Experiment 26 Refraction

Transcription

1 Expement 26 Refacton Advanced Readng: (Knght 4 th Edton) Chapte 34 secton 34-3 Equpment: 1 Plex-Ray 2 cokboads 2 potactos 2 30 cm ules Objectve: The object of ths expement s the phenomena of efacton. Theoy: The Law of Reflecton states that the angle of the ncdent ay equals the angle of the eflected ay, o θ = θ. See Fgue In geneal the speed of lght n any mateal s less that the speed of lght n a vacuum, so the ndex of efacton defned as the ato speed of lght n vacuum n = c speed of lght n medum v fgue 26-1 Fgue 26-2 Fgue 23-2 Reflecton fom a plane mo s geate than one. The Law of Refacton (Snell's Law) elates how a ay of lght wll behave when passng fom one meda to the othe. It s gven by snθ 1 = n 2 snθ 2 n 2 whee and n 2 ae the ndces of efacton fo the two dffeent meda and θ 1 and θ 2 ae the angles measued wth espect to a lne nomal to the nteface. See Fgue Fgue 26-3 Fgue 23-3 Refacton when changng meda Pocedue: 1. Place the plex-glass squae at the cente of anothe sheet of clean whte pape and tace aound t. In the uppe-left cone of the taced squae, daw a lne nomal to the squae 1 cm fom the cone of the squae that cosses the sde of the squae. See fgue 26-4 to the ght. fgue 26-4

2 7. Daw a lne left of the nomal and place two pns on the lne. Label these pns P1 & P2. Look though the plexglass squae fom the sde opposte pns and place pns two pns n the page so that t appeas that all the pns ae collnea. Label these two ponts P3 and P4. Refacted pns (n glass) n fgue 26-5 ae not collnea. Pns (both eal and efacted) n fgue 26-6 ae collnea. Take the pns out and daw a lne though the ponts begnnng at the edge of the squae. Whee the lne jons the edge of the squae, daw anothe nomal lne that cosses the sde of the edge of the squae. See Fgue below. Fgue 26-5 Pns above ae not collnea 8. Measue the angles that the ncdent and tansmtted (efacted) ay makes to the nomal when the ay entes the block. Repeat these measuements fo the ay as t leaves the block Use these angles to calculate the ndex of efacton of the mateal. 9. Repeat fo a dffeent set of ponts. Questons/Conclusons: 1. Why does most of the efacton n the eye take place at the a-conea nteface (ndex of efacto.0003 & espectvely) athe than at the lens- vteous humo nteface (ndex of efacto.44 & 1.34 espectvely.) 2. Calculate the speed of the lght the block that you used n lab (based upon you ndex of efacton. Show calculaton. Fgue 26-5 Pns above ae collnea

3 Expement 26 Reflecton and Refacton Advanced Readng: (Knght 4 th Edton) Chapte 34 secton 34-3 Equpment: 1 Plex-Ray 2 cokboads 2 potactos 2 30 cm ules Objectve: The object of ths expement s to study the laws of eflecton and efacton. Theoy: The Law of Reflecton states that the angle of the ncdent ay equals the angle of the eflected ay, o θ = θ. See Fgue In geneal the speed of lght n any mateal s less that the speed of lght n a vacuum, so the ndex of efacton defned as the ato speed of lght n vacuum n = c speed of lght n medum v fgue 26-1 Fgue 26-2 Fgue 23-2 Reflecton fom a plane mo s geate than one. The Law of Refacton (Snell's Law) elates how a ay of lght wll behave when passng fom one meda to the othe. It s gven by snθ 1 = n 2 snθ 2 n 2 whee and n 2 ae the ndces of efacton fo the two dffeent meda and θ 1 and θ 2 ae the angles measued wth espect to a lne nomal to the nteface. See Fgue Fgue 26-3 Fgue 23-3 Refacton when changng meda Pocedue: 1. Place the plex-glass squae at the cente of anothe sheet of clean whte pape and tace aound t. In the uppe-left cone of the taced squae, daw a lne nomal to the squae 1 cm fom the cone of the squae that cosses the sde of the squae. See fgue 26-4 to the ght. fgue 26-4

4 7. Daw a lne left of the nomal and place two pns on the lne. Label these pns P1 & P2. Look though the plexglass squae fom the sde opposte pns and place pns two pns n the page so that t appeas that all the pns ae collnea. Label these two ponts P3 and P4. Refacted pns (n glass) n fgue 26-5 ae not collnea. Pns (both eal and efacted) n fgue 26-6 ae collnea. Take the pns out and daw a lne though the ponts begnnng at the edge of the squae. Whee the lne jons the edge of the squae, daw anothe nomal lne that cosses the sde of the edge of the squae. See Fgue below. Fgue 26-5 Pns above ae not collnea 8. Measue the angles that the ncdent and tansmtted (efacted) ay makes to the nomal when the ay entes the block. Repeat these measuements fo the ay as t leaves the block Use these angles to calculate the ndex of efacton of the mateal. 9. Repeat fo a dffeent set of ponts. Questons/Conclusons: 1. Why does most of the efacton n the eye take place at the a-conea nteface (ndex of efacto.0003 & espectvely) athe than at the lens- vteous humo nteface (ndex of efacto.44 & 1.34 espectvely.) 2. Calculate the speed of the lght the block that you used n lab (based upon you ndex of efacton. Show calculaton. Fgue 26-5 Pns above ae collnea

