident angle, 6, = reflected angle, and 0, = transmitted angle.) Our earlier discussion is really about as far as anyone would normally need to go With the subject, but we are going to do it all over again a different way Why " One reason is that we assumed before that the indexes were real (no ab- sorption in the materials) But another reason is that you should kn0w h0w to deal With what happens to waves at surfaces from the point of view of Maxwell’s equations. We'll get the same answers as before, but now from a straightforward solution of the wave problem, rather than by some clever arguments. We want to emphasize that the amplitude of a surface reflection is not a property of the material, as is the index of refraction It is a “surface property," one that depends preCisely on how the surface is made. A thin layer of extraneous junk on the surface between two materials of indices I11 and n2 will usually change the reflection. (There are all kinds of pOSSlbliltICS of interference here~like the colors of 011 films Suitable thickness can even reduce the reflected amplitude to zero for a given frequency; that’s how coated lenses are made.) The formulas we Will derive are correct only if the change of index is sudden—within a distance very small compared with one wavelength. For light, the wavelength is about 5000 A, so by a “smooth” surface we mean one in which the conditions change in 33—1 33-1 Reflection and refraction of light 33—2 Waves in dense materials 33—3 The boundary conditions 33—4 The reflected and transmitted waves 33—5 Reflection from metals 33—6 Total internal reflection Review. Chapter 35, Vol. I, Polarization SURFACE “2 Fig. 33—1. of light waves at a surface. directions are normal to the wave crests.) Reflection and refraction (The wave WAVE caasrs \ \ Fig. 33—2. For a wave moving in the direction It, the phase at any point P is (wt — k - r). going a distance of only a few atoms (or a few angstroms). Our equations will work for light for highly polished surfaces. In general, if the index changes grad- ually over a distance of several wavelengths, there is very little reflection at all. 33—2 Waves in dense materials First, we remind you about the convenient way of describing a sinusmdal plane wave we used in Chapter 36 of Volume 1. Any field component in the wave (we use E as an example) can be written in the form E = EOeW—k'“, (33.6) where E represents the amplitude at the point r (from the origin) at the time t. The vector k points in the direction the wave is travelling, and its magnitude [k] = k = 27r/>\ is the wave number. The phase veIOCity of the wave is m, = w/k, for a light wave in a material of index n, up], = c/n, so can k — —ce- (33.7) Suppose k is in the z-direction, then k - r is just kz, as we have often used it For k in any other direction, we should replace 2 by rk, the distance from the origin in the k-direction; that is, we should replace k2 by krk, which is Just k r. (See Fig. 33—2.) So Eq. (33.6) is a convenient representation of a wave in any direction. We must remember, of course, that k‘r= k1x+kyy+kzz, where k,, k,,, and k; are the components of k along the three axes. In fact, we pointed out once that (w, k,, ky, k2) is a four—vector, and that its scalar product With (1, x, y, z) is an invariant. So the phase of a wave is an invariant, and Eq. (33.6) could be written E = Eoelkm‘. But we don’t need to be that fancy now. For a sinusoidal E, as in Eq. (33.6), 6E/at is the same as in, and aE/ax is — lsz, and so on for the other components. You can see why it is very convenient to use the form in Eq. (33 6) when working with differential equations—differentia- tions are replaced by multiplications. One further useful pomt: The operation V = (6/6x, 6/0y, 8/62) gets replaced by the three multiplications (—lk1, —ik,,, —ikz). But these three factors transform as the components of the vector k, so the operator V gets replaced by multiplication with —ik: 6 u ~—> 1w, at v —» —ik. (33.8) This remains true for any V operation—whether it is the gradient, or the diver- gence, or the curl. For instance, the z-component of V X E is 615, 613,. 