Ebook Vật lý lý sinh y học: Phần 2

3.4.3. M gt ung dung ddc biet cua ddng m ot chieu . M ay k h i/ru n g

Mot trong nhiing trang bi thiet yeu cua mot phong hoi siic cap ciiu tieu
chuan la may khii rung ma thtidng dtidc biet den dtidi ten goi pho thong hdn “may soc dien” .
Mot dong dien nho di qua tim co the gay nen rung that, nhting mot dong
dien ldn hdn nhieu co the gay nen mot sti co “cting” tam that (co nghia la mot sti
co cd dtidc bao toan). Khi rung that dang de doa sti song, mot dong dien ldn trong
khoanh khac truyen qua tim se chan rung that lai va nhip dap tim binh thtidng
dtidc phuc hoi khi dong dien ket thuc. Bien phap kha “soc” nay da chting minh
dtidc hieu qua cua no trong viec chan dting hoat dong hon loan cua cac buong tim
va du lau de cac xung dien tim tti nut SA lay lai sti kiem soat. Bien phap “soc” nay
con dtidc dung trong cap ctiu ngting tim.
May khii rung (defibrilator) thtic chat la mot tu dien ldn. Dien tich dtidc nap
va ltiu giti trong cac ban tu tao nen mot hieu dien the rat cao -7500 V, nhting doi
khi cao tdi 10.000 V. The hieu mot chieu rat cao nay dtidc nap tti nguon xoay
chieu qua mot may bien the va mot diode (den dien tii hai ctic). Hieu dien the tti
bien the ra la xoay chieu nhting diode chi cho dong dien di qua theo mot chieu, do
do cac ban cua tu da dtidc tich dien trai dau mot cach ben vting. Noi vdi hai ban tu
la hai dien ctic phang, rong (8 -12 cm dtidng kinh). Hai dien ctic rong co tac dung
phan phoi dong dien rat ldn de tranh bong. Sd do mot may khii rung dtidc bieu
dien tren hinh 5.21

Hinh 5.21

May khu" rung

Neu co dtidng dan truyen dien gitia hai dien ctic cua may th i tu se phong
dien. Doi vdi benh nhan ngting tim hoac rung that, trtidc het ngtidi ta boi ldp gien
(jely) dan dien len hai vung d hai phia tren ngtic benh nhan. Tiep theo ngtidi ta ap
hai dien ctic vao hai cho do va dong mach cho tu phong dien. Dong dien rat ldn
nhting chi keo dai vai miligiay. Sau khi phong dien. tu can dtidc nap lai trong

vong vai giay trtidc khi co the dtidc dung lai.

201


C in luu y rang, dong dien diidc san sinh cao hdn miic nguy hiem nhieu nen
benh nhan khong diidc tiep xue vdi cac vat noi dat th i du nhii gnidng sat va cung
khong diidc tiep xue vdi ngifdi khac.
Sii phong dien cua tu co the difdc phat dong theo y muon bang tay. nhung
trong dieu tri mot sd" roi loan tim ngifdi ta dung phoi hdp vdi mot may ghi ECG
hien dai va sii phong dien difdc dieu khien bang cd che khdp nhip (synchronized)
vdi tin hieu cua dien tim. Trong che do phoi hdp nay, sii phong dien cua may khu
rung khong dien ra tiic thdi khi nut phong dien difdc bat ma se tri hoan cho den
pha xuong cua song R (song ldn nhat cua ECG). Che do nay khong dung difdc neu
nhif sdng R vang mat trong ECG.
3.4.4. Ifng dung cua ddng dien xoay chieu trong vat ly tri lieu
3.4.4.1. O’n g dung cua ddng dien xoay chieu ha tan, trung tan

Trong y hoc con goi cac dong dien xoay chieu la cac dong xung dien thay doi
chieu (cd the thay doi ca bien do), trong so" dd dang tieu bieu nhat la ddng dien
bien thien dieu hoa hinh sin quen thuoc.
I —I MAX

Trong dd: co la t in so goc; f

2/r

COS CO t

la tan so" cua dong dien.

Ngifdi ta phan loai ddng dien theo tan so" cua no nhif sau:
-

Ddng dien xoay chieu ha tan cd tan sd" nho hdn 1000Hz

-

Ddng dien xoay chieu trung tan cd tan sd" trong khoang 1000Hz den
300 000Hz

Khac vdi ddng dien mot chieu, ddng dien xoay chieu khi tang khi giam; lam
cd co va met nhanh do dd tao len sii tap luyen va liic cd diidc tang ciidng. Tac dung
nay the hien rd ret nhat d ddng dien xoay chieu cd xung n g in va tan so" tif 40 den
180Hz. Trong cac benh thoai hoa than kinh van dong, cac cd lien quan trd nen bat
dong. Sii hoi phuc trong qua trin h dieu tri than kinh thoai hoa doi hoi thdi gian
kha dai (nhieu thang) vi cac cd de bi teo di. Ddng dien ha tan difdc iing dung de
kich thich cac cd trong triidng hdp ke tren de chong teo cd. Ngoai ra khi cac cd bi
co giat thi sii lifu thong mau cung diidc tang ciidng, do dd sii dinh diidng cd cung
diidc hoi phuc.
Ddng dien trung tan cd kha nang kich thich van dong yeu hdn ddng dien ha
tan, do dd phai dung ciidng do cao hdn. Dieu thuan Idi la vdi nhiing t in so vao
khoang 5000Hz trd len, tac dung kich thich van dong trd nen tiidng doi rd ret hdn
tac dung kich thich cam giac. Ndi khac di cd bi co nhiing khdng cd cam giac dau.
3.4.4.2. Ong dung cua ddng dien xoay chieu cao tan

Ddng dien cao t in la ddng dien xoay chieu vdi t in so f >300 000Hz. Ngoai ra
ngifdi ta con dung dai liidng biidc sdng X de dac triin g cho ddng cao tan. >.=V/f
trong dd V = 3.108 m/s la tdc do ddng dien. Ngiidi ta chia ddng dien cao t in ra:
202

- Song ngan co budc song khoang 10m, tan so khoang 30MHz
- Song sieu n gin co bade sdng khoang 70cm, tan sd" khoang 400MHz
- Sdng ciic ngin cd bade sdng khoang 10cm, t in sd" khoang 2500MHz
De tao ra ddng dien cao t in trong cO the, ta khong c in phai dung day dan
triic tiep. Nang lUdng dien cao t in d dang sdng dien tir cd the truyen di xa trong
khdng gian ma ft bi mat mat. Vi vay khdng phan biet trong vat ly tr i lieu cac khai
niem: ddng dien cao tin , sdng cao t in hay dien tu: triidng cao tin .
Trong viec dieu tri bang sdng cao tin , thiic chat ngiidi ta dat cO the benh
nhan theo each nhat dinh trong mot dien trUdng manh va bien thien vdi t in so"
hOn 300 KHz (cung g in giong nhii viec dat thiic pham vao lo vi sdng). Day la
phudng phap dieu tr i bang dien cd tii sdm nhat.
Khi ddng dien cao t in tac dung vao cO the ta thay khdng cd hien tUdng dien
phan, cO va th in kinh cung khdng bi kich thich.
Nang ludng cua ddng dien cao t in dufOc bien thanh nhiet nang trong khu viic
cO the cd ddng dien di qua, theo dinh luat Joule. Sii phan phoi nhiet trong c0 the
khdng dong deu va phu thuoc vao tan sd" ddng dien. Vdi sdng ngan nhiet giU nhieu
d to chiic md va it d to chiic cO, theo ti le 9/1. Vdi sdng sieu ngan ti le nay la 1/4
hoac 1/3, tac dung nhiet sau hdn nhiing chiia tdi mang xUdng. Vdi sdng ciic ngan,
ti le nay la 1/1, trong triidng hdp nay c in de phong tac hai den mat.
3.4.5. Phau thuat dien va dot cat dien

