Giáo trình vật lý đại cương phần quang học ths nguyễn hoàng tuyến, ths nguyễn thị ngọc vân

Trong trifdng hop tong quat, xet anh sang truyen trong mot moi trufrng khong dong tinh, gia suf tai thdi diem tx dnh sang truyen den mat , tai thcfi diem t2 anh sang truyen tdi mat S2 th

Nguyen Homing Tuyen - Nguyen Thi Ngoc Van

TM&G DaI HQCeiSUFOlU LOW

Trttfng Dai hoc Dan lap Cftu Long muon khang dinh la trung tarn dao tao nguon nhan lite co trinh do cao cho khu vile dong bang song Cifu Long de phuc vu cho cong cuoc cong nghiep hoa, hien dai hda dat nifdc thi phai nang cao chat lifcrng dao tao toan dien Mot trong nhtfng yeu to trong yeu la doi ngu thay co, he thong cac giao trinh va trang thiet bi day hoc.Cung vdri cdc nganh cdc cap trong toan quoc dang day nhanh tien do xay difng va cung co vi the trong xu the hoi nhap, tnrcrng Dai hoc Dan lap Cdu Long dan thng biidc hoan thien de difctag dau vdi nhtfng thach thtfc do, gop phan difa nen giao due dai hoc Viet Nam dtfng vifrig, ngang tarn khu vifc va the gidi.

Ke hoach xay difng ve mot Bo giao trinh suf dung cho cac giang vien lam tai lieu co ban de giang day, cho sinh vien nghien cdu tham khao hoc tap la nhu cau cap bach cua nha trifefng

An pham nay la giao trinh Vat Ly Dai Cifcfng - Phan Quang Hoc cua Khoa Khoa Hoc Ccf Ban, Tnfbng Dai hoc Dan lap Cufu Long, do ThS Nguyen Hoang Tuyen va ThS Nguydn Thi Ngoc Van bien soan

Lanh dao triferng Dai hoc Dan lap Cdu Long tran trong nhffng cong hien, dong gop cua Quy thay co mefi giang cung nhif cac thay co can bo - giang vien cP htfu cua nha trifefng

ThS.Nguyen Cao Dat

trong nha trufrng.

Tran trong gidi thieu giao trinh nay den Quy thay co va cac em sinh vien trildng Dai hoc Dan lap Cdu Long.

Vinh Long, thang 09 nam 2009

Q HIEU TRtfdNG

QUANG HOC

Quang hoc 1& mot phan cua vat ly hoc chuyen nghien ctfu

ve dnh sdng sir tifong tdc cua dnh sdng vdi cdc chat

• Vao cuoi the ky XVII, Newton diTa vao ti'nh truyen thang cua anh sang da dtfa ra thuyet hat don gian ve anh sang

Sy phan xa va khuc xa anh sdng diTtfc thuyet nay giai thich bang quan diem cd hoc.

• Cung thcri gian ay, Huygens lai difa ra thuyet song anh sdng nhSm giai thich cac hien tiTc/ng giao thoa, nhiSu xa va phan cite ma thuyet hat cua Newton khong giai thich noi

• Den nam 1865, Maxwell da chilng minh rang cac triTcrng dien tu" bien thien lan truyen trong khong gian vdi van toe anh sdng Nhif vay, ong da xac nhan anh sang la song dien tor - c6 birdc song ter 0,40pm den 0,76pm

Thuyet song dien tir ve anh sang da cho phep giai thich tot cac hien tirqng mang tinh chat song cua anh sdng (giao thoa, nhidu xa, phan cifc, tan sSc) Tuy nhien, thuyet nay khong the giai thich diTOc hieu ting quang di$n, sir tan xa Compton, trong do nhiing khia canh hat cua anh sang diTOc the hien

• Vao the ky XIX va dau the ky XX, xuat hien khai niem hat anh sang : anh sdng gom mot dong hat goi la photon lan truyen theo cac dinh luat lan truyen cua song dien tir Do

la thuyet liTOng td anh sang cua Einstein Theo do, anh sang phat ra hay hap thu nhiing photon, day la nhiing liiong gian doan md nang liiqng cua chung phu thuoc vao tan so” Nhii vay theo thuyet liiong td ve anh sang, anh sdng vda co tinh song vda co tinh hat, goi la liidng tinh song - hat

- Trong mot so' hien tirong nhii giao thoa, nhieu xa, phan ciic, anh sdng the hien tinh chat song

- Trong cac hien tiiOng khac nhii hi$u dng quang dien, hieu dng Compton anh sang lai the hien tinh hat

Chitcfng 1

nhau, nghia Id tdc dung cua mot chum sang nay khong phu

moi

i = i'

QUANG HOC SONG

3 ;

Descartes

Khi tia sang SI tdi mat phan each (£) gitfa hai trtfcfng trong suot, dong chat va ding hifdng thi no bi tach thanh hai tia: tia phan xa IR1 va tia khuc xa IR2 Hai tia nay tuan theo hai dinh luat sau day :

a) Dinh luat phan xp anh sang

Tia phan xa ndm trong mdt phdng tai va goc tai bang goc phan xa

I NHUNG CO Sd CUA QUANG HINH HOC

Quang hinh hoc la mon hoc nghien cufu cac hien tiTcfng anh sang difa tren quan niem ve tinh truyen thing cua dnh sdng No dat cd sd tren nhang dinh luat sau :

1 Dinh luat truyen thang ciia anh sang

Trong moi truang trong sudt, dong chdt vd dang hudng, anh sang truyen di theo ditdng thang.

