8. Technical Information
40
8. Technical Information
8-1. Terminology and References
About Refractive Index
Refractive index (n) is the ratio of wave velocity of light ray in vacuum to that of the
medium. For convenience sake, air is used as reference medium.
When light comes from the air into water,
the light changes its direction at the
boundary.
(Optically “from air to water” means from
“thin to thick” medium.)
Suppose the incoming angle of light is
( α )and refraction angle( β ), these two
angles are related as follows (Snell’s law):
( Eq. )
n
n
1
2
＝
sin
sin
If light travels from thick to thin medium,
the light also changes its direction. As the
angle of incidence increases( β ), some rays
do not pass through thin medium reaching
critical angle. ( refraction α=90° at this
point )Those rays beyond critical angel are
called Reflection.
Critical angle ( β ) is used to compute RI.
( Eq. ) α
＝
90
ﾟ
» sin α
＝
1
( Eq. )
sin
2
1
n
n
Refractive index depends on wave length of light and temperature. Measuring condition
needs to be specified. For example, measure with Na-D line ( 589.3nm)at 20 degrees
Celsius is expressed by n
D
20
.
Summary of Contents for RA-620
Page 2: ......
