CHAPTER VI

INDEX OF REFRACTION
Theory.—When a ray of light passes from one transparent
medium into another of different density, at the surface of nqMuni-
tion the ray is always bent from its first course, unless it strikes
this surface at an angle of 90°. This phenomenon in known as
optical "refraction." The angle i, included between th<»
incident ray and the normal to the separating surface (we Fig.
30) is the angle of incidence while the angle r, between the
refracted ray and the normal, is the angle of refraction. The ratio
n
- is the index of refraction.
If s and .5' represent, respectively, the speed of light in the*

medium from -which the light
emerges and in that into whirh it
passes, then the Index of refraction
of the latter medium with respect
to the former ib greater than 1 if .v
is greater than #', mid conversely.
In a general way the* speed of light
varies inversely with the* density
of the medium, although this is not
a strict mathematical relation.
fig. 30.—Illustrating refraction
of light.
It will thus be neon that the* con-
ception of index of refraction must
involve two   suhBtunww  and  that
its  value   will   depend   upon   the
density   of   each.       ah   ordinarily
used,   it   is   undcrHtcxw!   thitt   the
light  emerges   from   ordinary  air
into the substances  who«c index in
being measured and the term "index of refraction'^1 therefore
signifies  the  ratio  of the angle of incidence  from air, to flic
angle of refraction in the substance under cozu*itlc;rutum.
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