PhiWng truyen cua dnh sang goi la tia sdng M6i chum sang chtfa vo so" tia sang

2 Dinh luat ve tac dung dpc lap cua tia sang

Tdc dung cua cac chum sang khde nhau Id doc lap ddi vdi nhau, nghia la tdc dung cua mot chum sang nay khong phu thuoc vdo su co mdt hay khong cua cac chum sang khde.

Dinh luat phan xa va khuc xa anh sang ciia

Ly thuyet va thilc nghiem chtfng to : n2i = —,

N

b) Dinh luat khuc xq anh sang

Tia khuc xq ncim trong mat phdng tai va tl so giUa sin goc tai va sin goc khuc xq la mot hang sd'.

Ri

R2

(2)sin i

-7 - - n21

sinr

• (S) la mat phan edeh 2 moi trifcrng

• IN 1 (E), IN la phdp tuyen cua (Z) ■

• Mat phdng (SI, IN) goi la mat phdng tdi

• Hang so” n2i goi la chiet suat ty doi cua moi trifdng (2) ddi ¥di moi trufrng (1) 1121 phu thuoc ¥ao ban chat cua hai moi trifbng nay ¥a mau sac cua tia tdi SI

DSc biet doi vdi khong khi v^ « c nen n^ « 1.

Goi n],n2 la chiet suat ty dol cua moi trirdng (1), moi trtfcfng (2) doi vdi chan khong, tdc chung la cac chiet suat tuyet doi cua cac moi tru’dng nay

n21 = — = — : — = —v2 v2 vx

Neu moi trifdng (1) la khong khi nj « 1 => n21 a n2

Vay chiet suat tuyet doi cua mot moi triTcfng cung diftfc coi

la chiet suat ty doi cua moi trifdng d6 doi vdi khong khi

The (2) vao (1), ta dutfc :

sin i _ n2 sin r n}

Quy udc ndi tat : nj la chiet suat moi trufrng tdi, n2 la chiet suat moi trdcrng khuc xa

AB ma anh sang di duttc trong moi treferng ay

Ky hieu quang lo : L hay [AB]

L = [AB] = nl

A

Bdl

Neu anh sang lan Ihtft di qua nhieu moi truhng dong chat

co chiet suat , n2, n3, tren cac doan dhbng thcfng hng

AM, MN, NO, co do dhi lan Ihqt la , 12, I3, thi quang

lo tren doan dhbng tong cong la :

[AMNO ] = [AM] + [MN] + [NO] +

L = n^j + n2l2 + n313 +

L = Xni1i

AKhi do :

L= ndlJa

Y nghia cua quang 16 :

rB q fl

L= I -dl = c [

Ja v J/

Trong trifdng hop tong quat, xet anh sang truyen trong mot moi trufrng khong dong tinh, gia suf tai thdi diem tx dnh sang truyen den mat , tai thcfi diem t2 anh sang truyen tdi mat S2 thi Zi,S2 chinh la cac mat song hinh hoc Ta thay ro

t la thdi gian dnh sdng di dirqc doan dadng dl

Vay quang 16 L cua dnh sang tren doan cung AB trong mot moi trifdng la doan difcfng ma anh sang se di dirqc neu nhtf

no truyen qua chan khong trong cung khoang thdi gian r ma anh sang da dung de di qua moi triTcfng do

b) Mat song hinh hoc

Mat song hinh hoc cua mot chum tia sdng la tap hop nhtfng diem ma anh sang cua chum tia do truyen den tai ciing mot thdi diem

Trong moi trildng trong suot, dong chat va dang hutfng anh sdng truyen di theo moi phuOng vdi cung van toe Do do, neu nguon diem phat sang, thi :

■ 6 gan nguon, cac mat song hinh hoc la nhifrig mat cau

dong tarn, co tarn la nguon

■ O rat xa nguon, cdc mat song cau gan nhu" la nhifrig mat phang song song - vuong goc vdi tia sang

Mat song

hinh hoc

ly Fermat Noi dung cua nguyen ly nay nhu" sau :

Gina hai diem A vd B, dnh sang se truy&n di theo con dttdng ndo md quang Iq la cite tri (ate dqi, cite tieu hay khdng ddi).