Cac dung cu phau thuat dien ngay nay diidc dung rong rai, de cat cung nhii
de “han” cac md. Nhiing dung cu nay tao ra cac ddng cao t in bang the hieu tdi
15.000 V.
Nhii da ndi trong 3.3.1, d t in sd" rat cao ngiidng nguy hiem cua ddng dien rat
ldn, do dd trong phau thuat hay dot cat dien, benh nhan tham chi khdng cam nhan
diidc ddng dien. Hieu iing chu yeu se la tac dung nhiet. Khi the hieu cao tin diidc dat
vao cac dien ciic rat mong, nhon thi mat do ddng dien ciic ldn tai ndi dat dien ciic se

lam “chay” md va vi vay trd thanh cong cu cat rat tot. Phau thuat dien cd nhiing Uu
the" nhat dinh so vdi phau thuat cd dien.
Tan sd" cd khoang 500 - 600 KHz diidc xac dinh la hieu qua cao trong viec
cat. Cac t in sd" cao hdn nhieu, khoang 2 - 4 MHz, hieu qua hdn trong viec lam
ddng (cung cd nghia la noi md) va dot.
Diem mau chot cua phiidng phap nay la mot mat tap trung ddng dien “di
vao” tai mot vung rat hep tai ndi c in cat, noi, b in g cach dat vao dd dien ciic rat
manh, nhon. Nhiing mat khac dien ciic con lai th i lai rat rong de phan tan ddng
dien “di ra” , tranh gay bong. Thiidng ngiidi ta dung mot tam dan dien lot ap deu
mong cho ddng “ra” . Cac tii “di ra” , “di vao” chi de cho de hinh dung, con ddng
dien la xoay chieu nen khdng cd khai niem ddng ra, ddng vao.

203


Bai 6

CAC DjNH LUAT QUANG HINH CU BAN - MAT
MUC TIEU HOC TAP
•

•

•

1. Trinh bay cac dinh luat quang hinh cd ban va ve dung diidng truyen
cua cac tia sang qua cac dung cu quang hinh chinh.
2. Mo ta he quang hinh cua m it va trin h bay diidc nguyen ly chung cua
cach siia tat quang hinh (tat khuc xa) cua mat b&ng kinh mat.
Nguon nang liidng chu yeu cua muon vat tren tra i dat la tii biic xa mat trdi.

Diidi tac dung cua anh sang mat trdi, sii song da phat sinh duy tri va phat trien.
Anh sang mang lai cac thong tin tii moi triidng song cho sinh vat noi chung
va con ngiidi noi rieng. Cac quy luat quang hinh hoc ap dung kha chinh xac cho
viec giai thich sii tao anh d vong mac.
Quang hinh hoc sU dung phiidng phap gan dung coi tia sang la rieng re. doc
lap vdi nhau va tuan theo cac quy luat phan xa va khuc xa de khao sat sii tao anh
cua cac he quang hoc.
Khai niem ve tia sang hinh thanh tii mot chum sang thiic, truyen qua mot Id
hep. Ta se co cam giac rang, lo cang hep thi chum sang cang gan vdi tia sang,
nhiing nghien ciiu hien tUdng nhieu xa anh sang ta lai thay rang, lo hep den mot
miic nao do thi chum sang chang nhting khong bi thu hep ma con trd nen rong ra.
Vi vay khong the bang cach giam klch thiidc lo de tach rieng biet mot tia sang. Do
do tia sang chi la mot khai niem toan hoc, khong phai la mot khai niem vat ly.
Tuy nhien, mot chum sang song song du hep co the coi gan dung la mot tia sang.
1. CAC DjNH LUAT CO BAN CUA QUANG HiNH HOC VA MOT SO DU^G CU
QUANG HINH
1.1. Cac djnh luat cd ban cua quang hinh hoc
1.1.1. Djnh luat truyen th in g cua anh sang
“Trong mot mdi triidng trong suot, dong tinh vd dang hiidng anh sdng
truyen theo diidng thang".
Khi nghien ciiu hien tiidng nhieu xa ta se thay dinh luat nay cd gidi han iing
dung cua no. Trong triidng hdp anh sdng truyen qua nhiing lo that nho hoac gap
nhiing vat can cd kich thiidc nho vao cd biidc sdng anh sdng thi dinh luat tren
khdng con dung niia.

1.1.2. Djnh luat ve tac dung doc lap cua cac chum tia sang
“Tdc dung cua chum tia sdng khac nhau thi doc lap vdi nhau. Dieu nay cd
nghia la tac dung cua mot chum tia sdng nay khdng phu thuoc vd sii cd mat cua
chum tia sdng khac\
204

Dinh luat nay cung chi g in dung. Khi lo nho, do roi d mot diem tren man
anh co the thay doi ro ret theo kich thiidc cua lo vi cac chum sang khong con doc
lap veil nhau ma co the giao thoa vdi nhau, do do tang ciidng lan nhau hoac lam
yeu nhau. Tuy nhien, khi tao anh nhd cac quang cu, chung ta thudng sii dung
churn sang rong nen khi tao anh khong co sii giao thoa hay dung hon sii giao thoa
khi dd cho ta anh diem cua tting vat. Vi vay trong quang cu ta hoan toan cd the ap
dung dinh luat ve sii doc lap cua cac chum tia sang.
1.1.3. Dinh luat phan xa anh sang

Neu tia sang truyen tif moi triidng
quang hoc nay sang moi triidng quang hoc
khac thi tai mat phan gidi cua hai moi
triidng, tia sang bi phan xa hoan toan hay
mot phan va tuan theo dinh luat phan xa anh
sang sau day:
-

Tia tdi vd tia phan xa cung nam
trong mat phang vdi phap tuyen cua
mat phan xa ve tit diem tdi.

-

Goc phan xa bang goc tdi i, = i ’,
Tia tdi vd tia phan xa co tinh thuan
nghich (Hinh 6.1).

Hinh 6.1

1.1.4. Djnh luat khuc xa anh sang

Neu mot tia sang tdi mat phan gidi cua hai moi triidng dang hiidng ma
truyen sang moi triidng th ii hai thi no tuan theo dinh luat khuc xa anh sang sau
day:
• Tia tdi vd tia khuc xa nam trong cung mot mat phang vdi phap tuyen cua
mat phan gidi ve tit diem tdi.

• Ty so giiia sin cua goc tdi vd sin cua goc khuc xa la mot dai liidng khdng
doi doi vdi hai mdi triidng quang hoc cho triidc:
s in h
— n2 \

( 6 . 1)

smi~>

Dai liiOng n21 diidc goi la chiet
suat ti doi cua moi triidng th ii hai
doi vdi moi trudng th ii nhat.
- Khai niem chiet suat:
Chiet suat cua moi triidng so
vdi chan khong (CK) du'Oc goi la
chiet suat tuyet doi cua moi
triidng dd (thiidng don gian goi la
chiet suat).