6x 8y If both Ey and Ex vary as e77" ’, then we get —zkxEy + zkyEr, which is, you see, the z—component of —Ik X E. So we have the very useful general fact that whenever you have to take the gradient ofa vector that varies as a wave in three dimensmns (they are an important part of physics), you can always take the derivations quickly and almost Without thinking by remembering that the operation V is equivalent to multiplication by —ik. 33—2 For instance, the Faraday equation 03 vxE——3? becomes for a wave —ik X E = —in. This tells us that _ k X E 0.) B . (33.9) which corresponds to the result we found earlier for waves in free space—that B, in a wave, is at right angles to E and to the wave direction. (In free space, w/k : c.) You can remember the sign in Eq. (33 9) from the fact that k is in the direction of Poynting’s vector S : enczE X B. If you use the same rule With the other Maxwell equations, you get again the results of the last chapter and, in particular, that II 1 ii I l k ‘ k (33 10) But since we know that, we won’t do it again. If you want to entertain yourself, you can try the following terrifying problem that was the ultimate test for graduate students back in 1890: solve Maxwell’s equations for plane waves in an anisotropic crystal, that is, when the polarization P 18 related to the electric field E by a tensor of polarizability. You should, of course, choose your axes along the principal axes of the tensor, so that the relations are simplest (then P, = aaEI, P,, = ahEu, and P2 = acEz), but let the waves have an arbitrary direction and polarization. You should be able to find the rela- tions between E and B, and how k varies with direction and wave polarization. Then you Will understand the optics of an anisotropic crystal. It would be best to start With the simpler case of a birefringent crystal—like calcite—for which two of the polarizabilities are equal (say, oq, 2 ac), and see if you can understand why you see double when you look through such a crystal If you can do that, then try the hardest case, in which all three a’s are different. Then you Will know whether you are up to the level of a graduate student of 1890. In this chapter, however, we will consider only isotropic substances I I Fig. 33—3. The propagation vectors I ‘ ~ ‘ . « - ‘ k, k’, and k” for the incident, reflected, and transmitted waves. We know from experience that when a plane wave arrives at the boundary between two different materials—say, air and glass, or water and Oil—there is a wave reflected and a wave transmitted Suppose we assume no more than that and see what we can work out. We choose our axes With the yz-plane in the surface and the xy-plane perpendicular to the incident wave surfaces, as shown in Fig. 33—3. 33—3 The electric vector of the incident wave can then be written as E, = Eoezm‘”k '). (33.11) Since k is perpendicular to the z-axis, k - r = kxx + kyy. (33 12) We write the reflected wave as E. = E{,e““"‘_""”, (33.13) so that its frequency is w', its wave number is k’, and its amplltude 1s E6. (We know, of course, that the frequency is the same and the magnitude ofk 15 the same as for the incident wave, but we are not going to assume even that. We W111 let 1t come out of the mathematical machrnery.) F1nally, we wr1te for the transmltted wave, E) = E6’e“‘“"’_"""). (33.14) We know that one of Maxwell’s equations gives Eq (33.9). so for each of the waves we have I N 13.: 5‘ X E" B. = {5 X,E', B, = Lia. (33.15) (U CO CO Also, 1f we call the 1ndexes of the two media ml and 112, we have from Eq. (33.10) 22 k2 = = (3316) C2 Since the reflected wave 1s 1n the same material, then [2 2 k’2 = (33.17) whereas for the transmitted wave, H2 2 W = E962”? (33.18) 33—3 The boundary conditions All we have done so far is to describe the three waves; our problem now 18 to work out the parameters of the reflected and transmitted waves in terms of those of the incident wave. How can we do that? The three waves we have de- scribed satisfy Maxwell’s equations 1n the uniform mater1al, but Maxwell’s equa- tions must also be satisfied at the boundary between the two d1flerent materials. So we must now look