Trong trifefng hop anh sang truyen tCr moi triTOng nay sang moi trufrng khac, nguyen ly Fermat dan tdi dinh luat phan xa va khuc xa anh sang

^2

• M6i dnh sang ting vdi mot gia tri xdc dinh cua butfc song X trong chan khong va co mot man s£c rieng biet goi la anh sang dcfn sdc

II NHUNG CG sd CUA QUANG HOC SONG

Quang hoc song la mon hoc nghien ctfu cac hien tiftfng anh sang dya tren quan niem ve ban chat song cua anh sang

Co sd cua quang hoc song la :

■ Thuyet dien tif anh sang cua Maxwell

■ Nhtfng nguyen ly ccr ban : nguyen ly chdng chat song, nguyen ly Huygens- Fresnel

1 Thuyet di£n tu* anh sang cua Maxwell

Theo thuyet dien th anh sang, dnh sang thay done la nhttng song dien tif co butfc song nSm trong khoang tir 0,40}.im den 0,76|xm va truyen trong chan khong vdi van tdc 3 x 108 m / s

• Anh sang la loai song ngang

• Khi truyen den mat, vectcf cirbng do dien trirdng E cua song anh sang tac dung len te bao than kinh trong vong mac

va gay nen cam gidc sang Tir do, vecto E cua song sang goi la vecto song sang

• Khi mo ta song anh sang, ta chi can bieu didn dao dong cua vecto E theo phiiong vuong goc vdi phitong truyen v cua nd

(= OM), phircfng trinh

x = acos(o(t - t) ,trong do

Cirhng do sang tai mot diem la dai li/Ong co tri so bang

nang liTOng truyen qua mot dcfn vi dien tich dat vuong goc vdi

phirong truyen sang trong mot dcfn vi thcfi gian

Ctfcfng do sang dac trong cho do sang tai mot diem

27tLX

27tLX

x = a cos co(t + t) hay x = a cos cot +

2tiLXNeu anh sang truyen theo chieu nguttc lai , ham song cua

dnh sang tai M se co dang :

t la thcfi gian anh sang truyen tut 0 den M

T ’ "

2 Ham song ciia anh sang

Neu tai 0 phifcfng trinh dao dong sang la :

x0 = a cos(cot)

Tai diem M each O mot doan r

dao dong sang se la :

vdi L la quang Id cua doan difcfng OM, T la chu ky dao dong cua song, X la butfc song anh sang trong chan khong

271 L 2tiL

cot = -= -

T c X

I = a2

4 Nguyen ly chong chat song sang

Tai giao diem cua hai hay nhieu song sang, cdc song thdnh phdn khong bi cdc song khdc lam nhiZu loan Sau khi giao nhau cdc song sang udn tiep tuc truyen di nhd cu Con tai diem giao nhau, vecta song sang tong hap bdng tong cdc veca song sang cua nhUng song thdnh phdn giao nhau.

Nguyen ly nay ditoc nghiem dung khi khao sat cac hien tifo'ng giao thoa va nhieu xa

5 Nguyen ly Huygens - Fresnel

Xet song sang th mot ngudn sang phat ra va truyen di khap moi trhdng Nguyen ly nay giai thich co che cua sif truyen sang nhif sau:

O Moi diem cua moi trifcrng cd song sang truyen tdi deu

cd the xem la mot ngudn sang thtf cap phdt ra nhiing song

sang gbi ve phia trifdc nd.

© Ngudn sang thtf cap cd bien do va pha dao dong la bien

dp va pha cua dao dong do ngudn sang thifc gay ra tai vi tri cua ngudn sdng thtf cap do

© Muon xac dinh dao dong sang do ngudn sang thoc S gay ra tai mot diem M bat ky, ta thay ngudn sang thifc S bang

Cobng do sang I tai mot diem ty le vdi binh phifOng bien do dao dong sdng tai diem do

I~a2 =>I = ka2,

S />

Chitong 2

SU GIAO THOA ANH SANG

I 7U |x2 - axcos cot - 2 — L2

1 Stf giao thoa anh sang

Hien tirqng hai hoac nhieu song anh sang giao nhau gay nen trong khong gian nhtfng mien sang va nhilng mien toi goi

la hien tuVng giao thoa anh sang Mien khong gian co sir giao thoa anh sang goi la triThng giao thoa

Ly thuyet va there nghiem cho thay chi co cac song anh sang ket hop mdi tao ra dirqc hien tirqng giao thoa

2 Dao dqng ket hdp va dao dpng khong ket hqfp

Xet hai nguon dao dong sang Oj;, O2 ciing tan so" va xay

ra ciing phiTOng

Phirong trinh dao dong sang cua 2 nguon nay la :

xCOi) = a! cos(cot)x(O2) = a2 cos(cot)

Tai M se nhan dirqc dong thd’i hai dao dong sang th hai nguon go'i tdi veri cac ham song titong ling la :

trong do : «

vi

Nhu' vay khong co hien tifOng giao thoa

Trircfng hop 2 : Hieu pha A(p = - (p2 khong bien doitheo thdi gian

Co the xay ra hai trObng hop :

Trifdng hop 1 : Hieu pha Acp = (pj - cp2 bien doi bat ky theo tho’i gian Khi do cosCcp! - (p2) lay moi gid tri bat ky tif -1 den 1, bien do a cung bien thien theo thcri gian t (rat nhanh), tufc la cObng do sang tai M cung bien thien theo t, no co tri trung binh nhif sau :

I = lx + I2 vi cosCcp! - (p2) = 0

vdi : lx = a^ , I2 = a2 la cac cifbng do sang do song sang truyen tu" O| va O2 den M

TR'JIjNCTHOyito

a = yjal +3-2 + 2a1a2 cos^ - cp2)

tg(p = ai sin(Pi+a2 sin(p2

ai cos (pi + a2 cos <p2

trong d6 AL = L2 - Lx goi la hieu quang 16 cua hai song sang tai M.

