How can I control light? (and rule the world?)

Transcription

1 How can I control light? (and rul th world?) "You know, I hav on simpl rqust. And that is to hav sharks with frickin' lasr bams attachd to thir hads! - Dr. Evil Phys 230, Day 35 Qustions? Spctra (colors of light) Discharg Lamps & lasrs Rmindrs: Lasrs Bonding and LEDs Survy on nxt wk up til tonight Modls of th Atom Thomson Plum Pudding Why? Known that ngativ chargs can b rmovd from atom. Problm: just a random guss Ruthrford Solar Systm Why? Scattring showd hard cor. Problm: lctrons should spiral into nuclus in ~0 - sc. Bohr fixd nrgy lvls Why? Explains spctral lins. + Problm: No rason for fixd nrgy lvls dbrogli lctron standing wavs Why? Explains fixd nrgy lvls Problm: still only works for Hydrogn. Schrodingr savs th day! Why? Explains vrything w know Problm: complicatd + +

2 Larning Goals. What on ss if bash atoms with anything, particularly lctrons, as in a discharg lamp. 3. What light coming from atoms ( spctra ) imply about bhavior of lctrons in atom. 3. Dscrib and dsign how to us atomic structur and intraction with light to mak lasrs Look at with diffraction gratings and atomic discharg lamps. Mrcury, Sodium, non Hold grating only by dgs...oil from hands ruins grating. Hold clos to y... S rainbow from lights. Turn so rainbow is horizontal. In atomic discharg lamps, lots of lctrons givn bunch of nrgy (voltag). Bash into atoms. ( Non lights, Mrcury strt lamps) 20 Volts voltag diffrnc or mor with long tub Cathod Moving lctrons Colliding with atoms 2

3 Discussion: Givn what w know about light, what dos this imply about lctrons in atoms? Atoms only chang btwn vry spcific nrgis. Only way for individual atoms to giv off nrgy is as light. Each tim a photon is mittd an atom must b changing in nrgy by that amount (rlasing nrgy). (basics, applications) Atoms ar lazy - always want to go back to lowst nrgy stat. 2. Excitd atom.. atom 3. Atom gos. Fast lctron gos to highr nrgy back to low hits atom nrgy Lss KE Highr nrgy Ground stat Excitd stat 0ns nrgy lvls of lctron stuck in atom 3 2 nrgy of colliding lctron G (ground) If many colliding lctrons hav an nrgy btwn that of lvl and lvl 3 whn thy hit th atom a. no lvls will b xcitd, and so no light will com out. b. color of light will com out c. 2 colors of light will com out d. 3 colors of light will com out. 4 colors com out. 3

4 Enrgy lvl diagrams- rprsnt nrgy lvls of atom diffrnt hight = diffrnt nrgy For Hydrogn, transitions to ground stat in ultraviolt! No light mittd with colors in this rgion bcaus no nrgy lvls spacd with this nrgy. In discharg lamps, on lctron bashs into an atom. d 0 V D -2 V -3 V -6 V If atom fixd at this point in tub, -0V list all th possibl nrgy photons (colors) that you might s? A. V, 2V, 3V, 4V, 7V, 8V B. 4V, 7V, 8V C. V, 3V, 4V D. 4V Answr is D. Elctron only gains about 5V! E. Impossibl to tll. Elctron nrgy = qδv = (Ed), whr E is th lctric fild = (battry V)/(total distanc D), and d is th distanc it gos bfor a collision. 4