at what happens right at the boundary. We w1ll find that Maxwell‘s equations demand that the three waves fit together in a certam way. As an example of what we mean, the y-component of the electr1c field E must be the same on both s1des of the boundary. This is required by Faraday’s law, _ VXE=—w Fig. 33—4. A boundary condition at EN = 5,1 is obtained from fr E ds = O. , (33.19) as we can see in the following way. ConSIder a little rectangular loop I‘ which straddles the boundary, as shown in F1g 33—4. Equation (33.19) says that the line 1ntegra1 of E around I‘ is equal to the rate of change of the flux of B through the loop: _ 6 fE-ds = ———/B'nda. 1‘ 6! Now imagine that the rectangle is very narrow, so that the loop encloses an 1n- finitesimal area. If B remams fimte (and there‘s no reason 1t should be 1nfin1te at the boundary!) the flux through the area is zero So the l1ne 1ntegral of E must 33—4 be zero. If EU] and EH2 are the components of the field on the two sides of the boundary and if the length of the rectangle 1s I, we have E111] — Eu‘ll : 0 or Eu] = E.,2, (33.20) as we have said. This gives us one relation among the fields of the three waves. The procedure of working out the consequences of Maxwell’s equations at the boundary IS called “determining the boundary cond1trons.” Ordinarily, it IS done by finding as many equations like Eq. (33 20) as one can, by making argu— ments about little rectangles hke F in Flg. 334, or by using little gaussian surfaces that straddle the boundary Although that is a perfectly good way of proceeding, it gives the impression that the problem of dealing With a boundary 15 different for every different physrcal problem For example, in a problem of heat flow across a boundary, how are the tem- peratures on the two SldCS related? Well, you could argue, for one thing, that the heat flow In the boundary from one stde would have to equal the flow away from the other side. It IS usually possible, and generally quite useful, to work Out the boundary conditions by making such physrcal arguments. There may be times, however, when in working on some problem you have only some equations, and you may not see right away what physical arguments to use. So although we are at the moment Interested only in an electromagnetic problem, where we can make the physical arguments“ we want to show you a method that can be used for any problem—at general way of finding what happens at a boundary directly from the differential equations We begin by writing all the Maxwell equations for a dlelectric—and this time we are very specrfic and write out explicitly all the components: VE:—Ff 6I) {)E 8E, 0E aP OB, 6P J m; a: : _ ,4 in ,J .2 6“<le + By + 8:) (fix + 6y + 02) (33 1) QB VXE"—a OE: _ “34 _ _ 9?: (3% 22a) 0)) ('32 — (it “ 0E 0E, OBJ Vt' _ 7r :2 _ .7; 33.22 OZ 6A a! ( b) 013, 81? 0t? , ' _ v 2 _ , .i 3 22 0x 6y (1; (3 c) V ‘ B ’7 0 £151 + .1er 01% : 0 (33 23) 0A d)’ dz 2 .- I 613 (1E, ( ‘- X B x (‘1) + , 03, 68, 1 OF 315, -“ ‘—~7 =v # V 3.24, ‘ (av 62) e” at + at (3 d) 9 03 OB~ l an 6E” .~ I" i ~ : 7 , fii .2 ( (02 0x) 60 61 + at (33 4b) ) 03,, at; 1 ap 6E: .~ _ 30 _ , “z W 3 .2 ‘ (Ox 6y) e0 61 + at (3 4C) 33—5 REGION 1 REGION 3' REGION 2 The fields in the transition Fig. 33—5. region (3) between two different ma- terials in regions (1) and (2). Now these equations must all hold in region 1 (to the left of the boundary) and in region 2 (to the right of the boundary). We have already written the solu- tions in regions 1 and 2. Finally, they must also be satisfied In the boundary, which we can call region 3. Although we usually think of the boundary as being sharply discontinuous, in reality it is not. The physical properties change very rapidly but not infinitely fast. In any case, we can imagine that there is a very rapid, but continuous, transition of the index between region 1 and 2, in a short distance we can call region 3. Also, any field quantity like P1, or E,,, etc., Will make a Similar kind of tranSition in region 3. In this region, the difierential equations must still be satisfied, and it is by following the differential equations in this region that