'It is the nature ofman to rise to greatness if greatness is expected of him." - John
Steinbeck. Reading: pages 1131 - 1136. Outline: - interactions ofphotons with ...
'
Physics4C (Spring 2013) Chapter40: AllAboutAtoms
"What we seedependsmainly on what we lookfor. " - JohnLubbock nLife is a mirror and will reflect backto the thinker what he thinksinto it - EmestHolmes " "Childrenare likely to live up to y)hatyou believeof them."- LadyBird Johnson 'It is the natureofman to riseto greatnessif greatnessis expectedof him." - JohnSteinbeck Reading: pages1131- 1136 Outline: - interactionsofphotons with matter absorption,scattering,andemission + lasers characteristicsof laserlisht how laserswork
ProblemSolvingTechniques Someproblemsrequireyou to understandthe operationofa laser.You may be askedto calculate the wavelengthor frequencyofthe laserlight, given the energylevelsor you might be askedto calculatethe populationsof the levels,in themrodynamicequilibrium or after pumpinghas occurred.If atomsin the laserjump from energyd to lower energyE7,then the frequency/of the light is given by hf : & - ^E7.lnthermodynamicequilibrium the numberof atomswith an energyE is proportioralto e'-*', whereZis the temperatureon the Kelvin scaleandf is the Boltzmannconstanl
Questionsand ExampleProblemsfrom Chapter 40 QuestionI Which (if any) ofthe following is essentialfor laseractionto occurbetweentwo energylevelsof an atom?@ Therearemore atomsin the upperlevel than in the lower.@ The upperlevel is metastable.(c) The lower level is metastable.(d) The lower level is the groundstateof the atom. (e; The lasingmediumis a gas.
..)\q+b
ProblemI 3.2 eV. Supposethat at a A hypothetibalatomhasonly two atomic energylevels,separated-by x 10r3/cm3 of theseatomsin the higher certainaltitudein the atmosphereof a startherearc 6.1 energystateand2.5 x 10"/cmi in the lower energystate.What is the temperatweof the star's aftnosphereat that altifide?
E^'Et=
3'At-/
Nt=N,
N^-- 6.1xtoe/e^a N,= 1.5rto'7.n2
K = t"zz*V-DVK
N/N, = e
\.
T
(€)-e,)/KT
A" ( Ny'r..r, ) = - (e.,-E,)/rt /=
T=?
e
(E,-E,)/rr
- (€*-E,)
K .o^( Ny'1,)
- ( =.a"v)
-
l/=l.o*loqKl t, Problem2 A helium-neonlaseremits laserlight at a wavelengthof 632.8nm anda power of 2.3 mW. At what ratearephotonsemittedby this device?
' 7r,ah J0'-* p,}"jt^, a^* -a*...r-!t-d-- '"2.fo"* r"qiJh''^&'+Y srn^t*
A= 7/e=' f= h+ = hy'>
A=
A:
(t-:, to-3w) (oar.r n.) t*:ltl-=; (raro "-V"o*)(
P -)---
( \cl't
B{wdi.l
F"l,t pA,&\
=
P \ t''.- ' _,
hc-
A = 7.3" lo's i'
Problem3 A hypotheticalatomhastwo energylevels,with a transitionwavelengthbetweenthem of 580nm. In d particularsampleat 300K, 4.0 x I 020suchatomsarein the stateof lower energy. (a) How manyatomsarein the upperstate,assumingconditionsof thermalequilibrium? (b) Suppose,instead,that 3.0 x 1020of theseatomsare'pumped" into the upperstateby an external process,with 1.0 x 1020atomsremainingin the lower state.What is the maximumenergythat could be releasedby the atomsin a singlelaserpuiseif eachatomjumps oncebetweenthosetwo states(eithervia absorptionor stimulatedemission).
. \---
N^= N,
3*Y frl, = 4.o^ 16'o
A.E=
lJ4
N*=?
(e"-E')/rr " E
.1
>
- E4 r
= L t€t
)
-
Er-E,=64
l]'{o eU-nrrt \
N,:N, e- *?-r --
llaoeV.nrq
l!"o'.;(r.ar*ro-a)srlp)(1*.,;.o.) C3*r)
(tl"o^ lo*) e
fu ^.,a-y fu^*A (b)
aj-o44-1;/ar.ir/c/Nct --)
4AA
l.o* 1o-ob,
=)
,a,ffia
EW
3,ox1p1o "b,.4yhpA^4
alnab
< p*ot
= (1.o"[o'") hf = (]..oxto"o) ny'X
E 8o nr",
A /-- tug
+ /,^*p .rIli
oX A.o* 16ao pt*b,"a,v^ep'aurL;
= (Y1o'") ( tr.toeV.,,*) -
4. ? lo"o.v/ " 6sf
l4;l^pLjq,t
^;Al"
o 3.rfl,h gr^,"1
Fr,.u
