- Anh do chi cd thi quan sat bdng mdt dat d vi tri thu nhan dupc chiim tia phdn xa hoae khiic xa (Hinh 29.10).
- Anh that cd the hiing dupc trdn man anh (Hinh 29.11).
a) De tdng quat hod khdi niem anh, ta quy udc gpi chum tia sang truydn ra khdi bd mat sau ciing ciia he quang hpc la chiim tia Id.
Cd the dinh nghla tdng quat nhu sau ve dnh trong Quang hpc :
-Anh diem Id diem ddng qui cua chiim tia Id hay dudng keo ddi ciia chung.
- Mot dnh diem Id :
+ that neu chum tia Id la chum hdi tu ; + do neu ehiim tia Id Id chiim phdn ki.
b) Tuong tu, ta ciing cd the' tdng quat hod khai niem vat. Thdng thudng, vat phdt ra chCim tia tdi. Dd la vat that.
Mdt each tong quat :
- Vdt diem Id diem ddng qui cua chum tia tdi hay dudng keo ddi ciia chung.
- Mot vdt diem Id :
+ that neu chum tia tdi la chiim phdn ki;
+ do neu chum tia tdi Id chum hdi tu.
Ta khdng xet vat do trong chuong tiinh hpc.
S'
A F A O
~ \ . ^'^^'^"^^
>' \ s . ^
Hmh 29.12 Anh ao tao bdi thiu kinh hdi tu
2. Cach dung anh tao bdi thau kinh
a) Thuat ngii dung dnh (ve anh) ed nghla la ve dudng truydn cua mdt chum tia sdng bieu didn su tao anh cua mdt vat diem (didm B a Hinh 29.12). BE
Ta thudng ve cdc tia tdi sau day : - Tia tdi qua quang tam O ciia thdu kfnh.
- Tia tdi song song vdi true chfnh ciia thdu kfnh.
- Tia tdi qua tieu diem vat chfnh F (hay cd dudng keo dai qua F).
b) Trong trudng hpp phai ve mdt tia bdt ki thi ta xac dinh true phu song song vdi tia tdi. Tia Id tuong iing (hay dudng keo dai eua nd) se qua tidu diem anh phu trdn true phu dd (Hinh 29.13).
H I Khi tao anh ^o, th^'u kfnh hoi tu cho chum tia Id phan kl. Ket qua nay c6 mau thuan vdi tfnh chat cua thau kfnh khong ? Giai thich.
F; Trgcphy
8 . .^^^-.^.^^
A I
A/A
F O
\ F ' A
"--•^\ 1 r
B' Hmh 29.14
Di cd anh rd (ddng dang vdi vat), vat phai cd dang phdng, nhd, dat vudng gdc vdi true chfnh vd dupe bidu didn tupng tnmg bdi mdt mui tdn AB liin net (vat that).
Trong di^u kien dd, dnh A'B' cua vat cung vudng gdc vdi true chfnh. Anh dupc bieu didn bdng miii tdn lien net neu la anh that va miii ten diit net neu la anh do (Hinh 29.14 va 29.15).
3. Cac trudng hop anh tao bcri thau kinh
Anh ciia mdt vat tao bdi mdi loai thdu kfnh cd nhiing dac diem xac dinh vi tfnh chat (that, do), chieu va dp Idn.
Bdng each thay doi vi tri ciia vat, ta cd the dung anh tuong iing va nhan ra cac dac diem nay.
BANG T 6 M TAT
(chi xet vat that)
\ Thau
\ kinh
\
Anh
Tinh chat (that, do)
: Do Idn i (so vdi vat)
H6itu(f>0)
I F O F' r
(Oi=or = 2f)
t Anh • That: vat ngoai OF
• Ac : vat trong OF Anh ao > vat
[ > vat: vat trong FI Anh that: • = vat; vat o / (^nh 6 /';
[ < vat: vat ngoai FI
Chieu I (so vdi vat)
Phanki(f<0)
Vat va anh • cung chieu <-ằ trdi tfnh chat
• cung tinh chat <^ trai chieu
O F
Anh luon luon ao
i A n h < vat
Anh cung chieu so vol vat
186
V - CAC CONG THCrc VE THAU KINH
Cdc cdng thiic anh dupc thid't lap dd' xde dinh vi tri, tfnh chdt (that, do), chidu va dd Idn ciia dnh tao bdi thau kfnh.
Xet hai trudng hpp cua dnh nhu ve d cac hinh 29.14 vd 29.15.
a) De thid't lap edc cdng thiic tdng quat dp dung cho mpi trudng hpp, ngudi ta dat cdc gid tri dai sd cho khodng each nhu sau :
0A=dv6i quy udc vat that :rf>0
vat ao : </ < 0 (khdng xet)
OA' = rf' vdi quy udc anh th$t: rf' > 0 anh ao:d' <0
b) Ngodi ra, chieu vd dp Idn ciia dnh dupc xde dinh bdi ti so :
^ E Dung cdng thiic xac djnh v|
trf anh, hay chdng to rang, ne'u giijr thau kfnh cd dinh va ddi vat doc theo true chfnh thi dnh va vat ludn di chuyen ciing chieu.
JB = k
k dupe gpi la so phdng dqi dnh.
- Ne'u k > 0 : v&t va anh ciing chieu.
- Ne'u ^ < 0 : vat va anh ngupc chieu.
Lap cdc ti sd theo tfnh ddng dang ciia cdc tam gidc (OAB va OA'B') cung nhu cdc tam gidc {F'ON va FAB), ddng thdi xem xet tat cd cdc trudng hpp ciia anh, ta ed hai kdt qua :