we can arrive at the needed “boundary conditions.” For instance, suppose that we have a boundary between vacuum (region 1) and glass (region 2). There IS nothing to polarize in the vacuum, so P, = 0. Let‘s say there is some polarization P2 in the glass. Between the vacuum and the glass there is a smooth, but rapid, transition If we look at any component of P, say P7,, it might vary as drawn in Fig. 33—5(a). Suppose now we take the first of our equations, Eq (33.21). It involves derivatives of the components of P with respect to x, y, and z. The y- and z-derivatives are not interesting; nothing spec- tacular is happening in those directions. But the x-derivative of PI Will have some very large values in region 3. because of the tremendous slope of PI. The derivative ()PJ/ax Will have a sharp spike at the boundary, as shown in Fig. 33—5(b). If we imagine squashing the boundary to an even thinner layer, the spike would get much higher If the boundary is really sharp for the waves we are interested in, the magnitude of BPI/ax in region 3 Will be much, much greater than any contribu— tions we might have from the variation ofP in the wave away from the boundary—— so we ignore any variations other than those due to the boundary. Now how can Eq. (33 21) be satisfied if there is a whopping big spike on the right-hand side? Only if there is an equally whopping big spike on the other side. Something on the left-hand Side must also be big. The only candidate is aE,/ax, because the variations with y and z are only those small effects in the wave wejust mentioned. So —eo(aE/6x) must be as drawn in Fig. 33—5(c)—just a copy of (am/ax. We have that €9£e=_apx. :3 06x 6x i If we integrate this equation With respect to x across region 3, we conclude that 60(Ex2 — Em) = _(Px2 In other words, thejump in 60E, in gOing from region 1 to region 2 must be equal to the jump in —Px. We can rewrite Eq. (33.25) as EoEw 4‘ P352 = 60E“ ‘i’ P11, (33-26) which says that the quantity (60E, + Pr) has equal values in region 2 and region 1. People say: the quantity (60E,r + P,) is COHIII’ILIOHS‘ across the boundary. We have, in this way, one of our boundary conditions. Although we took as an illustration the case in which P1 was zero because region 1 was a vacuum, it is clear that the same argument applies for any two materials in the two regions, so Eq. (33.26) is true in general. Let’s now go through the rest of Maxwell’s equations and see What each of them tells us. We take next Eq. (33.22a). There are no x-derivatrves, so it doesn’t tell us anything. (Remember that the fields themselves do not get espeCially large at the boundary; only the derivatives With respect to x can become so huge that they dominate the equation.) Next, we look at Eq. (33 22b). Ah' There is an x-derivative! We have aEz/ax on the left-hand Side. Suppose it has a huge de- rivative But wait a moment! There IS nothing on the right-hand side to match it With; therefore E3 cannot have any jump in gomg from region 1 to region 2. [If it did, there would be a spike on the left of Eq. (33.22a) but none on the right, 33~6 and the equation would be false ] So we have a new condition: E:2 = Ezl. (33.27) By the same argument, Eq (33.22c) gives Ell/2 : EUl. This last result is just what we got in Eq. (33 20) by a line integral argument. We go on to Eq. (33 23) The only term that could have a spike is (am/ax. But there’s nothing on the right to match it, so we conclude that 8,2 = 8,1. (33.29) On to the last of Maxwell’s equations! Equation (33 24a) gives nothing, because there are no A-dCl‘lVElthCS Equation (33 23b) has one, —c2 6B2/6x, but again, there is nothing to match it with. We get B22 = B“. (33.30) The last equation is quite similar, and gives 31/2 : Byl- The last three equations gives us that BZ = 3;. We want to emphasize, however, that we get this result only when the materials on both Sides of the boundary are nonmagnetic—or rather, when we can neglect any magnetic effects of the materials. This can usually be done for most materials, except ferromagnetic 0;;‘65 (We Will treat the magnetic properties of materials in some later chapters.) E