Khi do bien do a va cifbng do sang I tai M co gia tri Loan to&n xac dinh Tai nhffng diem khac nhau, a va I se 16n hay be toy thuoc gia tri A<p Do do, trong khong gian se co nhtfng diem sang hoac tdi, nghia la co hien tifo’ng giao thoa anh sang xay ra

6 Ketluan :

Dieu kien cdn vd du de co giao thoa anh sang la hai song sang giao nhau phdi co cung tdn so, cung phitung doo dong vd

cd hieu pha ban ddu khong ddi theo that gian.

Cac song sdng co cung tan so' va hieu pha ban dau khong do’i theo thbi gian duVc goi la s6ng ket hop Nhttng nguon phat

ra cac song ket hop goi la nhtfng nguon ket hap

Nhif vay, hien tiTOng giao thoa anh sang xay ra khi cac song sang phat ra tif cac nguon ke't hop va c6 cung phifOng dao dong

3 Cach tao ra hai song ket hprp

Ta biet rang khong co mot nguon sang thong thirhng nao

co the phat ra song anh sang hoan toan don sac, cac song ay

co bien do va pha luon luon thay doi Sd di nhif vay la vi nguyen td chi phat xa trong mot khoang thbi gian ngan chifng

10 8s Do do, moi lan phat xa, moi nguyen td phat ra mot xung song ngan lan truyen co dang sin Mdi xung sang nay difOc goi la mot doan song Do dai cua doan song diTOc xac dinh bdi thbi gian phat xa t cua nguyen td

Bien do va pha cua cac doan song do mot nguyen td phdt

ra td lan phdt xa nay sang lan phat xa khac, cung nhd do cdc nguyen td khdc nhau phat ra trong mot lan phat xa, co the rat khac nhau, khong co lien he gi vdi nhau, nghia la cac pha ban

Lx;.; -• ■ ■ ■’ ■

S

(E)(Hinh 1)

^2 |(P)

dau cpi va (p2 cua hai song sang luon luon thay doi va co moi gia tri bat ky Do do cufrng do sdng tong hop cung thay doi rat nhanh mot each hon loan den noi khong mot may thu dnh sang nao du la nhay nhat lai c6 the ghi nhan diTOc nhtfng trang thai tile thefi nay cua cuftng do sdng

Su" thay doi hon loan cua nhtfng do sang chdp nhoang nhil the tai mot diem khong gian cung khong diftfc mSt thu cam, nhifng gay ra cam gidc ve quang thong deu, khong thay doi theo thdi gian Song sang do cac nguon phat ra rieng biet la cac song khong ket hop

The nhtfng, neu ta dung mot thiet bi quang hoc de tach song phat ra til mot nguon sang thanh hai song di theo nhilng dildng truyen khac nhau, roi sau do lai cho chung gap nhau thi hieu so' pha cua hai song ay se khong phu thuoc thbi gian va chung lai co cung tan so' la tan so' cua nguon sang ban dau Luc

do, hai song nay la hai song ket hop

Nhif vay, nguyen t£c tao ra hai song ket hop la til mot song duy nhat tach ra thanh hai song rieng biet VI du de tao

ra cac song ket hop, ta co the dung cac thiet bi sau day :

a) Khe Young

Khe Young la mot dung cu gom mot nguon sang S dat trifde mot man chan sang (P) co due hai 16 tron S]^ va S2 gan nhau Sau (P) dat mot man quan sat (E) (Hinh 1)

Sj , S2 la hai nguon ket hop, chung phat ra hai song ket hop.

De hinh anh giao thoa ro net, ngifcfi ta thirhng thay nguon diem 8 bang nguon khe hep va cdc 16 8! va S2 cung diTOc thay bang cac khe hep song song vdi khe nguon (cac khe nay vuong goc vol mat phang hinh ve)

b) Giicfng Fresnel

Day la dung cu gdm hai gOOng phang G-l , G2 dat nghieng vdi nhau mot goc a rat nhd (chiing vai phan ngan radian)

Mot nguon sang 8 dat trade hai gOOng se cho hai anh ao

Si, S2 kha gan nhau Hai chum sang xuat phat th 8 phan xa tren hai gOOng va dap len man quan sat (E) Hai chum tia phan xa nay taa nha daqc xuat phat td hai nguon sang ao Sx, S2 ddn xuat ttf mot nguon sang 8, nen chung la hai chiim sang ket kqp 8!, S2 la hai nguon sang ket hop (hinh 2)

Tren man (E), tai mien giao nhau cua cac tia phan xa th hai gaong ta quan sat daqc cac van sdng giao thoa

Man (Q) ngan tia sang trqc tiep th nguon 8 dap len man quan sat (E)

L! quang 16 tren doan dufrng ij (= S1M),

M

x0

trong do :

(E)Dao dong tong hop tai M co phuOng trinh :

x = a cos(cot + <p)

a = 'Ja^ + a2 + 2a^a2 cos((pi — (p2) ;

rl

r2 Si

L2 la quang 16 tren doan dircrng r2 (= S2M).