Hinh 6.2
205

Dat n10 = n1( n20 = n2 ta co the de dang tim moi lien he gitia chiet suat ti doi
n21 cua hai moi trUOng vdi chiet suat tuyet doi nt va n2cua chung (Hinh 6.2)
sin i
sin ii

= nw = ni

sinzsin/2

= " 20 = ” 2

Chia hai bieu thiic tren cho nhau va chu y i j = i 2 ta cd:
sini,

n7

sinil2

n,
'*/

---- - = —

= n21

( 6 . 2)

Thtic te neu day moi triidng hai lai gan moi triidng mot, khi tiep sat nhau

th i i! trung I2.
Vay chiet suat ti doi cua hai moi triidng bang ti so" gitia cac chiet suat tuyet
doi cua chung. Moi triidng cd chiet suat tuyet doi ldn hdn goi la moi triidng chiet
quang hdn.
Cd the viet bieu thtic (6.2) diidi dang doi xting nhti sau:
n,.sini,

= n2.sini2

(6.3)

- Phan xa toan phan:
Tti cong thtic (6.2) ta thay rang khi mot tia sang truyen tti mot moi triidng
(1) chiet quang hdn vao moi triidng (2) kem chiet quang hdn th i tia khuc xa bi lech
xa phap tuyen cua mat phan gidi hdn (Hinh 6.3a). Cang tang goc tdi i 1( goc khuc
xa i 2 cang tang nhanh va khi i1dat den gia tri:
igh = arcsin

thi

7T
l2 = ~

2

2

Goc i gh diidc goi la goc
gidi han. Theo dinh luat bao
toan nang ltidng, nang ltidng

cua chum tia tdi dtidc phan
bo" cho chum tia phan xa va
chum tia khuc xa. Khi goc tdi
cang tang th i ctidng do cua
chum tia phan xa cang tang
va ctidng do cua chum tia
khuc xa cang giam va giam
den 0 khi goc tdi bang goc
gidi han.
206

(6.4)

Hinh 6.3a


Khi goc tdi ldn hdn goc
gidi han, anh sang khong di
vao moi triidng thti hai,
ciidng do cua chum tia phan
xa bang ciidng do chum tia
tdi. Hien tiidng nay diidc goi
la sii phan xa toan phan va
Hinh 6.3b
diidc iing dung rong rai
trong cac dung cu quang
hoc. Ong dan sang (sdi dan quang) diidc dung trong thiet bi noi soi la mot th i du
dien hinh. Diidng di cua tia sang trong long sdi dan quang dac bang polymer diidc
minh hoa d hinh 6.3b.
Trong ky thuat truyen tin hieu cua biiu dien thi sdi cap quang ddng vai trd

quan trong trong viec truyen tin hieu di xa. Trong y hoc dac biet trong ky thuat
noi soi ngiidi ta dung ong dan anh sang de dan anh sang cua cac hinh anh noi
tang ra ngoai cd the giup thay thuoc quan sat de dang doi tiidng can quan sat.
Nguyen ly Fermat iing dung cho quang hinh hoc cho rin g anh sang di tii
diem nay den diem khac theo diidng doi hdi thdi gian ngan nhat. Hoan toan cd the
chiing minh de dang rang dinh luat phan xa va dinh luat khuc xa anh sang cd the
suy ra tif nguyen ly Fermat.
1.2. Mot so dung cu quang hinh
1.2.1. G i/ang phang - Girting cau

1.2.1.1. GifOng phang
/
/
/

Giidng phang la mot phan mat
phang, phan xa diidc hau nhii hoan
toan anh sang chieu tdi no.
Chum sang phan ky tif nguon
sang diem S sau khi phan xa tren
giidng p hing (Hinh 6.2) van la mot
chum sang phan ky.
Diem S’ la anh ao cua nguon
diem S vi no dude tao ra bdi phan keo
dai cua chum tia phan xa chti khong
phai chinh chum tia phan xa. Diidng
SS’ vuong goc vdi mat phang giidng va
a = a’ trong dd a va a’ lan liidt la
khoang cach tif nguon diem S va anh
S’ cua no tdi giidng.

//

\N

a
/
7^

/

/s '
7
/
/

s'

^

Hinh 6.4

Kich thiidc hinh hoc cua nguon sang va anh cua no la nhii nhau vi anh cua
mot vat doi xting vdi vat qua giidng.

207


1.2.1.2. G trong cau

(b)

Hinh 6.5
Gtidng cau la mot chom c iu phan xa anh sang. Khi mat phan xa lom (quay
ve phia tam cua mat cau) ta co gtidng cau lom. Khi mat phan xa loi (quay ve phia
tam cua mat cau) ta co giidng cau loi.Tam cua mat cau C goi la tam giiOng; dinh
cua mat cau O goi la dinh giidng; diidng thang noi tam va dinh giidng OC goi la
true chinh cua gtidng; cac dtidng thang khac di qua tam gtidng goi la true phu.
Goc tao bdi true phu di qua mep giidng dtidc goi la goc md cua giidng khi no chtia
true chinh cua gtidng (tat ca nam trong tiet dien thang cua gtidng). 0 day ta chi xet
triidng hdp goc md cua gtidng rat nho.
De ve anh cua mot diem qua gtidng cau ta co the dung 2 trong 4 tia sau:
- Tia di song song true chinh cua gtidng khi phan xa di qua F la trung diem
cua OC goi la tieu diem chinh cua giidng (OF = FC =R/2). OF = f goi la tieu cti
cua gtidng.
- Tia di qua F gap gtidng, phan xa thanh tia song song true chinh.
- Tia di qua dinh gtidng 0 khi phan xa thanh tia doi xting vdi tia tdi qua
true chinh.
- Tia di qua tam giidng C khi phan xa lai qua chinh tam gtidng.
Anh cua mot diem la anh that khi no la giao cua cac tia that, la ao khi no la
giao cua cac dtidng keo dai them cua cac tia that ve phia tia that khdng truyen
den. Xem hinh 6.5a va 6.5b.
1.2.2. Ban m at song song
Doi vdi ban mat song song (hinh 6.6) goc tdi vd goc lo cua cac tia sdng la
bang nhau. Ban mat song song lam dich chuyen tia sdng song song vdi chinh tia
sang do mot khoang A.

208

I 1 - sin2 z

A = d.sini

b
V

2

(6.5)

• 2•

V n - s in

i

y

Trong do d la chieu day ban, i la goc tdi cua tia sdng do, n la chiet suat ti doi
cua vat lieu lam ban doi vdi moi tritdng chtia ban.
Vi vay khi nhin qua ban mat song
song se thay diem S cua vat hinh nhti nam
tai diem S' (diem S' chinh la do anh cua
diem S khi nhin qua ban) ttic S' co ve gan
vdi be mat cua ban hdn S mot khoang:

A' =
sin/

Hoac A ' = d

V

1- sin2i
2 • 2■
Vn -s in i y

(6.6)
Hinh 6.6

Khi goc tdi i *• 0 , A & 0 vd A ' = d

n -1

. Tinh dong quy cua chum tia ditdc

n
bao toan khi chum tia tdi lech vdi phap tuyen cua ban nhting goc nho.

1.2.3. Lang kinh

Lang kinh
ba mat la mot khoi dong nhat trong suot*> diidc gidi han bdi tting
9
cap mat phang lam vdi nhau mot goc nao dd. Cac mat phang nay diidc goi la mat
lang kinh. Goc gitia hai mat lang kinh la goc chiet quang cua lang kinh. Giao
tuyen cua hai mat la canh lang kinh. Mat doi dien vdi canh la day lang kinh.
Bay gid ta se dung dinh luat khuc xa anh sang de khao sat sii truyen cua tia
sang qua lang kinh.