Our program has netted us the six relations between the fields in region 1 and those in region 2. We have put them all together in Table 33—1. We can now use them to match the waves in the two regions. We want to emphasize, however, that the idea we have just used Will work in any phySICal situation in which you have dilTerential equations and you want a solution that crosses a sharp boundary between two regions where some property changes. For our present purposes, we could have easin derived the same equations by usmg arguments about the fluxes and circulations at the boundary. (You might see whether you can get the same result that way.) But now you have seen a method that will work in case you ever get stuck and don't see any easy argument about the phySlCS of what IS happen- ing at the boundary—you can Just work With the equations. 33—4 The reflected and transmitted waves Now we are ready to apply our boundary conditions to the waves we wrote down in Section 33—2. We had: 5, = Eoe'(W’—’tr“—W’, (33 32) ET : Efiertw'l—Agzwki’fl), El : E;;y<w”'~ké’x-ké’v>, (33.34) B] : 52:79, (33.35) Br : Egg/E, (33.36) 0.) Bi : I: 6:; 9. (33 37) We have one further bit of knowledge: E is perpendicular to its propagation vector k for each wave. 3 3—7 Table 33—1 Boundary conditions at the surface of : dielectric (GOEi + Pl); : (60152 + P2); (El)7/ : (E2)rl (E i)z = (E2): BI L B; (The surface is in the yz-plane) The results Will depend on the direction of the E-vector (the “polarization”) of the incoming wave. The analySIS is much Simplified if we treat separately the case of an inCident wave With its E—vector parallel to the “plane of incidence" (that is, the xy-plane) and the case of an inCIdent wave with the E-vector perpendicular to the plane of incidence. A wave of any other polarization is just a linear combina- tion of two such waves. In other words, the reflected and transmitted intenSIties are different for different polarizations, and it is easiest to pick the two Simplest cases and treat them separately. We will carry through the analysis for an incoming wave polarized per- pendicular to the plane of inCidence and thenjust give you the result for the other. We are cheating a little by taking the simplest case, but the prinCiple is the same for both. So we take that E, has only a z-component, and Since all the E-vectors are in the same direction we can leave off the vector signs. So long as both materials are isotropic, the induced oscillations of charges in the material will also be in the z—direction, and the E—field of the transmitted and radiated waves will have only z—components. So for all the waves, Ex and E1, and Pr and Pg are zero. The waves will have their E- and B-vectors as drawn in Fig. 33—6 (We are cutting a corner here on our original plan of getting everything from the equations. This result would also come out of the boundary conditions, Fig 334,. polarization of the re_ but we can save a lot of algebra by usmg the phySical argument When you have flecfed and transmitted waves when the some spare time, see if you can get the same result from the equations. It is clear E-fleld of the incident wave is perpendicu- that what we have said agrees With the equations; it lSJUSt that we have not shown lar to the plane of incidence. that there are no other pOSSibilities.) Now our boundary conditions, Eqs. (33 26) through (33.31), give relations between the components of E and B in regions 1 and 2. For region 2 we have only the transmitted wave, but in region 1 we have two waves. Which one do we use? The fields in region 1 are, of course, the superposition of the fields of the inCident and reflected waves. (Since each satisfies Maxwell’s equations, so does the sum.) So when we use the boundary conditions, we must use that 'E12E1+Ers E2:Ets and Similarly for the B’s. \ For the polarization we are considering, Eqs. (33.26) and (33.28) give us no new information; only Eq (33.27) is useful. It says that El + Er : E; at the boundary, that is, for x = 0. So we have that Eoeuwtwkuy) + Ebemm’l—kuu) : E(;)/gi(t1i"l—/,1r[rll)’ which must be true for all t and for ally. Suppose we look