5 Important Idas ) Atoms hav spcific, fixd nrgy lvls 2) Diffrnt st of nrgy lvls for diffrnt atoms 3) photon mittd pr lctron jump down btwn nrgy lvls. Photon color dtrmind by nrgy diffrnc. 4) atom spnds vry littl tim (0-8 s) in xcitd stat bfor hopping back down to lowst unfilld lvl. 5) If lctron not stuck in atom, it can hav any nrgy. Hydrogn Lithium Enrgy Elctron nrgy lvls in 2 diffrnt atoms Lvls hav diffrnt spacing. Atoms with mor than on lctron lowr lvls filld. (not to scal) Applications of atomic spctroscopy (how it is usful). Dtcting what atoms ar in a matrial. (xcit by putting in discharg lamp or hating in flam to s spctral lins) 2. Dtcting what sun and stars ar mad of. Look at light from star with diffraction grating, s what lins thr ar- match up to atoms on arth. diffraction grating tlscop 3. Making much mor fficint lights! Incandscnt light bulbs wast 88% of th lctrical nrgy that gos into thm! (2% fficint) Strtlight discharg lamps (Na or Hg) 80% fficint. Fluorscnt lights ~ 40-60% fficnt. star 5

6 Incandscnt light (hot filamnt) Tmpratur = K Hot lctrons jump btwn many vry closly spacd lvls (solid mtal). Produc all colors. Mostly infrard at tmp of normal filamnt. 88% is worthlss IR IR = longr than 680nm IR λ 2% of nrgy is usful visibl light Discharg lamp Enrgy lvls in isolatd atom. kick up, only crtain wavlngths whn com down. 20 V or mor Right atom, right prssur and voltag, mostly visibl light. Strtlight discharg lamps (Na or Hg) 80% fficint. Florscnt Lights. Similar ida, but littl mor complicatd to gt out light that looks whit to y. Convrting UV light into visibl photons with phosphor. Phosphors block all but UV, convrts to visibl. phosphor coating 254 nm far UV Hg nrgy of lctron in phosphor molcul 6

7 Qustions on applications of discharg lamps to lighting? Lasrs: ( light amplification by stimulatd mission of radiation ). What is diffrnt/spcial about lasr light. 2. Physics of intractions of atoms with light. (how us to mak whol bunch of idntical photons) 3. How to build a lasr (you ll hav to find your own shark) 3 sourcs of light (traditional): light bulb filamnt Hot lctrons. vry larg # clos nrgy lvls (mtal) Radiat spctrum of colors. Mostly IR. atom discharg lamps Elctron jumps to lowr lvls. Only spcific wavlngths. 20 V or mor with long tub P λ IR Light from xtndd sourc Going diffrnt dirctions Rang of wavlngths photon viw wav viw lasr light-- all xactly th sam whol bunch of idntical photons (actually on top of ach othr) big lctric fild narly prfct sinwav 4 7

8 Light from lasrs is much mor likly to damag th rtina of th y than light from a bulb bcaus a. lasr is at a mor dangrous color. b. has lots mor powr in th bam. c. light is concntratd to a much smallr spot on th rtina. d. light from bulb is turning off and on 60 tims pr scond so light is not as intns. 5 How to produc lasr light? - photons xactly sam color - sam dirction - in phas Bas on how light intracts with atoms! 6 8

9 incrasing nrgy allowd nrgis Figur from txtbook. ground stat d. arrows on lft rprsnt absorption of light, arrows on right rprsnt mission. Wak rsonant xcitation absorption, Light ONLY absorbd if it matchs ΔE thn spontanous mission ~ sc latr what happns if strongr xciting light? 7 Stimulatd mission of light. First ralizd by A. Einstin Photon hits atom alrady in highr nrgy lvl. original photon continus and atom mits scond idntical on G photon atom in xcitd stat scond idntical photon coms out. Atom jumps down. Cloning photon. 8 9