Dat : (p! =Z|^L1

x(S1) = aj cos(cot), x(S2) = a2 cos(cot).

1 Vi tri cac ctfc dai va cu’c tieu giao thoa

Tai diem M tren man (E) trong giao thoa trifcrng, c6 hai dao dong sang nhan tCr hai ngudn vdi cac ham song

va :

(1)

Vay, nhu'ng diem co cufrng do sang ciTc tieu (toi nhat) la nhtfng diem ma hieu quang lo cua hai song sang giao nhau bang mot so nguyen le lan nda bildc song Nhtfng diem nay goi

la cac cite tieu giao thoa

amin

^I = a2 =(a1-a2)2 =Irmn

Khi do : —AL = 2k7i => AL = kX

Vay, nhffng diem co ciTdng do sang cifc dai (sang nhat) la nhu’ng diem ma hieu quang lo cua hai song sang giao nhau bang mot sb nguyen lan birdc song anh sang Nhu’ng diem nay goi la cac cifc dai giao thoa

tg(p = al sincpj! +a2 sin(p2

aj cos epi + a2 cos <p2

Gid tri cua bien do a diftfc xdc dinh bdi hieu sb pha :

A2

(k = 0, ±1, ±2, ±,3, )

r 2 - r l =

2 Hinh dang cac van giao thoa

Gia stf bai todn giao thoa tren xay ra trong moi trufbng khong khf (chiet suat n = 1) Khi do, dieu kien cifc dai va cifc tieu giao thoa (1) va (2) difOc viet lai nhuf sau :

• M co cifc dai giao thoa khi :

■ Quy tich cac cifc dai giao thoa la mot ho hyperboloid

So, Xi, Xi, X2, X2, - (tfng vdi k = 0,1,-1,2,-2, ) cd hai tieu diem la Si, S2 va true dd'i xdng la difdng thang SiS2

Dac biet, mat Xq (^ng vdik = 0) la mat phing trung trife cua doan SiS2

■ Quy tfch cac cifc tieu giao thoa cung la mot ho hyperboloid xen ke vdi hp mat tren

De quan sat hinh anh giao thoa, ta dat man (E) song song vdi SiS2 Giao tuyen gitfa man (E) vdi hai hp hyperboloid

la cac difdng hyperbol sang toi, xen ke va doi xtfng nhau qua mot van sdng chinh gitta, do la giao tuyen cua (E) vdi mat phing Xo ■

den (E) (hinh ve).

3 Vi tri cac van giao thoa

x = OM, D la khoang each tu” cac nguon

0 /

>0Sir

be nen cac van giao thoa difpc xem la cac doan thang song song

va each deu nhau

^2

Trong cong thde (3):

• Vdi k = 0 -> xs = 0 (tai O), ta co van sang trung tarn

• Vdi k = ±1, ta cd van sang bac 1

• Vdi k = ±2, ta cd van sang bac 2

Td (3) va (4) ta thay cac van sdng, van toi ndm xen ke

va edeh deu nhau

Khoang van i la khoang edeh gitfa hai van sang (hoac hai van toi) lien tiep :

Cac van sang each diem gitta 0 khoang each

man dieu kien:

-> r2 - rf = 2xa -> r2 - ^ = — —a — x

rl+r2

1

2a

D

4 Giao thoa vdri anh sang tr&ng

Vi i ty le vdi X, nen khi Sr, S2 phat ra anh sang trang (X = 0,40pm - 0,76|j.m) thi mdi anh sang dcfn sac trong anh sang trang se cho mot he van giao thoa cd mau sSc khac nhau

• Tai O, xs = 0 la van trung tam cua moi he van don sac, ket qua la tai O ta cd mot van sdng trdng trung tam

• O hai ben O, cac van sang dcfn sSc khdng trung kbit vdi

nhau, chung tao thanh van mau Nhiing van mau khac nhau tfng vdi mot gid tri cua so' nguyen k deu cd bb trong vien tim,

bb ngoai vien do Gang xa O, cdc van mau cang chong chat len nhau gay nen mau bang bac

III GIAO THOA ANH SANG VClI NGUON SANG BONG

Trong each bo' tri there nghiem d tren, ta deu dung hoac mot ngubn sang diem hoac mot khe hep di/pc chieu sang manh Vdi Idle thi do rong khdng deftfe Idn qua mot gidi han xac dinh Khi do, cd dat man quan sat (E) trong giao thoa triThng la ta nhan dilpc he thd'ng van He thong van nhif the gpi la van

trdbng giao thoa

Mdc du ngudn sang cd cdbng do manh nhitag he van tren van khdng dirge sang De dirqc he van kha sang vdi cac ngudn sang khdng manh, ta cd the dung ngudn sang rdng Khi do man quan sdt (E) phai dat tai mot so' diem nhat dinh, va hai