first at y = 0. Then we have t ’1 i”! E06”) ‘1” E68”) : Eifem This equation says that two oscfllating terms are equal to a third oscfllation. That can happen only if all the oscillations have the same frequency. (It is im- poss1ble for three—0r any number~of such terms With different l‘requenCIes to add to zero for all times.) So to” = w’ = 0.). (33.39) As we knew all along, the frequenCies of the reflected and transmitted waves are the same as that of the inCident wave. We should really have saved ourselves some trouble by putting that in at the beginning, but we wanted to show you that it can also be got out of the equations. When you are doing a real problem, it is usually the best thing to put everything you know into the works right at the sta rt and save yourself a lot of trouble. By definition, the magnitude of k is given by k2 = n2w2/c2, so we have also that k/IZ kl2 k2 err = W = a (33.40) n3 n? nf 33—8 Now look at Eq. (33.38) for I = 0. Usmg again the same kind of argument we have Just made. but this time based on the fact that the equation must hold for all values of y, we get that k1] : k;, 2 kg. (33.41) From Eq. (33.40), k’2 2 k2, so k;2 + kff = k: + k3. Combining this with Eq. (33.41), we have that kll : kf, or that k’, = ikx. The positive Sign makes no sense; that would not give a reflected wave, but another incident wave, and we said at the start that we were solving the problem of only one incident wave. So we have M = —k.. (33 42) The two equations (33.41) and (33.42) give us that the angle of reflection is equal to the angle of inCidence, as we expected. (See Fig. 33—3 ) The reflected wave is E, = (,e'<w‘—kzx+kyy>. (33.43) For the transmitted wave we already have that kg; = k. and k”2 k2 4* = 4- .4 "g "f , (33 4) so we can solve these to find k;’. We get 2 kg? = W — kg,” = g; k2 — k3. (33.45) Suppose for a moment that ml and I13 are real numbers (that the imaginary parts of the indexes are very small). Then all the k’s are also real numbers, and from Fig. 33—3 we find that k, _ . k’y’ _ . 7; — Sin 0,, F, — Sin 0). (33.46) From (33.44) we get that 112 sin 6; = n1 sin 0,, (33.47) which is Snell’s law of refraction—again, something we already knew. If the indexes are not real. the wave numbers are complex, and we have to use Eq. (33.45). [We could still define the angles 6, and 0) by Eq. (33.46), and Snell’s law, Eq. (33.47), would be true in general. But then the “angles” also are complex numbers, thereby losing their simple geometrical interpretation as angles. It 15 best then to describe the behaVior of the waves by their complex km or k’,’ values ] So far, we haven’t found anything new. We havejust had the Simple-minded delight of getting some obvious answers from a complicated mathematical ma- chinery. Now we are ready to find the amplitudes of the waves which we have not yet known. Using our results for the (0’5 and k’s, the exponential factors in Eq. (33.38) can be cancelled, and we get E, + E{, 2 E6’. (33 48) Since both Et’, and E{)' are unknown, we need one more relationship. We must use another of the boundary conditions. The equations for E, and E, are no help, because all the ES have only a z-component So we must use the conditions on B. Let’s try Eq. (33 29): BIZ = le- 3 3—9 Fig. 33—7. when the E-field of the incident wave is parallel to the plane of incidence. Polarization of the waves From Eqs. (33.35) through (33.37). Recalling that w” = w’ w and kj’,’ = kl, = k,,, we get that H / w II E0 ‘l' EU _ 0- But this isjust Eq. (33 48) all over again‘ We’vejust wasted time getting something we already knew. We could try Eq. (33.30), Bzg 2 So there’s only one equation left: Eq. (33.31), 8,,2 : B21, but there are no z-components of B‘ B“. For the three waves. / - I/ B _ B i kLE, B g k, E, 111 H _ ‘_" ’ yr _ _ TT/i ’ y! _ _ T /7 ‘ (.0 (,0 0.) (33.49) Putting for E,, E,, and E, the wave expression for x = 0 (to be at the boundary), the boundary condition is k k5; k t—k iv ’i—k’i ”i—A” i Enema) V11) + ‘7 E‘ljeuu) yJ) : _/7 Ezjleuw U ll). w w (.0 Again all w’s and ky’s are equal, so this reduces to kIE” + kLEE, = k;’E(,’. (33.50) This gives us an equation for the ES that is different from Eq. (33 48). With the two, we can solve for E6 and E6’. Remembering that k: : —k,, we get kg, — kfi/ Et’) 2 Riki/a, Em (33 51) 2k " = IA 33. 