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1. Interferencea.Double slit Interference
sinar matahari
S1
S2
So
A B C
The narrow slits, S1and S2 act
as sources of waves
The waves emerging from theslits originate from the same
wave front and therefore are
always in phase
Sun ray
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Ray come from S1dan S2construc at P. Wave front come to
screen B is reputed parallel
L
S1
S2
a
b
)
r1
r2
y
P
dO
!
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"or a bright fringe, produced by constructive
interference, the path difference must be either
#ero or some integral multiple of of the
wavelength$ % d sin &bright% m '
m % (, )1, )2, *
m is called the order number
+hen m % (, it is the #eroth order maimum
+hen m % )1, it is called the first order maimum
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+hen destructive interference occurs, a dar-
fringe is observed
This needs a path difference of an odd half
wavelength
$ % d sin &dar-% m / 0 '
m % (, )1, )2, *
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"or bright fringes
"or dar- fringes
2,1,(mmd
ybright =
=
2,1,(m21m
dydar- =
+=
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Pber3ara- r1dari S1dan r2dari S2.
Tari-lah garis dari S2-e bsehingga 4S2%4b
5i-a d 66D, ma-a S2b hampir r1dan r2.
7-ibatnya S1S2b 4a8 %
7gar di titi- 4 ter3adi ma-simum, ma-a
S1b %d sin harus -elipatan bilangan bulat
S1b=m , dengan m= (, 1, 2,......5adi,
d sin % m , dengan m % (,1,2, . ... ma-simum
d sin % m/192 , dengan m % (,1, 2, ..... minimum
Dengan:d % 3ara- -edua celah% 4an3ang gelombang cahaya
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Gambar 1.3
Interferensi ole refle!si pada selaputtipis den"an sumber caaya dian""ap
lebar
udara
udara
selaput n
a#
P # P
sumber cahayaS S
mata
a
Interference
in Thin "ilms
air
air
film
ight source
eye
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;ay 2 also travels an additional distanceof 2t before the waves recombine
"or constructive interference
2nt % m / 0 ' m % (, 1, 2 *
This ta-es into account both the difference inoptical path length for the two rays and the 1
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Bila !edua sinar dian""ap datan"nya normal$ ma!a syarat a"ar
diperoleintensitasmaksimumadala%
2 d & 'm(12 )n den"an n& n $
sein""a diperole%2 d n & 'm ( 12) den"an m &*$ 1$ 2$ ..... 'ma!simum)
Syarat untu! intensitas minimum adala%
2 d & m n den"an n& n $
sein""a diperole%
2 d n & m den"an m &1$ 2$ ..... '+inimum)
,en"an
n& Pan-an" "elomban" caaya dalam at cair
& Pan-an" "elomban" caaya di udara
n & inde!s bias at cair 'lapisan tipis)
d & tebal at cair 'lapisan tipis)
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Contoh soal 1
Sebuah selaput air n%1,>> di udara tebalnya >2( nm. 5i-a
selaput disinari oleh cahaya putih dalam arah normal, warna
apa-ah yang a-an muncul pada cahaya yang direfle-si-an?
Jawab: &(
&
(
&(
2 dn
m
m
mnm ma!simum
12
1
2
12
2 /2* 1 //
0*
' )' )' $ )
' )
3adi pan3ang gelombang yang memberi-an ma-simum dan
minimum adalah
@anya ma-simum pada m % 1 sa3a yang terleta- pada daerah
cahaya tampa- A((BC(( nm dan berwarna hi3au -e-uningan.
)'min01*
imumnmm
=
m 0(maks) 1 (min) 1(maks) 2(Min) 2(maks)
(nm) 1700 850 570 425 340
Warna (nm)
ungu 425
nila
biru 475
hijau 525kuning 575
0ranye 625
merah 675
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2.2. DifraDifractionction
First Dark
Incident
ave
P1
Po
(
B
r1r2
) bab
12
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Single Slit i!!ra"ti#nSingle Slit i!!ra"ti#n$ %he light !r#m #ne
rti#n #! the slit "aninter!ere 'ith light
!r#m an#ther rti#n$ %he resultant
intensity #n thes"reen e&ens #nthe ire"ti#n
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$ *ll the 'a+es that #riginate at the slit are in&hase
$ Wa+e 1 tra+els !arther than 'a+e 3 by anam#unt e,ual t# the &ath i!!eren"e (a-2) sin
$ .! this &ath i!!eren"e is e/a"tly hal! #! a'a+elength the t'# 'a+es "an"el ea"h #theran estru"ti+e inter!eren"e results
$ .n general destructive interference#""urs !#r asingle slit #! 'ith a 'hen sin (ark m - a
m 1 2 3
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Titi- 41, minimum pola pertama pola difra-si, memili-i
intensitas nol.
bb % a92 sin % 92
atau,
a sin =
Dengan:a % lebar celah
% pan3ang gelombang cahaya
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elah ibagi atas 4 'ilayah yang samaelah ibagi atas 4 'ilayah yang sama
Keadaan pada minimum kedua pola difraksi
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Eeadaan digambar-an di atas mengharus-an bahwa:
atau
Dengan perluasan cara di atas, rumus umum titi-
minimumdalam pola difra-si pada layar , yaitu:
F dengan m % 1, 2, >, ...
a
3 2sin =
= 2sina
ma =sin
Di se-itar titi- tengah antara dua minimum yangberdampingan terdapat titi- ma-simum.
)4'sin += ma F dengan m % (, 1, 2, >, ...
S
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4olari#ation by Selective 7bsorption
The most common techniGue for polari#ing light
Hses a material that transmits waves whose electricfield vectors in the plane parallel to a certain directionand absorbs waves whose electric field vectors areperpendicular to that direction
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The intensity of the polari#ed beamtransmitted through the second polari#ingsheet the analy#er varies as
I % Iocos2& Iois the intensity of the polari#ed wave incident on
the analy#er
This is -nown as Malus Lawand applies to anytwo polari#ing materials whose transmission aesare at an angle of & to each other DJ8 withpolari#er
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>. 4olarisasi
Cahaya yang tak terpolarisasi tidakditrasmisikan oleh pelat-pelat polarisator yangbersilangan
Pengutub
Penganalisa
P1
P2
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I = Imcos2
I % Intensitas setelah melewati plat
Im % Intensitas ma-simum
% Sudut antara -edua plat
Contoh soal 1Dua lembar pengutub mempunyai arahBarah
pemolarisasi yang se3a3ar sehingga intensitas Imdari
cahaya yang ditransmisi-an adalah ma-simum. Jelalui
sudut berapa-ah salah satu lembar tersebut harus
diputar agar supaya intensitasnya turun sebanya-
setengah?
5 b
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5awab:
8leh -arena I % (,K Im,
ma-a :
(,K Im% Im cos2
% ) AK(
F 1>K(
fe- yang sama a-an didapat-an ta- peduli lembarmana-ah yang diputar-an atau di dalam arah mana
pun lembar tersebut diputar-an.
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