10 Thr procsss by which light intracts with atoms in out in out in out absorption (of light) stimulatd mission (of light) Surprising fact. Chanc of stimulatd mission of xcitd atom EXACTLY th sam as chanc of absorption by lowr stat atom. Critical fact for making a lasr. spontanous mission (of light) (Aftr lc. coll. or light xcitd atom) Lasr-- just us stimulatd mission to clon photon many tims (~0 20 /sc) Light Amplification by Stimulatd Emission of Radiation 9 Chanc of stimulatd mission of xcitd atom EXACTLY th sam as chanc of absorption by ground stat atom. Glass tub blow, full of atoms, lik discharg lamp. Som xcitd som not xcitd (as shown) For th condition abov: what do you xpct? a. Mor photons will com out right hand nd of tub, b. Fwr photons will com out right hand nd of tub c. Sam numbr as go in, d. Non will com out. 20 0

11 LASER - Light Amplification by Stimulatd Emission of Radiation Nd to clon lots of photons! LOTS of idntical light. Thr procss, all play important rols: in out in out in out stimulatd spontanous absorption mission mission Basic rquirmnts for lasr: ) Nd mor atoms in an uppr lvl than a lowr on ( Population Invrsion ) (hard part of making lasr) 2) Nd mthod of r-cycling photons to clon mor tims ( fdback ) (mirrors) 2 To incras numbr of photons aftr going through th atoms nd mor in uppr nrgy lvl than in lowr. Nd a Population invrsion (This is th hard part of making lasr, b/c atoms jump down so quickly.) N uppr > N lowr, (mor rproducd than atn) N uppr < N lowr, fwr out than in. 22

12 Can you gt a population invrsion in a two lvl systm? 23 Gtting a population invrsion nd at last on mor nrgy lvl involvd. Trick: us a scond color of light (why two lvls (on color) won t work as HW problm (mayb)) 2 also can kick up by bashing with lctron t t 2 To crat population invrsion btwn G and lvl would nd: a. tim spnt in lvl 2 (t 2 ) bfor spontanously jumping to is long, and tim spnt in lvl (t ) bfor jumping to G is short. b. t =t 2 c. t 2 short, t long G d. dos not mattr pumping procss to produc population invrsion 24 2

13 Lasr-- Light Amplification by Stimulatd Emission of Radiation lots of cloning of photons- LOTS of idntical light. Figur out conditions for l.a.s..r. Important rols all playd by: absorption stimulatd mission spontanous mission Rquirs ) mor atoms in an uppr lvl than a lowr on ( population invrsion ) (hard part of making lasr) 2) Mthod of r-cycling photons to clon mor tims ( fdback (mirrors) 25 Amplifying light: Population invrsion giv amplification of photons from lft. But much asir if not all light scaps. Rus. Us mirror to rflct th light. (sim) If 3 in bcoms 6 at nd, What dos 6 bcom? 26 3

14 On photon bcoms two, 2 bcoms 4, 4 bcoms 8, 8 sixtn.. Etc Lasr Gain Do you know th words of Al Bartltt? (th lack of undrstanding th xponntial function is th grat failur of th human rac) May b bad for human population. Good for photon population. Numbr of photons btwn th mirrors, n = n 0 Gt gain G >0 xponntial incras. Vry quickly incrass until narly all input powr is going into lasr light. Us partially rflctiv mirror on on nd. Lt som of lasr light insid lak out --- that s what w s. 27 Two typs of lasrs: H-N and Diod Gas lasr lik Hlium Non. Just lik non sign with hlium and non mixtur in it and mirrors on nd. Diod lasr- Sam basic ida, but light from diod at P-N diod junction. Mirrors on it. 28 4

15 Many applications of lasrs High nrgy small ara: Cutting: surgry, lasr wlding communication (and wapons) Focus light into xtrmly small spot: (diffraction limit, bcaus in phas!) CDs, DVDs, Collimatd bam Tracking, lvling, Pur color LIDAR. 29 End of gnral atomic spctra. Undrstanding of what has bn obsrvd, how implis lctrons in atoms only in crtain nrgy lvls. Whn hop from highr to lowr giv off light. Applications: non lights, lasrs Qustions? Nxt: Band structur / LEDs Build from singl atom / nrgy lvls to mor complx what happns to nrgy lvls whn atoms intract 30 5