Ket hop (3), (4) va (5), ta ddde : x.s = ki; xt =

e

Tia tdi SI to'i mat tren cua ban cho 2 tia :

* Tia phan xa IR

* Tia khuc xa IJ den mat dutfi cua ban phan xa thanh tia

JK vd khuc xa qua mat tren cua ban thanh tia KRi

D£ dang thay rang hai tia IR va KR! song song nhau Do

do chung gap nhau cf vo ctfc Hieu quang lo AL giffa hai tia IR

Ta se xet hai loai van dinh xtf quan trong hay xay ra trong thifc te la van dong do nghieng va van dong do day

1 Van cung dp nghieng

Trirdc mot nguon sdng rong co dat mot ban mong trong sudt, hai mat song song, be day e (e cd vai phan tram mm , vi

du nhu” mang mong xa phong, vang dau xang tren mat nifdc, )

Vi vay van cua ban hai mat song song duttc goi la vdn

ciing do nghieng va co the quan sdt vdi nguon sang rong

Muon quan sat dirqc van giao thoa nay, ngirdi ta dung mot thau kinh hoi tu dat song song vdi ban mong, khi do cac chum sang thi ban dutfi cung goc tdi i se cho cap tia giao thoa nfim xung quanh true chinh cua thau kinh, chung hoi tu tai cac diem nam tren mot dufrng tron co tarn tai tieu diem F cua thau kinh Ctrdng do sang tai moi diem tren difcrng tron do deu nhif nhau

vi cung cd mot gia tri cua AL Dat man quan sat (E) song song vdi thau kinh va chtfa tieu cif F ta se difcrc cac van tron sang tdi xen ke dong tarn F

Thifc nghiem cho ta thay khi tia sang phan xa tren mat gidi han cua mot moi trifdng chiet quang manh hefn moi truftng tdi thi quang lo cua tia se tang them nda bifdc song (X/2)

(E) F M

V

JNeu nhQng chum tia tdi la anh sang trang, thi he van giao thoa cung do nghieng se la mot he thong van nhieu mau

2 Van cung dp day

Ban mo ng trong suot cd hai mat song song chi la trifdng hap rieng va thabng diftfc tao ra vdi muc dich ky thuat nao do Hau het cac ban mdng trong suot thiTbng gap trong thifc te deu

cd be day thay doi tCr diem no sang diem kia Khi chieu anh sang th nguon sang rong len chung, ta khdng cd van giao thoa cung do nghieng, ma lai cd mot loai van khdc, do la van giao thoa cung do day

a)Vdn nem

Nem la Idp moi trifd'ng trong suot gidi han bdi hai mat phSng (Gj), (G2) tao vdi nhau mot gdc A rat nhd (chtrng vai phut)

Xet mot chum tia sang dan sac song song tdi roi vudng gdc vdi mat dirdi (G2) cua nem khdng khi (chiet suat n « 1).Tia tdi SI tren mat (Gj), se bi tdch thanh hai tia :

I khong khf

quang manh hon moi trifdng khong khf cua nem

• e la do day cua nem tai diem I

Hieu quang 16 cua cdp tia IR^ va IKIR2

AL = [IKI] + - = 2KI + -

* Tia phan xa IR!

* Tia khuc xa IKI'R2 va 16 trd lai moi trtfcrng tdi ban dau

Cap tia IRi va IKTR2 la hai tia ket hop

Ri \S

b) Van iron Newton

Xet mot thau kfnh phang loi T, mat Idi co ban kinh cong

R dat ap len mot tarn kinh phang P Ldp khong khi rat mong nam gitfa T va P tac dung nhii mot bdn khong khi mong

cd do day e thay doi Nd cung gidng nhii mot nem khong khi, nhOng mat tren cua ban la mot mat cau (hinh 1)

▼ Cac van sang tai cac diem tdi cd do day es (cua nem) thoa dieu kien:

12

r H

P0

Hieu quang 16 cua cap tia giao thoa dirqc tinh :

Tiep diem O la van to'i ting vdi k = 0

Ta co : HM2 = OH.OH'; 00' = 2OC (dtfdng kinh mat cau cua T) DSt p = HM

es=ik 41’(k=1’2> - )

e

P2 =e.(2R-e)

pI = 2Rk— -> p = Vk.^RX

2 BAI TAP CHVONG 2

1 Mot nguon sdng don sac phat ra dnh sdng co bode sdng

A, = 0,6pm Chieu anh sang tren v&o hai khe hep song song edeh nhau 1 = 1 mm va edeh deu nguon sdng Tren man anh dat song song va each mat phang chda hai khe mot doan

D = 1 m, ta thu difOc mot he thong van giao thoa

a/ Tinh khoang van neu toan bo he thong dat trong khong khi

b/ Xdc dinh vi tri cua ba van tdi dau tien

c/ Dat triidc mot trong hai Idle mot ban mong phang trong suot co hai mat song song, do day e = 12 pm va co chiet suat

n = 1,5 Khi do he van giao thoa co gi thay doi? Xdc dinh do dich chuyen cua he van giao thoa

d/ Neu khong dat ban mong ma lai do vao khoang gitfa man anh va mat ph&ng chtfa hai khe mot chat Idng thi ngiicfi

p2 = 2 Re,Gia sir M thuoc van tdi thu” k cd do day e, khi do :

A 1A

AL = 2ne cos r + — = k + — 2 i 1 A,

2

ta thay be rong cua moi van giao thoa la i' = 0,45 mm Tmh chiet suat cua chat long.

