<s>
In	O
optics	B-Algorithm
,	O
a	O
ray	O
is	O
an	O
idealized	O
geometrical	O
model	O
of	O
light	O
or	O
other	O
electromagnetic	O
radiation	O
,	O
obtained	O
by	O
choosing	O
a	O
curve	O
that	O
is	O
perpendicular	O
to	O
the	O
wavefronts	O
of	O
the	O
actual	O
light	O
,	O
and	O
that	O
points	O
in	O
the	O
direction	O
of	O
energy	O
flow	O
.	O
</s>
<s>
Rays	O
are	O
used	O
to	O
model	O
the	O
propagation	O
of	O
light	O
through	O
an	O
optical	O
system	O
,	O
by	O
dividing	O
the	O
real	O
light	O
field	O
up	O
into	O
discrete	O
rays	O
that	O
can	O
be	O
computationally	O
propagated	O
through	O
the	O
system	O
by	O
the	O
techniques	O
of	O
ray	B-Algorithm
tracing	I-Algorithm
.	O
</s>
<s>
Ray	B-Algorithm
tracing	I-Algorithm
uses	O
approximate	O
solutions	O
to	O
Maxwell	O
's	O
equations	O
that	O
are	O
valid	O
as	O
long	O
as	O
the	O
light	O
waves	O
propagate	O
through	O
and	O
around	O
objects	O
whose	O
dimensions	O
are	O
much	O
greater	O
than	O
the	O
light	O
's	O
wavelength	O
.	O
</s>
<s>
Ray	O
optics	B-Algorithm
or	O
geometrical	O
optics	B-Algorithm
does	O
not	O
describe	O
phenomena	O
such	O
as	O
diffraction	O
,	O
which	O
require	O
wave	O
optics	B-Algorithm
theory	O
.	O
</s>
<s>
A	O
light	B-Device
ray	I-Device
is	O
a	O
line	O
(	O
straight	O
or	O
curved	O
)	O
that	O
is	O
perpendicular	O
to	O
the	O
light	O
's	O
wavefronts	O
;	O
its	O
tangent	O
is	O
collinear	O
with	O
the	O
wave	O
vector	O
.	O
</s>
<s>
Light	B-Device
rays	I-Device
in	O
homogeneous	O
media	O
are	O
straight	O
.	O
</s>
<s>
Geometric	O
optics	B-Algorithm
describes	O
how	O
rays	O
propagate	O
through	O
an	O
optical	O
system	O
.	O
</s>
<s>
A	O
slightly	O
more	O
rigorous	O
definition	O
of	O
a	O
light	B-Device
ray	I-Device
follows	O
from	O
Fermat	O
's	O
principle	O
,	O
which	O
states	O
that	O
the	O
path	O
taken	O
between	O
two	O
points	O
by	O
a	O
ray	B-Device
of	I-Device
light	I-Device
is	O
the	O
path	O
that	O
can	O
be	O
traversed	O
in	O
the	O
least	O
time	O
.	O
</s>
<s>
An	O
is	O
a	O
ray	B-Device
of	I-Device
light	I-Device
that	O
strikes	O
a	O
surface	O
.	O
</s>
<s>
A	O
meridional	O
ray	O
or	O
tangential	B-Device
ray	I-Device
is	O
a	O
ray	O
that	O
is	O
confined	O
to	O
the	O
plane	O
containing	O
the	O
system	O
's	O
optical	O
axis	O
and	O
the	O
object	O
point	O
from	O
which	O
the	O
ray	O
originated	O
.	O
</s>
<s>
A	O
skew	B-Device
ray	I-Device
is	O
a	O
ray	O
that	O
does	O
not	O
propagate	O
in	O
a	O
plane	O
that	O
contains	O
both	O
the	O
object	O
point	O
and	O
the	O
optical	O
axis	O
.	O
</s>
<s>
The	O
principal	O
ray	O
or	O
chief	B-Device
ray	I-Device
(	O
sometimes	O
known	O
as	O
the	O
b	O
ray	O
)	O