2 E0 kr + kit! EO' ( 5 ) These, together With Eq. (33.45) or Eq. (33 46) for kf’, give us what we wanted to know. We will discuss the consequences of this result in the next section. If we begin With a wave polarized with its E-Vector parallel to the plane of inCIdence, E will have both x- and y-components, as shown in Fig. 33—7. The algebra is straightforward but more complicated (The work can be somewhat reduced by expressmg things in this case in terms of the magnetic fields, which are all in the z-direction.) One finds that 2 _ 2 // lEril = IEol <33 53> l’lg r "1 x and {Em : i£”1”~’lf,£ lEUi. (33 54) 2 2 "2k; + (11kg Let’s see whether our results agree with those we got earlier Equation (33 3) is the result we worked out in Chapter 35 of Volume I for the ratio of the intensity of the reflected wave to the intenSity of the 1nc1dent wave Then, however, we were considering only real indexes For real indexes (and k’s), we can write k, = kcos 0, : :0? cos 0,, kg = k” cos 6; = 9% cos 6; Substituting in Eq. (33.51), we have 1% flLcos 6, — *1; cos 6; (33.55) _. iiiiiii 7 , E0 n1 cos 6, + 112 cos 0, 33—10 which does not look the same as Eq. (33.3). It will, however, if we use Snell’s law to get rid of the n’s. Setting n2 2 n1 sin HL/sin 6,, and multiplying the numerator and denominator by sm 6!, we get 6 cos 0, sin 6; — sin 0, cos 6; E0 cos 6, sm 6; —l— Sin 0, cos 0, The numerator and denominator are just the sines of (0, — 6t) and (671 —l— 0,); we get §§ _ 3195— at) E0 _ Sm (gitl' 0:) (33.56) Since EX, and E0 are in the same material, the intensities are proportional to the squares of the electric fields, and we get the same result as before. Similarly, Eq. (33.53) is the same as Eq. (33.4). For waves which arrive at normal incidence, 0, = 0 and 6, = 0. Equation (33.56) gives 0/0, which is not very useful. We can, however, go back to Eq. (33.55), which gives Q 2 (fly = <L:Q>2. 1, Eu "1 + "2 This result, naturally, applies for “either” polarization, since for normal incidence there is no speCial “plane of incidence.” (33.57) 33—5 Reflection from metals We can now use our results to understand the interesting phenomenon of reflection from metals. Why is it that metals are shiny? We saw in the last chapter that metals have an index of refraction which, for some frequenc1es, has a large imaginary part. Let’s see what we would get for the reflected intenSity when light shines from air (with n = 1) onto a material with n = —im. Then Eq. (33.55) gives (for normal incidence) E6 _ 1 + [H1 F0 — li— in] . For the intensity of the reflected wave, we want the square of the absolute values of E6 and EU: Q = iEg|2 _Ll+in1|2 I. lE0l2 _ |1 —in112’ 01‘ 2 _1: 2 Lily. I 1 (33 58) 1: 1 + n? For a material With an index which is a pure imaginary number, there is 100 per- cent reflection' Metals do not reflect 100 percent, but many do reflect Visible light very well. In other words, the imaginary part of their indexes is very large But we have seen that a large imaginary part of the index means a strong absorption. So there is a general rule that if any material gets to be a very good absorber at any frequency. the waves are strongly reflected at the surface and very little gets inSide to be ab- sorbed You can see this effect with strong dyes Pure crystals of the strongest dyes have a “metallic” shine. Probably you have noticed that at the edge of a bottle of purple ink the dried dye Will give a golden metallic reflection, or that dried red ink Will sometimes give a greenish metallic reflection. Red ink absorbs out the greens of transmitted light, so if the ink is very concentrated, it will exhibit a strong surface reflection for the frequencies of green light. You can easin show this eflect by coating a glass plate with red ink and letting it dry. If you direct a beam of white light at the back of the plate, as shown in Fig. 33—8, there Will be a transmitted beam of red light and a reflected beam of green light. 