2 Cho mot liTOng lang kmh Fresnel, gom hai lang klnh giong nhau, cac day diftfc ddn vdi nhau bang mot chat nhifa trong suot M6i l&ng kinh co goc chiet quang A = 1° va co chiet suat

n = 1,5 Tnrdc luf&ng lang kinh, co dat mot khe sang hep S song song vdi dufcfng canh cua cdc iSng kinh va nam trong mat phang chda day cua cac lang kinh Khoang each ttf khe sang S den lifdng lang kinh la di = 20cm Cdch luftng lang kinh mot khoang d2 = 6m dat mot man anh (P) vuong goc vdi true doi xdng cua he thong Day cua lang kinh co be day khong dang ke

a/ Chdng minh rang li/Ong lang kinh Fresnel tiTOng diTOng vdi he thong khe Young Ve mien giao thoa va tinh be rong cua

3 Cho mot thau kinh hoi tu L, tieu cif f = 50cm, khau do cd ban kinh R = 3cm Cach thau kinh mot doan d = 75cm ngifcfi

ta dat mot Idle sang thing ddng S Anh sang do khe phat ra

cd bifdc song A = 0,5pm Thau kinh nay duVc cufa doc theo true chinh thanh hai nda thau kinh L}, L2 roi tach chung ra thanh mot khe hd thing dtfng song song vdi khe sdng S va cd

be rong a = 1mm He thong nay diftfc goi lifting thau kinh Billet

//////////////

>

5cm

6 Hai nguon ket hap Si, S2 dat each nhau mot khoang

d = 2mm phdt ra anh sdng defn s^c co bifdc sdng X = 0,5pm Khoang each t€r man quan sat (E) den nguon ket hop la

5 Mot giXOng phang dai 5cm diTOc chieu sang bang tia don sac

co babe song X = 0,54pm tif mot Idle hep S dat song song vdi

canh gilcrng, each gifo’ng 0,1cm va xa canh gifting mot doan 5cm (hinh ve) Xac dinh khoang each van tren man dat xa khe

mot doan 120cm va vuong goc vdi gifting Tmh be rong cua giao

thoa triftlng He quang hoc nay goi la gifting Lloyd

hai gifting la r = 0,10m Anh sang phan xa tren hai gifting Gj^, G2 tdi giao nhau va cho anh giao thoa tren man anh (E) dat each giao tuyen cua chung mot khoang D = 1,5m Hay xac dinh khoang edeh van tren man (E), biet anh sang cd biftlc sdng

X = 0,5pm

a/ Cach lifting thau kinh mot doan bdng s, cd man (P) quan sdt vuong gdc vdi chum tia sang phat ra til lifting thau kinh Chtfng minh rdng lifting thau kinh tren lifting diftlng vdi

he thong khe Young Bat dau tb gid tri s0 nao cua s ta cd the quan sat diftlc cdc van giao thoa tren man (P) ?

b/ Tim sif phu thuoc cua khoang van i vao khoang each

s Tinh gid tri cua i khi s = 3m

c/ Vdi s = 3m, tong so- van sdng tren man quan sat bang bao nhieu?

D = 2m Diem A nSm tren man sao cho tia SjA vuong goo vdi man (hinh ve)

a/ Tai diem A tren m&n se quan sat ditoc van tdi hay van sdng?

b/ Neu bay gib dat mot ban mat song song bang thuy tinh

co do day e = 10,5^m vuong goc vdi tia S2A thi tai diem A se quan sdt dirqc van toi hay van sdng ?

7 Trong thi nghiem Young ta dat ban thuy tinh that mong, hai mat song song tren dufdng truyen cua mot trong hai tia giao thoa, sao cho tia sang vuong goc vdi hai mat song song Khi do van sang trung tarn dich chuyen den vi tri cua van sang thtf nam ban dau Cho biet chiet suat cua thuy tinh la n = 1,5 bifdc song anh sang dtfqc dung X = 6xl0-5cm Hay tinh do day e cua ban

8 Trong thi nghiem giao thoa vdi gtfcfng Lloyd (tiTOng tq nhu” bai 5) song anh sang tdi trifc tiep tif ngudn S (khe hep) giao thoa vdi song phan xa tren gircfng phing M Ket qua 1& mot he van giao thoa di/qc tao ra tren man anh (E) Khoang each tif nguon den man anh la 1 = 100cm Ung vdi mot vi tri nao do cua nguon, be rong cua van giao thoa tren man anh la

i = 0,25mm, con sau khi nguon dich ra xa mat giTcfng doan

n

10 Chieu mot chiim tia sang dem sac, song song va thang goc vdi mat dildi cua mot nem khong khl Anh sdng tori co bifdc song X = 0,6pm Tim goc nghieng cua nem, biet r&ng tren 1cm cua mat nem co 10 van giao thoa

