18. n sin θi = ncore sin θr
= ncore sin(90o − θc)
= cos θc
= ncore sin(1 − sin2 θc)1/2
•
¤
!
¦'
¦§
1'
I
p
!
§
§
¨
!0
f
(
d
sin θc =
nclad
ncore
•
!0
E
d
n sin θi = ncore 1 −
nclad
ncore
2
19. = n2
core − n2
clad
§
NA = n sin θi
)
0
E
d
NA = n2
core − n2
clad