Gia sti tia SI dap vao mat
AB cua lang kinh bi khuc xa va
truyen theo phtidng II' nam trong
tiet dien ABC cua lang kinh. Den
I ’ tia sang lai bi khuc xa tai mat
AC va ra khoi lang kinh.
Nhti vay sau hai lan bi
khuc xa tai hai mat AB va AC tia
sang di ra khoi lang kinh da bi
lech so vdi phtidng cua tia tdi mot
goc 5 nao dd, goi la goc lech (Hinh
6.7). Do ldn cua goc lech 5 cd lien
he vdi goc tdi i,. goc khuc xa i 2 cua
tia ra khoi lang kinh va goc chiet
quang a bdi he thtic

^

l_Un^ g y

209


5 = ix + i2- a (6.7)

Neu ta thay doi goc tdi ij dan d in cho den khi tia tdi va tia ra khoi lang kinh
do’i xting nhau qua mat p hing phan giac cua goc chiet quang A, th i goc lech 5 se
cd gia tri ctfc tieu (5 = 5min). Khi dd tren hinh (6.7) ta cd A l = A l’ va = i 2 = i. Tti
(6.7) ta cd:
+a

sini - sin -----------

2

Nhting theo dinh luat khuc xa anh sang th i sini = n . s i n i trong dd i ’ = i ’, = i’2
do dd:
8
+a
n .sin i' - s i n - ^ ------

(6.8)

cc

Theo hinh ve th i i ’ = — cho nen (6.8) dtidc viet lai la:
2
n. sin — = sin —— ------

2

(6.9)

2

Vdi goc lech ctic tieu 5min thoa man dieu kien (6.9) lang kinh lam sai lech
chum tia sang truyen qua no it nhat, ttic la anh quang hoc cho bdi lang kinh khi
dd cd chat ltidng tot nhat.
Neu goc chiet quang a be th i 5 cung be, vi vay cd the dat:
ex

cc

S

+ cc

sin — - — va sin —^
2
2
2

va (6.9) trd thanh:

S

+ cc

-----2

5min = (n-l)a

Lang kinh cd goc chiet quang a be dtidc goi la nem quang hoc.
Cong thtic (6.9) cho phep ta xac dinh chiet suat cua chat cau tao nen lang
kinh, khi biet goc chiet quang va do dtidc goc lech 5min. De do goc chiet quang a
cung nhti goc lech § ngtidi ta dung giac ke.
Lang kinh diidc dung rong rai trong cac may quang pho de phan tich chum
btic xa phtic tap thanh cac thanh phan ddn sac. Muon the lang kinh trong may
quang pho° phai diidc dat sao cho phan gitia cua quang pho nghien ctiu di qua lang
kinh dtidi goc lech ctic tieu. Ngoai ra lang kinh con diidc dung de doi phtidng tia
sang, dac biet la lang kinh phan xa toan phan. Hinh 6.8 bieu dien cac lang kinh

phan xa toan phan. Trong trtidng hdp (a) anh bi quay di mot goc 90°. Trong
trtidng hdp (b) anh bi doi chieu do hai lan phan xa toan phan tren hai mat ben
cua lang kinh. Trong triidng hdp (c) anh cung bi doi chieu do phan xa toan phan cl
mat day cua lang kinh.

210


Hinh 6.8
1.2.4. Mat cau khuc xa

1.2.4.1. Khai niem m$t cau khuc xa

Mat cau khuc xa la mot phan mat
cau phan cach hai moi triidng trong
suot co chiet suat khac nhau. Do la
dang quan trong nhat trong tat ca cac
mat gidi han cua thau kinh. Sii khuc xa
anh sang qua mat cau la hien tiidng cd
ban dan den sii tao anh cua cac he
quang hoc.
Gia sii ta cd mat cau khuc xa ban
kinh R, tam C ngan cach hai moi
triidng cd chiet suat n (ben trai) va n’
(ben phai). True doi xiing cua mat cau
di qua tam C cua no la quang true
chinh. Giao diem cua quang true chinh
vdi mat cau la dinh 0 cua mat cau khuc
xa. Diidng thang bat ky di qua C la
quang true phu (hinh 6.9)

Hinh 6.9

He cac mat cau cd tam cung nam tren mot diidng thang tao thanh mot he
quang hoc dong true.
De tien Idi cho viec chiing minh cong thiic sau nay, ta diia vao khai niem
chum tia gan true. Chum tia gan true diidc xem nhii mot chum dong quy di rat
gan quang true chinh hay cac tia trong chum lam vdi quang true chinh nhiing goc
be. Trong triidng hdp nay cd the coi sin cua cac goc giiia tia sang va quang true
bing tang cua chung va bang chinh cac goc dd. N hii vay cong thiic tim diidc vdi
dieu kien chum tia sang gan true se cd dang toan hoc ddn gian.
1.2.4.2. Qui udc ve dau trong quang hinh hoc

t
Trong quang hinh hoc de tim cac cong thiic cd tinh chat tong quat hdn. ngiidi
ta thiidng dung quy lidc ve dau cho cac doan thang va cac goc.

211


A

O

(c)

i
o

<

A'
V

A'

-y'
(e)

(b)
Hinh 6.10

Neu coi chieu diidng la chieu truyen anh sang tif tra i sang phai va chon cac
goc tai dinh th i ta co quy lidc ve dau sau day:
- Do dai cac doan thang theo true chinh diidc xem la diidng neu co chieu
trung vdi chieu truyen anh sang, neu ngiidc lai la am (Hinh 6.10a)
- Do cao cua vat va anh diidc xem la diidng neu chung hiidng len phia tren
quang true chinh, la am neu hiidng xuong diidi (Hinh 6.10b).
- Ban kinh chinh khuc cua mat cau difdc xem la diidng neu tam chinh khuc
d ben phai mat cau, la am neu tam chinh khuc d ben tra i mat cau (Hinh 6.10c).
- Do day cua thau kinh va khoang cach giiita hai mat khuc xa luon diidc xem
la diidng (Hinh 6.10d).
- Goc giiia tia sang va quang true chinh hay phap tuyen cua mat khuc xa
diidc xem la diidng neu ta quay quang true hay diidng phap tuyen den gap tia
sang theo chieu kim dong ho, neu theo chieu ngiidc lai la am (Hinh 6.10e).
- Khi cd sii phan xa th i doi dau chiet suat n’ va khoang cach giiia mat phan
xa vdi mat tiep sau.
Sau day chung ta se dung dinh luat khuc xa anh sang va cac quy ifdc ve dau
de tim cong thiic cua mat cau khuc xa doi vdi cac chum tia gan true.
1.2.4.3. Cong thCfc cua mat cau khuc xa

Gia sii mat cau khuc xa EE’ cd ban kinh R ngan cach 2 mdi triidng quang
hoc cd chiet suat n (ben trai) va n’ (ben phai). Chiing ta hay xet mot tia sang trong
chum tia gan true phat ra tii nguon diem P n im tren quang true chinh CO. ching
han tia PM. Tia nay den mat cau tai diem M, diidi goc tdi i, khuc xa qua mat ciu
theo phiidng MP’ diidi goc khuc xa i va cat quang true chinh tai diem P' (xem
hinh 6.9).