11 Chieu mot chum tia dem sfic song song va thang goc vdi ban thuy tinh phang Idi cua mot he thong cho van tron Newton Dufdng kfnh cua van toi thd tit do dilcfc b&ng d = 9mm, (coi tarn cua he thong la van tdi bac 0)

Tim bifdc song cua anh sang tdi, biet rang ban kfnh mat Idi cua thau kinh R = 8,6m, gitfa thau kinh va ban thuy tinh la khong khf

12 Chieu mot chum tia sang dem sac vuong goc vdi ban cho van tron Newton va quan sat anh sang phan chieu Bdn kinh cua hai van tdi lien tiep lan hint bang 4,00mm va 4,38mm, ban kfnh cong cua thau kinh bang 6,4m Tim so thd tif cua cac van toi tren va bifdc song cua anh sdng tdi

Ay = 0,60mm thi be rong cua van giam di N = 1,5 lan Tim budc song cua anh sang

9 Tren mot ban thuy tinh phang (chiet suat n = 1,5), ngildi ta phu mot mang mdng cd chiet suat n' = 1,4 Mot chum tia sang song song vdi A = 0,6pm diitfc chieu thing goc vdi mat ban Tinh be day cua mang mdng, biet ring do hien titong giao thoa, chum tia phan chieu cd do sang cifc tieu

Chitong 3

SU NHlfiU XA ANH SANG

A(P)

0

B(E) Hinh 2

Hinh

A A-B,

’ B(E)

1 Thi nghi^m

a) Thi nghiem 1

Cho anh sang xuat phat tif mot nguon sang diem 0 truyen qua 10 tron rat nho trade man chan (P) (hinh 1) Ta thay tren man anh (E) xuat hien nhieu van tron sang va td'i cung tarn diem, nam xen ke nhau or ca trong vung toi hinh hoc (vung tod hinh hoc la vung nam ngoai (AB))

Ket qua nay cho thay dnh sang da truyen ca den nhilng diem A', B' n&m ngoai hinh tron sang hinh hoc (AB), nghia

la, cac tia sang da bi lech khdi phifcfng truyen thing

Hien tuqng giao thoa anh sang da bieu hien ro ndt tfnh song cua anh sang Tuy nhien, de thuyet song that hoan chinh,

nd con phai giai thich dirge cac dinh luat cP ban cua quang hoc

ve sit truyen thing, sir phan xa va sir khuc xa anh sang Chinh trong hien tiTOng nhidu xa inh sang, quang hoc song da tim thay mot nguyen ly co ban cua minh, do la nguyen ly Huygens

- Fresnel, ma tir do cho thay quang hinh chi la trirdng hop rieng cua quang song.

I HIEN TUONG NHlfiu XA ANH SANG

doan day K

b) Thi nghiem 2

DSt mot doan day kim loai K rat manh song song vdi mot khe sdng hep F D&t man (E) phia sau doan day de quan

sat bong cua nd (hinh 2) Ta thay trong vung toi hinh hoc (AB)

tren man (E) xuat hien cac van toi va van sdng xen ke nhau

Cac van nay la cac doan thang song song vdi doan day K

Nhif vay, trong thi nghiem nay anh sang cung khong

truyen thang

2 Giai thich

Hien titong cac tia sang bi lech khdi phifcrng truyen thing khi chung di gan cdc vat can, gay nen cac van sang va van toi trong ca vung toi hinh hoc ditqc goi la hien tittmg nhi&u xa dnh sang Cac van sdng va van toi nay goi la vdn nhilu xa.

Muon giai thich hien tuqng nhiiu xa anh sang, ta phai

van dung nguyen ly Huygens - Fresnel Theo nguyen ly nay, mdi diem cua 16 tron (thi nghiem 1) hocic moi diem tren be m£t cua doan day K (thi nghiem 2) nhan diftfc anh sdng tuf ngudn sang gdi tdi difpc xem la ngudn sang thtf cap phat ra nhilng

song sang truyen den diem M bat ky tren man anh (E) Dao dong sang tai M se la tong hop nhtfng dao dong sang do cac

ngudn thuf cap goi tdi diem M Tuy theo hieu pha gitfa cac dao

dong song nay, tai diem M co the sang hoac toi

3 Bieu thtfc giai tich cua nguyen ly Huygens - Fresnel

De tinh bien do va pha cua cac ngudn thd cap, ta co bd de

Fresnel nhu sau:

Bien do va pha cua ngudn thd cdp la bien dp va pha do ngudn thUc g&y ra tai vi tri cua ngudn thd cdp.

Xet ngudn sdng O, mat song thtf cap nam tai mat kin (S)

bao quanh O, ta se tim bieu thdc dao dong sang tai diem M