in	O
an	O
optical	O
system	O
is	O
the	O
meridional	O
ray	O
that	O
starts	O
at	O
the	O
edge	O
of	O
the	O
object	O
,	O
and	O
passes	O
through	O
the	O
center	O
of	O
the	O
aperture	O
stop	O
.	O
</s>
<s>
As	O
such	O
chief	B-Device
rays	I-Device
are	O
equivalent	O
to	O
the	O
rays	O
in	O
a	O
pinhole	O
camera	O
.	O
</s>
<s>
The	O
distance	O
between	O
the	O
chief	B-Device
ray	I-Device
and	O
the	O
optical	O
axis	O
at	O
an	O
image	O
location	O
defines	O
the	O
size	O
of	O
the	O
image	O
.	O
</s>
<s>
The	O
marginal	O
and	O
chief	B-Device
rays	I-Device
together	O
define	O
the	O
Lagrange	O
invariant	O
,	O
which	O
characterizes	O
the	O
throughput	O
or	O
etendue	O
of	O
the	O
optical	O
system	O
.	O
</s>
<s>
A	O
sagittal	O
ray	O
or	O
transverse	B-Device
ray	I-Device
from	O
an	O
off-axis	O
object	O
point	O
is	O
a	O
ray	O
that	O
propagates	O
in	O
the	O
plane	O
that	O
is	O
perpendicular	O
to	O
the	O
meridional	O
plane	O
and	O
contains	O
the	O
principal	O
ray	O
.	O
</s>
<s>
All	O
other	O
sagittal	O
rays	O
are	O
skew	B-Device
rays	I-Device
.	O
</s>
<s>
When	O
discussing	O
ray	B-Algorithm
tracing	I-Algorithm
this	O
definition	O
is	O
often	O
reversed	O
:	O
a	O
"	O
paraxial	O
ray	O
"	O
is	O
then	O
a	O
ray	O
that	O
is	O
modeled	O
using	O
the	O
paraxial	O
approximation	O
,	O
not	O
necessarily	O
a	O
ray	O
that	O
remains	O
close	O
to	O
the	O
axis	O
.	O
</s>
<s>
A	O
meridional	O
ray	O
is	O
a	O
ray	O
that	O
passes	O
through	O
the	O
axis	O
of	O
an	O
optical	B-Architecture
fiber	I-Architecture
.	O
</s>
<s>
A	O
skew	B-Device
ray	I-Device
is	O
a	O
ray	O
that	O
travels	O
in	O
a	O
non-planar	O
zig-zag	O
path	O
and	O
never	O
crosses	O
the	O
axis	O
of	O
an	O
optical	B-Architecture
fiber	I-Architecture
.	O
</s>
<s>
A	O
guided	B-Device
ray	I-Device
,	O
bound	B-Device
ray	I-Device
,	O
or	O
trapped	B-Device
ray	I-Device
is	O
a	O
ray	O
in	O
a	O
multi-mode	B-Device
optical	I-Device
fiber	I-Device
,	O
which	O
is	O
confined	O
by	O
the	O
core	O
.	O
</s>
<s>
For	O
step	B-Device
index	I-Device
fiber	I-Device
,	O
light	O
entering	O
the	O
fiber	O
will	O
be	O
guided	O
if	O
it	O
makes	O
an	O
angle	O
with	O
the	O
fiber	O
axis	O
that	O
is	O
less	O
than	O
the	O
fiber	O
's	O
acceptance	B-Device
angle	I-Device
.	O
</s>
<s>
A	O
leaky	B-Device
ray	I-Device
or	O
tunneling	B-Device
ray	I-Device
is	O
a	O
ray	O
in	O
an	O
optical	B-Architecture
fiber	I-Architecture
that	O
geometric	O
optics	B-Algorithm
predicts	O
would	O
totally	O
reflect	O
at	O
the	O
boundary	O
between	O
the	O
core	O
and	O
the	O
cladding	B-Device
,	O
but	O
which	O
suffers	O
loss	O
due	O
to	O
the	O
curved	O
core	O
boundary	O
.	O
</s>
