Ondas Electromagneticas

The beauliful ,...,Ion; from lhe surf;,,,,, of (his w;'p buhble can be nicdy c~plain~d by (he w;I\'c Ihcory of light. A wall' buhhle is ;' vcry (hin ,pherieal film filled with air: light rcncctcd fron' (he outer and inncr ,urfaces of (his Ihin filo, of ro;,p), wakr imerlcro:s constructi,'tly to produce the hright colo"" Which color we See at any poin! derends on the thic~ne>s of the soapy watcrfilm at thal point and al'" on the ,-iewing angle. We cover fundamental asrects of the wave nalUre of light. induding inlerference and diffraction, thin·film imerferen"", and polarizalion, We also cowr man\' applications, from speelrometry to LCD monitor scrL..,n~

C H APTER24
The Wave Nature of Light
hat ligh( cMries energy is obvious (0 anyone who has focused the Sun's rays with a nwgnifyitlgglass on a piece of paper and burned a hole in i1. Bill how docs light travel, and in what form is this ene rgy ca rried'! In Ollr discllssion of waves in Chapter II. we noted that ene rgy can be carried from place to place in basically two ways: by particles o r by waves, In the fi rst case, material objects or particles can carry etlergy, such as an avalanche or rushitlg waler. In the ,econdcase, willer W;I\'es and so un d waves. for e~;'m p le, C;IO cil"y en~rgy over long disl;lOces c'-en lhough the osdll;'ling p;,rticle, of lhc medium do nol lravel lhese distances. In view of lhis, wh;'l can we s;'y ;,bout lhe nature of light: docs lighl travel a, ;' >lream of particles ;",'ay from ils source, or does lighll ra\'cl in lhe form of W;I\'es lh;'l spread o ulwa rd from lhe source?

T66.

Historically, Ihi8 qU~8tion h", turncd OUI 10 bc a difficult onc. For onc Ihing, light doc8 nol re"c,,1 ilsel f in ,my obvious w"Y "8 bcing m"dc up of tiny pa rticlcs: nor do wc ,ec tiny light wa"cs pil"ing by as we do w"lcr w,lves. Th~ cvidencc secmed 10 favor fi rsl onc sidc and lhcn thc olh er un Iii "bout 1830, wh~n mosl physicislS had acceplcd lhc wavc lhcory. By Ih~ c nd of lhcninetccnlh ccnlury, light was considcred 10 be an e/eClrOml!Wlflic !t'1!~' wcll, as we shall discuss in Ch,'plcr 27. Nonethclcs~, the W,l\'C lhcory of light rcmains v"lid and ha, proved vcry su~'CcssfuL We now im'e81ig"tc Ihe cvidence for lhc wavc Iheory and how il has h"en used to ex plain a wide rangc nf phenomena.

Waves Versus Particles; and Diffraction
Thc DUICh scicnti,l ChriSli,," l'luygen,(1629-1695), " conlcmpor,,,y of Newlon, proposed a w',,'e Iheory of light lhal had much m~ri\. Slill u>cful today is a lech nique Huygc n8 dcveloped for predicling lhe futurc posilion of a Wave front when an c,lrlie r po8ition is known. By a W,l\'C fronl, wc mean alilhe poinls along a lwo or Ihre"·dim,, nsional waw that form a W1l\'" crest-what We simply cnll a "waw" as sec n on the oeiplc if weRoy

FIGURE 24-3

RcfraCliun ~~plained_ using Huygens' principk \Va"" lronts are pcrpcndiculM to the rays-

A

Medium t

c

, , , ,

Mcdium2 (I'l < V,)

"

,,'

Jssume the sp c;m be reproduccd using Ih ree prim;,,), colo~ They ;lre red, green, ;md blue for direct source viewing such as TV ;md computer monilo~ For in ks uso:d in printing, Ihe prim;lrYcolors are 9'an (thecolor of the m;lrgin nole> in this book), yellow, and m;lgenta (Ihe color we use for light r;,ys in di;'gr;,m»_

Diffraction

a

Slit or Disk

Young'S double-slit experiment pll1 the wave theory of light on a fi rm footing. But full acceptance came only with studies on diffraction (Section 24 - 1) more than a decade Imer. in the 18lOs and 18205. We have already discussed diffraction brieflywith rega rd to water waves (Section 11 - 15) as well as for light (Section 24 - 1), and we have seen that it refers to the spreading or bending of waves around edges. Let's look in more detail. In 1819 Augustin Fre>nel (1788-1827) preso:nled to the French Ac;'demy ;' W;l\'e theory o f light th;'1 predicted ;md e~plained interfe rence ;md diffraClion effeCIs. Almo>t immediately Simcon Poisson...