212


Ta hay xac djnh vi tr i cua P’ doi vdi dinh 0 cua mat cau.
Ta hay dat OP = s, OP’ = s’, OC = R va goc M OC = cp. Tren hinh ta ghi tri so
sd" hoc cua doan th in g va goc.
Theo dinh luat khuc xa doi vdi chum tia sang gan true ta cd:
ni= n’i ’

(6.10)

Tit cac APMC vd AP’M C ta co:
(■i) = (-u) + (p hay i - u - q>
(-i’) =

11

- cp

Thay cac gid tri cua i vd i vao (6.10) ta ditdc:
n(u -

(6.11)

Ve ditdng M H vuong goc vdi true chinh. Vi ta dang xet chum tia gan true nen
co the coi :

H P = s ; H P ’ = s '; H C = R
Cac goc u, u' vd (p deu be nen ta co:
tg (-u ) = (~ii) = — hay u ——

-s

s

t ' =U ' = —
h
tgu
S
tg(p = (p = —

(6 . 12)

(6.13)

(6.14)

R
Tit ( 6 .1 2 ) vd ( 6 .1 3 ) thay cac gid tri u, u' vd cp vao (6 .1 1 ) vd ddn gian h, ta
ditdc:

1

n

\s

1\

—n

1
R'

Rj

Ta cd cong thiic mat cau khuc xa
n'

n _

s'

s

n

n

R

(6.15)

Cac dai liidng s, s’ va R trong cong thiic nay deu la nhiing do dai dai so". Bieu
thiic (6.15) la cong thiic cd ban cua mat cau khuc xa. No cho phep tim diidc
khoang cach s’ = OP' neu cho biet s = OP tiic la xac dinh diidc vi tr i cua diem P’
tuy theo vi tr i cua diem P. Doi vdi hai mdi triidng va mat cau cho triidc (n. n’ va R
cho triidc) s’ chi phu thuoc vao s. N hii vay tat ca cac tia trong chum tia gan true
phat ra tif nguon diem P sau khi khuc xa qua mat cau se cat nhau tai cung mot
diem P’. Do dd P’ la anh diem cua nguon P.
213


Cong thiic (6.15) dung vdi mat cau co be loi hiidng ve phia nao cung diidc,
mien la trong do phai chu y den dau cua R. Neu la mat cau loi (be loi hiidng ve
ben trai) thi R > 0 neu la mat cau lom (be loi hiidng ve phia ben phai) thi R < 0.
Cong thiic (6.15) con cho thay neu nhii vat dat tai P’ thi anh cua no se nam
tai P, nghia la cd tinh chat thuan nghich.
1.2.4.4. Do tu. Tieu cu cua m$t cau khuc xa

Tut cong thiic (6.15) ta suy ra:
Khi s — -oo thi s’ = f=Khi s’ = oo thi

n’R

(6.16)

n - n

s = f =

-n R

(6.17)

n -n

Nghia la f va f chi phu thuoc
vao ban kinh R cua mat cau, chiet
suat n va n’ cua hai mdi triidng. Neu
n, n’ va R cho triidc thi f va f la cac
dai liidng khdng doi dac triin g cho
cau khuc xa, f va f diidc goi la hai
tieu cii chinh cua mat cau khuc xa, f
la tieu cii chinh th ii nhat, con goi la
tieu cii vat va f la tieu cii chinh th ii
hai con goi la tieu cii anh. Cac diem

Y
p " " —
°

f;

E’

F va F’ (O F = / ; 0 F ' —f ) diidc goi
la tieu diem chinh th ii nhat va th ii
hai tiidng iing (Hinh 6.11)

Hinh 6.11

Nhu: vay tieu diem chinh cua mat cau khuc xa la diem hoi tu cua chum tia
tdi song song vdi quang true chinh sau khi khuc xa qua mat ciu.
Neu chum tia tdi khong song song vdi quang true chinh ma song song vdi
quang true phu, th i diem hoi tu cua no la tieuj diem 9phu.* Doi vdi chum tia gin
true, th i mat chiia cac tieu diem chinh th ii nhat cd the xem la mat vuong goc vdi
quang true chinh va di qua tieu diem chinh th ii nhat F. M at nay diidc goi la tieu
dien chinh th ii nhat. Tiidng tii nhii the mat vuong goc vdi quang true chinh va di
qua tieu diem chinh th ii hai F’ diidc goi la tieu dien chinh th ii hai cua mat ciu
khuc xa.
9

Neu chia (6.16) cho (6.17) ta cd:
n

L f

'

(6.18)

n

Vay hai tieu cii cua mat cau khuc xa ty le vdi chiet suat tiidng iing cua hai
mdi triidng va bao gid hai tieu diem cung nam hai ben mat cau.


Khi n, n’ va R cho triidc th i bieu thiic diing 0 ve phai cua (6.15) la mot dai
liidng khong doi. Dai liidng nay diidc goi la do tu cua mat cau khuc xa.
,

n -n


m

Ta co

n'

n

d = — = ---r
f

T ii cong thiic cua mat cau khuc xa ta co the suy ra cong thiic cua giidng cau
(mat cau phan xa tot anh sang) bang cach thay doi n’ = -n trong (6.15). Khi do ta
co:
1 1 2
—+- =—
s'

s

(6.19)

R

Do la cong thiic quen thuoc cua giidng cau dung diidc cho ca giidng cau 15m
cung nhii giidng cau loi mien la phai chu y den dau cua R (R < 0 doi vdi giidng cau

lom, R > 0 doi vdi giidng cau loi).
T ii cong thiic cua giidng cau (6.19) ta suy ra cong thiic cua giidng ph&ng,
bang cach cho R = oo. Khi dd ta cd : s’ = -s
Nghia la neu vat dat triidc giidng phang th i anh cua no se n im doi xiing vdi
vat qua giidng. Anh cho bdi giidng phang la anh ao. Tieu cii cua giidng cau diidc
xac dinh tii (6.16) hay (6.17):
/ - *
2
Do dd (6.19) cd the diidc viet lai:
—/ + - = —
rt
1

s

1

s

1

(6.20)

f

1.2.4.5. Dinh ly Lagrange - Helmholtz
Tren day chung ta mdi
xet tdi sit tao anh cua mot diem
sdng nam tren quang true
chinh bdi mat cau khuc xa.

Bay gid ta hay khao sat sit tao
anh cua mot doan thang ngan
AB vuong goc vdi quang true
chinh (Hinh 6.12). Gid sit A B '
la anh ditng ditdc cua A B vdi
chum tia gan true.
\

Ky hieu chiet suat cua
mdi tritdng ben trdi vd ben

y

Hinh 6.12

215


phai cua mat cau tiidng ting la n vd n\ Tren hinh (6.12) ta ghi cac tri so so hoc cua
do cao cua vat AB = y, do cao cua anh A'B' = y \ OA = -s, OA' = s' goc tcti cua goc
khuc xa la i vd i, goc md ciic dai cua chum tia tdi mat cau la -u (khdu do) vd goc
md cua chum tia khuc xa tiidng ting (chum tia anh) Id u'.
Bay gid ta tim moi lien he gitia u, y vd u\ y\ n’:
Tii AABO vd AA'B'O ta co:
tgi = —
-s

tgi =

(6.21)

s

Vdi gid thiet y vd y' deu be vd chu y tdi dinh luat khuc xa anh sdng co the
viet:
tgi

sini

n’

tgi '

sin i '

n

( 6 . 22 )

Phoi hdp (6.21) vd (6.22) ta co:

^

=^

sy '

(6.23)

n

Mat khac tti AAMO vd AA'MO ta co:

/

v /

v

^

tg (-u ) = ( -u ) = —
-s
Do do:

s

u

s

u

—=—

/ h
tgu = u = —
s
(6.24)

Tti (6.23) vd (6.24) ta dtidc :

ynu = y ' n ' u '

(6.25)

He thtic (6.25) bieu dien dinh ly Lagrange - Helmholtz. Bieu thtic nay co
dang mot dai ltidng khdng doi nen con goi la bat bien Lagrange - Helmholtz.
Cong thtic nay co the viet tong quat cho mot he quang hoc phtic tap gom
nhieu mat cau. Khi do:

y \ = y 2 • • •y \ - i = y k
n ' j = n 2 . . . n'kl = nk
u 1 = u2 . . . uk l = uk
ta co:

W ' l =>2"2»2

=

(6.26)

Cac he thtic (6.25) vd (6.26) chi dung vdi tia gan true.
Khi dung chum tia rong vd muon co anh rd net thi phai thoa man dieu kien:

216


ynsinu = y ' n' sinu'

(6.27)

He thiic nay diidc goi la dieu kien sin Abbe. Bat bien Lagrange - Helmholtz
vd dieu kien sin Abbe gidi han sii tii do bien doi cac chum tia sdng bang cac he
quang hoc. Chung lien he khau do vd kich thiidc cua vat vdi khau do vd kich thiidc
cua anh.
1.2.4.6. Do phong dai dai

Ti so giiia do cao cua anh va do cao cua vat diidc goi la do phong dai dai P
(hay do phong dai ngang):
/

(6.28)

P =—

y
Tif dinh ly (6.25) ta co:
y'
nu
P = ^ = - r -§
y
n u

(6.29)

Thay u/u tif cong thiic (6.24) ta diidc:
ns'
P = ----n' s

(6.30)

Bdi vi n va n’ la nhiing dai liidng diidng nen dau cua P diidc xac dinh bdi dau
cua ti so" s’/s. Neu s < 0 va s’ > 0 thi P < 0, ta co anh thiic va ngiidc chieu vdi vat.
Neu s va s’ cung dau, th i P >0 ta cd anh ao va cung chieu vdi vat. Doi vdi giidng
cau thi n/n’ =-1 do dd:
/

P = ~—

(6.31)

Trong triidng hdp cho anh thiic, s va s’ cung dau do dd P < 0 anh va vat
ngiidc chieu nhau. Trong triidng hdp cho anh ao thi s < 0 va s’ > 0 nen P > 0. anh
cung chieu vdi vat.
Doi vdi giidng phang s’ = -s do dd P =1 tiic la anh ao cung chieu vdi vat va
bang vat.
1.2.5. Thau kinh mong
1.2.5.1. K hai niem thau kinh m ong

Thau kinh mong la mot khoi trong suot dong chat bang thuy tinh hay thach
anh diidc gidi han bdi hai mat trong dd cd mot mat cau. con mat kia cd the la mat
cau hay mat phang. Goi 0 ! va 0 2 la dinh cua hai mat cau cd ban kinh Rj va Rv, thi
khoang cach d = C^Oo diidc goi la do day cua thau kinh. Neu d rat nhd so vdi R, va
R2 th i thau kinh coi la mong. Khi dd cd the coi O! va 0 2 trung nhau va ki hieu la
0, diem 0 diidc goi la quang tam cua thau kinh. Dudng thang di qua quang tam

217


cua thau kinh diidc goi la quang true. Neu quang true di qua ca 2 tam cua m§t
cau goi la quang true chinh, cac quang true khac la quang true phu.

Bay gid ta se khao sat sii khuc xa cua anh sang qua thau kinh mong va cung
chi gidi han xet nhiing tia gan true.
1.2.5.2. Cong thtic tao anh qua thau kinh mong

Gia sii ta xet sii khuc
xa anh sang qua thau kinh
mong 2 mat loi (Hinh 6.13).
Ta dung cac ky hieu sau day:
chiet suat cua moi triidng
ben trai thau kinh la n! d ben p
phai thau kinh la n2, con
chiet suat chat lam thau
kinh la n; cac khoang cach tii
vat va anh den thau kinh la s
va s’ tiidng iing (diidc tinh tii
quang tam).
Hinh 6.13
Muon tim cong thiic
thau kinh mong ta ap dung cong thiic (6.15) lan liid t cho hai mat cau. Khi xet sii
khuc xa qua mat cau th ii nhat, ta coi nhii khong cd mat cau th ii hai, va khi dd coi
chiet suat cua moi triidng th ii hai la n. Anh cua nguon diem P nam tren quang
true chinh cho bdi mat cau 0 ! la P^ Theo (6.15) th i s, va s’, la khoang cach tii vat
va anh den dinh 0 , cua mat cau th ii nhat lien he vdi nhau bdi he thiic:
n
s\

n.
n —n.
' 1
s,

R,

(6.32)

Chi so' 1 chi cac dai liidng thuoc mat cau th ii nhat.
Anh P \ se la vat ao doi vdi mat cau th ii hai 0 2. Khi xet sii khuc xa qua mat
cau 0 2 ta coi nhii khong cd mat cau th ii nhat, tiic la coi chiet suat cua moi triidng
triidc mat cau 0 2la n, sau mat cau 0 2la n2. Anh cua P \ cho bdi mat cau 0 2la P’2
n im cach dinh 0 2 mot khoang s’2 cd lien he s2 = s^ (vi thau kinh mong) bdi he
thiic:
n
S2

s\

~

w, 2

n

(6.33)

R2

Chi so' 2 chi cac dai liidng thuoc mat cau th ii hai. Cong (6.32) va (6.33) ta
diidc:
n7 n,
n-n,
tu-n

— ------ L = ------ + ^ —
s'2
/?,
R2

(6.34)

Neu goi khoang cach tii nguon diem P den quang tam 0 cua thau kinh la s
(s, = s) va khoang cach tii anh P’2 den 0 la s’ (s’2 = s’) thi (6.34) se cd dang:
218


n.

n2-n

n-n,

n,

(6.35)

Day la cong thiic tong quat cua thau kmh mong. Thong thiidng thi thau
kinh difdc dat trong moi triidng dong tinh tiic la n, = n2 cho nen ta cd:
n,

Hay:

s

(I

1

- - \= ( n - n . )

s

( I

I

v*.

R2

J

(6.36)

n,

R2

v*.

J

n

Neu diia chiet suat ti do'i N = — thi cong thiic (6.36) se viet lai thanh :
n

,

\

(6.37)
V

R\

^2 J

Cong thiic (6.37) chi dung vdi thau kmh mong cd dang bat ky va dat trong
moi triidng dong tinh bat ky. Can chu y rin g cac dai liidng s, s’, R, va R2 diidc tinh
tii quang tam cua thau kinh va tuan theo cac quy lidc ve dau. Neu thau kinh diidc
dat trong mdi triidng khdng khi (n, = 1) th i N = n do dd (6.37) trd thanh:
r
1

1

s

s

/

I

n

(6.38)

R2

1.2.5.3. Tieu c i / - Do tu cua thau kinh mong

Tif (6.37) ta thay :
Khi s =

- oo

th i

r
rI
5 = f = ---------

1

(6.39)

L - L

(N-l)
V

R\

*2j

tiic la khi nguon diem P d vo ciic, cac tia sang den thau kinh la chum song
song vdi quang true chinh, thi anh cua no se nam tai diem F’ each quang tam cua
thau kinh mot khoang s’ - f va f difdc goi la tieu cii chinh th ii hai (tieu cii anh)
cua thau kinh con F’ difdc goi la tieu diem chinh th ii hai (tieu diem anh) cua thau
kinh.
Khi

1

s’ = oo t h i : s = f =
(N-l)

1

I* .