5 PHYS222 Expement 27 Thn Lenses Advanced Readng: (Knght 4 th Edton) Chapte 34 sectons 34-5 & 34-6 Equpment: 1 Optcal Bench 3 lenses holdes 3 optcal bench clamps 1 coss-haed lght souce 1 sceen 1 B-convex lens 1 B-concave lens Objectve: The object of ths expement s to obseve the chaactestcs of (thn) convex and concave lenses and to measue the focal lengths of these two types lenses. Theoy: In a pevous expement lght was found to change decton when t passed (at an angle) between meda wth dffeent ndces of efacton. Ths popety can be utlzed to bend (efact) lght n useful ways. A convex lens (made of glass wth n > 1) can be used to focus lght ays and fom a eal mage as the lght tavels fom a ( n 1) to glass and back to a. A eal mage s one that can be pojected on a sceen. A concave lens foms a vtual mage that cannot be pojected on a sceen. An mpotant popety of a lens s ts focal length. The focal length f of a thn lens s gven by: 1 s + 1 s = 1 f Eq whee s s the object dstance and s s the mage dstance. Fo an object that s nfntely fa away, the mage dstance and the focal length ae the same. Fo lenses used n the lab ( f < 30cm ) objects ove 20 metes away can be consdeed nfntely fa away. Pocedue: Pat 1: Measuement of the focal length of a convegng lens Ths wll be done two ways: (1) Usng dstant objects (2) Usng the lens fomula Dstant Object method 1. Place a lens and sceen on the optcal als. Pont the appaatus towads the wndow. Move the sceen untl a clea mage s fomed of somethng outsde the wndow. You should detemne a ange of uncetanty. You TA wll explan ths. 2. Measue the dstance fom the lens to the sceen. Ths s s. If we assume that the object dstance s vey lage compaed to the mage dstance, then the mage dstance s equal to the focal length of the lens. ote the chaacte of the mage. Is t nveted o not? Is the magnfcaton > 1 (.e., mage sze lage than the object tself) o magnfcaton < 1 (.e., mage sze smalle than the object tself)? Lens Equaton Method 3. Plug n the lght souce. Adjust the set-up so that a clea mage of the object falls on the sceen. See data table gven n uncetanty webste). 4. Measue the mage and the object dstances. Use equaton 27-1 to fnd the focal length of the lens. Measue at least 4 mage/object dstances and calculate an aveage focal length. 5. Compae the focal lengths measued fom the two dffeent methods. Pat 2: Measuement of the focal length of a dvegng lens In ths pat of the expement a eal mage wll be used as a vtual object. Befoe statng step 8 below exploe the behavo of a negatve lens by lookng at an object fom vaous dstances. ote what you obseve. Does mage eve go out of focus. Is t s always upght?

6 6. Recod all postons n pat 2 n you lab data notebook. Place the lght box at some poston O. Fom an mage of the lght box on the sceen wth the convegng lens. Recod ths mage poston as I. Ths mage wll seve as a vtual object fo the next pat of the expement. 7. ext, place the dvegng lens n the system at some pont between convex lens and the locaton whee mage I was fomed on sceen. (See fgue 27-3.) Whle keepng the dvegng lens statonay, fom a clea mage on the sceen by movng the sceen away fom the lenses to a new poston I. The dstance fom the concave lens to the new sceen poston (at I ) s the mage dstance s. s s The dstance fom the concave lens to poston of the fst mage fomed (at poston I) s the (vtual) object dstance s. See fgue Fgue Calculate the focal length of the dvegng lens. Please note that s s a negatve value. s 8. Move the lens aangement and epeat two moe tmes. Calculate the aveage focal length. Questons/Conclusons: 1. Daw one ay dagam each fo the dstant object method and the lens equaton method used to detemne the focal lengths of the lenses. 2. A concave lens made out of a s mmesed n wate (two watch glasses glued to a pece of ppe so that a s nsde). Wll t fom a eal mage that can be focused on a sceen? Why o why not? Justfy you answe usng a efacton ay dagam. 3. If a convex lens wth an ndex of efacton of 1.5 and a focal length of 20 cm wee mmesed n a flud whose ndex of efacton s also 1.5, what s the new focal length of the lens. (Hnt- Ths s somewhat of a tck queston.) 4. In Ptfall Peventon 36.1 of Seway & Jewett 8 th Edton the authos state Magnfcaton does not necessaly mply that enlagement. Explan what ths means n the context of what you obseved n the expement.