SPH3UW Unit 7.1 The Ray Model of Light Page 2 of 5. The accepted value for the speed of light inside a vacuum is c m which we usually

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1 SPH3UW Unit 7. The Ray Model of Light Page of 5 Note Phyi Tool box Ray light trael in traight path alled ray. Index of refration (n) i the ratio of the peed of light () in a auu to the peed of light in a gien aterial (). n i the peed of light in a auu Experiental eidene ugget that light trael in traight line. Thi reaonable auption ha led to the ray odel of light, een though hitory ha preented u with two other odel: The Wae Model, and the Partile Model. The ray odel i a generi explanation and it ain adantage lie in i it ipliity. Atually, a ray repreent an infinitely narrow bea of light. Thu when we obere an objet, light (ray) reahe our eye fro eah point on the objet. We note that light ray leae eah point on an objet in any different diretion, only a all few eer reah our eye, and when we oe our head, a different et of ray fro the objet will enter our eye, gien u a different perpetie of the objet. We will ue the traight path of the ray and geoetry to deribe refletion, refration, and the foration of iage by irror and lene One of the earliet quetion aked by phyiit, one they hae an undertanding of the propertie of light, wa, what i it eloity?. Galileo attepted (unuefully) to eaure light peed by trying to deterine the tie for light to trael a fixed ditane (between two hill top) The firt ueful deterination of the peed of light ae fro the Danih Atronoer Ole Roeer who notied that there wa a light hange in the period of one of the oon of Jupiter (Io). He reaoned that the differene wa due to the hanging ditane between Earth and Jupiter, thu it took light longer to reah Earth when Jupiter wa further away, and he deterined that light did hae a finite peed.

2 SPH3UW Unit 7. The Ray Model of Light Page 2 of 5 Around 7, Albert A. Mihelon ( 52-93) began to deie other experient to eaure light peed and the preene of the aether. Aepting the aether wae theory, he deigned ynthei optial equipent, inluding interferoeter, range finder and a anning deie (illutrated) to eaure light peed. Thi final apparatu inorporate a rotating otagonal irror, iilar to thoe found in uper wide angle iage anner and point of ale produt identifiation ode anning yte. Light fro the oure trike the anning irror and reflet. Light oing fro the parallel diretion reflet into the teleope. Anything within the bea' range and illuinated by the bea will exhibit a poition related intenity. Mihelon plaed a olliator on Mount San Antonio, 22.5 ile (line of ight) fro Mount Wilon, to the Eat-North-Eat of Mount Wilon, a round trip total of 45 ile. The ue of the teleope retrit the an to a all field of iew. One rotation of the irror an the objet tie. Obiouly, light ut trael fro M to M2 and return to M, through the Earth' highly refratie dene atophere. The peed of the otor, the geographi line-of-ight ditane, and the length of the eond need to be aurately realized. The troboopi effet ourred at "about 530 rp." The aepted alue for the peed of light inide a auu i whih we uually rounded to The ratio of the peed of light in auu to the peed in a gien aterial i alled the index of refletion, n, of that aterial. n The index of refration i neer le than (n>). The index of refration play a entral role in geoetri opti. Soetie the phrae optially dene i uually deribed a ediu in whih light trael lower than.

3 SPH3UW Unit 7. The Ray Model of Light Page 3 of 5 Exaple. What i the peed of light in water when index of refration in water i.33. n Exaple 2. How long doe it take light to reah u fro the Sun, whih i.50 0 k away. d t Notie that I onerted the Sun ditane fro k to. Alway ue unit analyi in your anwer Exaple 3. In a Mihelon-type experient, a rotating eight-ided irror wa plaed 50.0 k fro the refleting irror a diagraed below. The oberer found that in order to obere the return light ray, the irror had to rotate at 375 Hz. What i the peed of light alulated fro thi experient? oberer Stationary refleting irror Rotating eight-ided irror light oure 50.0 k

4 SPH3UW Unit 7. The Ray Model of Light Page 4 of 5 Veloity i ditane oer tie, and we an approxiate the ditane a 00 k. Our hallenge i to deterine the tie for thi journey. Thi an be aoplihed by noting that thi tie i idential to the tie it take for light to go and return to the refleting irror a the ided irror rotate. Or iply / th of the period of the rotating irror, a we need the next irror panel to be in the preie loation a the original irror panel a the light bouned off it, o the following irror panel will reflet the ignal bak to the oberer. That i, the irror ha only rotated / of a reolution to be perfetly lined up for the returning light ignal. Period of irror: T f yle 375 e Tie for light to go fro oure and return to oberer: t T The Speed of Light: d t

5 SPH3UW Unit 7. The Ray Model of Light Page 5 of 5 Extra Note and Coent