'
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<