(6.40)
J
R2 y

219


diidc goi la tieu cii chinh th ii nhat cua thau kinh (tieu cii vat). Diem F cach
quang tam cua thau kinh mot khoang s = f diidc goi la tieu diem chinh th ii nhat
(tieu diem vat) cua thau kinh (hinh 6.14)
9

Cac mat phang vuong goc vdi
quang true chinh va di qua cac
tieu diem chinh diidc goi la tieu
dien (hay cac mat phang tieu) cua
thau kinh. Giao diem cua bat cii
quang true phu nao vdi tieu dien
deu la tieu diem phu. T ii (6.39) va
(6.40) thay:
(6.41)

f = -f ‘

Vay neu thau kinh diidc dat
trong mdi triidng dong tinh se cd cac tieu cii bang nhau ve do ldn nhiing ngUdc
dau tiic la hai tieu diem F va F’ nam d hai ben thau kinh.
Tuy thuoc vao dau cua R,, R2 va (N -l) ma
tieu cii f cd the diidng hoac am. Neu tieu diem F
la thiic, nghia la chum song song sau khi khuc xa
qua thau kinh trd thanh chum hoi tu, thi thau
kinh dd goi la thau kinh diidng hay thau kinh hoi
tu. Neu tieu diem F’ la ao, nghia la chum tia song
song sau khi khuc xa qua thau kinh trd thanh
chum phan ky, thi thau kinh diidc goi la thau kinh
am hay thau kinh phan ky (Hinh 6.15). Neu chu y den
(6.39) thi (6.37) cd the viet lai:
1

F'

Hinh 6.15

1
s

/'

Cong thiic nay goi la cong thiic Gauss. Trong cong thiic nay s va s’ diidc tinh
t ii quang tam O.
Neu khoang cach tii vat va anh khdng phai diidc tinh tii quang tam O. ma tii
cac tieu diem F va F’ (FP =x va F’P’ = x’) thi cd the chuyen cong thiic Gauss thanh
dang khac, bang cach thay s = x + f va s’ = x’ + f
Ta cd:

1

1

x '+ f

x+f

1

(6.42)

f

Chu y rin g f = - f va sau khi ddn gian (6.42) ta diidc:
x.x' = / . / ’ = - / ' ■

(6.43)

Do la cong thiic Newton.
Tuy bai toan cu the. cd the chon cong thiic Gauss hay cong thiic Newton cho
ddn gian khi tinh toan.

220


Ta trd lai bieu thiic (6.37). Neu th&'u kinh cho triidc (n, Rl5 R2 cho triidc) va
mdi trudng dat th iu kinh cho triidc (nj cho triidc), th i bieu thiic d ve phai la mot
djnh luat khdng doi va diidc goi la do tu (j>cua thau kinh mong:
1

_ 1_

R2 y

/'

(6.44)

Vay do tu cua thau kmh cd gia tri bang nghich dao cua tieu cii.
Ddn vi de do do tu la diop (D). Diop la do tu cua mot thau ki'nh mong dat
trong khdng khi cd tieu cii bang 1 met:
1 D = 1/m
1.2.5.4. Cac dang thau kinh

Tren hinh (6.16) bieu dien cac

dang thau kmh hoi tu va phan ky khac
n
nhau. Cac thau kinh cd dang 1, 2, 3 la
cac thau kmh hoi tu (thau kinh hai mat
loi (1), thau kinh phang loi (2), thau
/
LA
kmh loi -lom (3)). Cac thau kmh cd
dang 4, 5, 6 la cac thau kinh phan ky
4
l
(thau kinh hai mat lom (4), thau kinh
phang - lom (5), thau kinh loi lom (6)).
Hinh 6.16
Ta thay rang kmh hoi tu cd mep mong
hdn khoang giiia con thau kinh phan ky cd mep day hon khoang giiia.

11

Neu mdi triidng dat thau kinh cd chiet suat ldn hdn chiet suat cua chat lam
thau kinh (n, > n th i cac thau kinh 1, 2, 3 trd thanh thau kinh phan ky con cac
thau kinh 4, 5, 6 trd thanh cac thau kinh hoi tu.
1.2.5.5. Cach di/ng anh. Do phong dai dai

Hinh 6.17

Hinh 6.18

Gia sii ta diing anh cua mot doan th in g AB vuong goc vdi quang true chinh.
Trong triidng hdp ddn gian naychi can diing anh B’ cua diem B.

De diing anh cua diem B ta thudng chon 2 trong 3 tia sau:
221


- Tia tdi song song vdi quang true chinh sau khi khuc xa qua thau kinh
mong se di qua tieu diem chinh th ii hai F’.
- Tia tdi di qua tieu diem chinh th ii nhat F sau khi khuc xa qua thau kinh
mong se di theo phUdng song song vdi quang true chinh.
- Ngoai ra neu thau kinh dat trong moi trudng dong tinh th i tia tdi di qua
tam O se khdng bi khuc xa.
Hinh (6.17) va (6.18) bieu dien each dung anh v&t AB cho bdi thau kinh hoi
tu va phan ky, A’B’ la anh cua vat AB.
Anh A’B’ cd the be hdn, ldn hdn hay b&ng vat AB tuy thuoc vao khoang each
tu! vat den thau kinh va loai thau kinh. De tinh do cao cua anh ta tim do phong
dai dai.
T ii hinh (6.17) va (6.18) ta cd the de dang tim dUdc cong thiic cua do phong
dai dai p. That vay tii cac tam giac dong dang AABF, I ’OF, IO F’ va A’B’F ta dUdc:
f

r

p = l - = - L =
y
x

i

x

(6.45)

/'

Xet hai tam giac dong dang ABO va A’B’O th i cd the bieu dien P theo s va s’:

y

(6.46)

s

Dau va do ldn cua P se cho biet chieu va do ldn cua anh so vdi chieu va do
ldn cua vat.
1.2.5.6. Thau kinh tru

True

True

x
day

X

—

___ _

Anh
---6

B

/
x’

222


Thau kinh tru la tap hdp cac thau kinh c iu mong giong nhau ghep chong
k h it len nhau (Hinh 6.19). Dudng th in g xx’ th in g goc vdi hai mat phing day va
di qua quang tam cua cac thau kmh c iu mong goi la true cua thau kinh. Chum
tia sang song song chieu th in g goc vdi mat day khdng bi khuc xa, chum tia sang
song song chieu theo phudng vuong goc vdi true thau kinh bi khuc xa va sau khi
di qua thau kinh se hoi tu tren dudng thang song song vdi true thau kinh (do'i
vdi thau kinh tru hoi tu) hoac se phan ky sao cho dudng keo dai gap nhau tren
dudng thang song song vdi true d phia trUdc thau kinh (do'i vdi tha'u kinh tru
phan ky). Nhu vay anh cua mot diem qua thau kinh tru se la mot doan nam tren
dudng tieu cU.
2.

m At v a d u n g c u b o t r o

2.1. Quang hinh h2.1.1. S o luge ve cau tao cua mat
Mat con goi la nhan cau, co dang hinh cau, diidng kinh do theo true triidc
sau khoang 22mm. Mat ngoai cua mat co sau bo cd van dong bdm vao de giup cho
mat co the quay diidc nhieu phia khac nhau vd dinh hiidng khi nhin. Vo cua con
mat diidc cau tao bdi cac ldp mang ddn hoi, bao gom ba loai, tinh tii ngoai vao
trong (Hinh 6.20) la:

- Cung mac la mang ngoai cung, bao kin 3 / 4 phia sau con mat. La ldp xd
day, dai, trang nhii sii, anh sdng khdng lot qua diidc.
- d 1 / 4 phia triidc la ldp md trong suot, anh sdng xuyen qua diidc goi la giac
mac. Giac mac co ban kinh cong nho hdn ban kinh cong cua cung mac.
- Mach mac con goi la
mang mach, nam trong cung
mac. Mang mach chiia nhieu
mach mau de nuoi diidng mat
vd co nhieu sac to den giii cho
ben trong nhan cau nhii mot
buong toi. Ngay phia sau giac
mac, mang mach co mot phan
ru xuong tao thanh mang chan
co mau den hoac nau, mang
chan co mot 16 hd hinh tron co
diidng kinh thay doi diidc, lo hd
nay goi la ddng tii. Anh sdng
sau khi xuyen qua giac mac se
di qua dong tii vao phia trong.
Do ddng tii tit thay doi ditdc ban kinh nen co kha nang tit dieu chinh thong
liidng cua anh sdng tdc dung vao vong mac. Vdi anh sdng co do roi ldn tdc dung
vao mat thi dong tit tit dong co lai de lam giam liidng anh sdng tdc dung vao mat,
vdi anh sang co do roi nho thi dong tit tit gian rong ra, do do thong litdng anh
sdng vao mat se tang len.