33—11 GLASS PLATE DRIED RED INK Fig. 33—8. A material which absorbs light strongly at the frequency (.0 also reflects light of that frequency. n2=0 n3=n‘.‘. If there is a small gap, Fig. 33—l0. internal reflection is not "total"; a trans— mitted wove appears beyond the gap. Fig. 33—9. Total internal reflection. 33-6 Total internal reflection If light goes from a material like glass, with a real index ll greater than 1. toward, say, air. with an index n2 equal to l, Snell‘s law says that sm 0; = )1 sin 0L. The angle 05 of the transmitted wave becomes 90" when the incident angle 0. is equal to the “critical angle" 0, given by nsm a0 = 1. (33.59) What happens for (91 greater than the critical angle" You know that there is total internal reflection. But how does that come about" Let’s go back to Eq (33.45) which gives the wave number k’,’ for the trans- mitted wave. We would have k5’“ = 25 kg. Now k,, = k sin 61 and k = wn/c, so 2 kg” = E:1)?“ ~ n2 Slnz 61). If n Sln BL 18 greater than one, kg’2 18 negative and k'r’ is a pure imaginary, say ilk]. You know by now what that means' The “transmitted” wave (Eq. 33.34) Will have the form Et 2 Ege::k[xei(ul—I.,ly). The wave amplitude either grows or drops off exponentially With increasing x. Clearly, what we want here is the negative Sign. Then the amplitude of the wave to the right of the boundary will go as shown in Fig. 33—9. Notice that A, IS of the order w/c—which is A”. the free—space wavelength of the light. When light is totally reflected from the mSide of a glass-air surface. there are fields in the air, but they extend beyond the surface only a distance of the order of the wavelength of the light We can now see how to answer the followmg question: If a light wave in glass arrives at the surface at a large enough angle, it IS reflected, if another piece of glass is brought up to the surface (so that the “surface” in effect disappears) the light is transmitted. Exactly when does this happen? Surely there must be con- tinuous change from total reflection to no reflection' The answer, of course, is that if the air gap is so small that the exponential tail of the wave in the air has an appreciable strength at the second piece of glass, it Will shake the electrons there and generate a new wave, as shown in Fig. 33—10. Some light Will be transmitted. (Clearly, our solution is incomplete, we should solve all the equations again for a thin layer of air between two regions of glass.) 33—12 (0) TRANSMITTER H) B DETECTOR (b) DETECTOR ii) ml Bl (c) A" TRANSMTTTER DETECTOR DETECTOR TRANSMITTER DETECTOR Fig. 33—1 1. A demonstration of the penetration of internally reflected waves. This transmission effect can be observed with ordinary light only if the air gap is very small (of the order of the wavelength of light, like 10’5 cm), but it is easily demonstrated with three-centimeter waves. Then the exponentially de- creasing field extends several centimeters. A microwave apparatus that shows the effect is drawn in Fig. 33—11 Waves from a small three-centimeter transmitter are directed at a 45° prism of paraffin. The index of refraction of paraffin for these frequencies is 1.50, and therefore the critical angle is 415°. So the wave is totally reflected from the 45° face and is picked up by detector A, as indicated in Fig. 33—ll(a). If a second paraffin prism is placed in contact with the first, as shown in part (b) of the figure, the wave passes straight through and is picked up at detector B. If a gap of a few centimeters is left between the two prisms, as in part (0). there are both transmitted and reflected waves. The electric field outSide the 45° face of the prism in Fig. 33—11(a) can also be shown by bringing detector B to within a few centimeters of the surface. 33—13 DETECTOR ...
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This note was uploaded on 06/18/2009 for the course PHYSICS none taught by Professor Leekinohara during the Spring '09 term at Uni. Nottingham - Malaysia.

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Feynman Physics Lectures V2 Ch33 1963-02-21 Surface Reflection

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