223


Vdng mac la ldp mang trong cung, ditdc cau tao bdi nhieu ldp te bao nhitng
quan trong nhat la ldp te bao than kinh thi giac. Cac te bao nay tap trung thanh

cac sdi than kinh nho noi lien vdi day than kinh thi giac. Co hai loai te bao than
kinh cam thu ditdc anh sdng, do la te bao non vd tebao que. O mat ngitdi co chitng
7 trieu te bao non vd 130 trieu te bao que. Sit phan bo cua hai te bao than kinh
nay tren vong mac la khac nhau, te bao non tap trung vao vung gan diem vdng
(phan vong mac nam gan giao diem giiia true chinh cua mat vdi vong mac). 0
diem' vdng vong mac mong hdn d cho khac do do bi lom xuong tao nen ho trung
tam, d day mat do te bao non rat cao (khoang 150.000 te bao / mm2). Khi nhin vat
thi anh cua vat se hien len d ho trung tam. Cang xa diem vdng thi mat do te bao
non giam dan vd mat do te bao que tang len, d vung xa diem vdng thi d vdng mac
chi con tebao que.
Dac diem vd chiic nang cua hai loai te bao than kinh nay khdng giong nhau.
Te bao non cam thu ditdc anh sdng co do roi ldn vd cd kha nang phan biet diidc
hinh the, mau sac vd chi tiet cac vat. Te bao que cam thu anh sdng co do roi nho
(co do nhay ldn hdn so vdi te bao non). Do vay vung xa diem vdng ta chi co cam
giac sang toi.
Mdi triidng ben trong nhan cau ditdc chia lam hai phan ngan cach nhau bdi
thuy tinh the. Thuy tinh the trong suo't, hai mat loi, mat cong phia triidc co ban
kinh ldn hdn mat cong phia sau. Nhd sit thay doi site cang cua day chang treo thuy
tinh the vd tinh ddn hdi cua ban than thuy tinh the md no cd kha nang thay doi ban
kinh cong mat triidc, sau ddn den sit hoi tu cua mat thay doi khi ta quan sat cac vat
d xa hoac gan. Thuy tinh the cd chiet suat khoang 1,43 vd do tu khoang 12+14 diop.
Khoang giiia giac mac vd thuy tinh the chiia dich trong suot. Khoang giita
thuy tinh the vd vdng mac chiia thuy tinh dich hay con goi la dich kinh. Do cac the
dich trong mat luon litu thong nen dp suat cua mat giit khdng doi d gid tri
12 + 2 5 mmHg.

2.1.2. Quang hinh hoc cua mat

Theo cau tao cua mat, ve
phUOng dien quang hinh thi

mat dUOc cau tao bdi cac mdi
trudng chiet quang ngan cach
nhau bang mat cau khuc xa
tao nen ba ludng chat cau:
Ludng chat cau giac
mac la quang he tao nen do
giac mac ngan cach mdi trudng
khdng khi vdi thuy dich. Giac
mac cd do day khoang 1mm.
ban kinh cong 8mm. chiet suat
thuy dich 1,336 va do tu cua
quang he la 45 diop.

224

Hinh 6.21


- Ludng chat cau thuy tinh the trUdc do mat cong triidc thuy tinh the ngan
cach thuy dich vdi thuy tinh the. Ban kinh cong mat triidc thuy tinh the khoang
10mm, chiet suat trung binh 1,388 va do tu cua he la 7 diop.
- Ludng chat cau thuy tinh the sau do mat cong phia sau thuy tinh the ngan
cach thuy tinh the vdi dich thuy tinh. Ban kinh cong mat sau thuy tinh the
khoang 8mm, chiet suat dich thuy tinh la 1,36 va do tu cua quang he khoang 12
diop.
Ba ludng chat cau tren cd cung true chinh va hop lai thanh mot he quang
hoc. He nay coi nhu dUdc ghep bdi ba he quang hoc cd chiet suat khac nhau. Anh
sang xuyen vao mat, no se bi khuc xa khi truyen qua ba ludng chat cau va cuoi
cung se tac dung len cac te bao than kinh d vdng mac. Nhu vay he quang hoc cua
mat se cd mot tam diem duy nhat. Do dac diem nay ma chung ta cd the thay the

ba he quang hoc gom ba ludng chat cau bang mot ludng chat cau tong hdp con goi
la con mat udc lUdc (Hinh 6.21).
Ludng chat cau tong hdp cd mat ciu ngan cach mdi trudng khdng khi vdi moi
trUOng ben trong mat. Mat cau khuc xa cd mat loi quay ra phia trUdc, ban kinh
cong 5mm, dinh cach giac mac 2mm, quang tam cach giac mac 7mm va cach vong
mac 15mm, chiet suat cua quang he la 1,333.
Do he quang hoc nay cd mdi trUOng th ii nhat la khdng khi, mdi trudng thU
hai la mdi trUOng ben trong mat cd chiet suat xac dinh la n nen tieu cU cua ludng
chat cau tong hdp dUdc tinh theo cong thUc sau:
/ = — = 1,333x5 = 20mm
n -1
1,333-1

(6.47)

trong dd f la tieu cU, R la ban kinh cong cua mat cau tong hdp, n la chiet
suat mdi trudng, chiet suat cua khdng khi bang 1.
Tii cac thong so quang hinh cua ludng chat cau tong hop va do ldn tieu cU ta
thay vong mac cua mat nam tren mat phang tieu cua ludng chat cau tong hdp.
Dudng thang di qua dinh mat cau va quang tam goi la true chinh, dudng thang di
qua quang tam va diem vang goi la dudng nhin thang. Khi nhin mot vat thi mat
hudng dudng nhin thang di qua vat ay. Neu dudng nhin thang hudng vao diem cd
dinh d phia trUdc thi khoang khdng gian ma mot mat nhin thay la mot hinh non
cd goc md ngang 160°, goc md dUng 130°. Khoang khdng gian nay goi la thi
trudng. ThUc te thi trudng rong ho'n nhieu do mat cd the xoay trong hoc mat lam
cho dudng nhin thang thay doi. Mat khac do luon nhin bang hai mat phoi hdp vdi
dong tac quay dau nen thi trudng dUdc md rong hdn. Viec ve anh cua vat qua 3 he
ludng chat cau tong hdp se trd nen ddn gian hdn nhieu.
2.1.3. Kha nang dieu tiet cua mat

M ilt binh thudng. khi nghi. cd tieu diem d dung tren vdng mac. Khi mat nhin
mot vat AB d vd cuc. thi anh A'B’ cua AB cung d dung tren vdng mac.
Cho vat AB tien dan lai gan mat, neu quang he cua mat khdng thay doi gi.
thi anh cua vat se lui dan ra sau vdng mac, va mat khdng trong rd vat nUa. De
225