Fig 1.4
Fig 1.4. Because light waves
travel at differently speeds in different mediums, at the boundary
of the higher index medium, the wave front must change speed,
in the case above, a beam of light enters a high - index material,
the wave front will slow down, and the beam will bend or what
is known as refraction. Refraction only occurs at the boundary
of a low - index medium to a high - index medium or vice versa.
When the beam is refracted, the wavelength changes, but is in
phase and they oscillate at the same frequency, this bends the
light in the direction shown. If the light beam passes back
out into the same low - index medium, then the waves become
longer (Speed up), and bend back to the same angle when it originally
entered the medium.
If light travelling in a medium
with a refractive index of n1 strikes the surface of a material with
a refractive index of
n2
at an angle of
q
1 to the normal the
light direction in the second material is given by
q
2 in the equation:
n1 sin
q 1 = n2 sin
q 2
The Above equation is known as
Snell's law
Snell's
Law of Refraction:
Where the arcsin is the
angle for which the stated value is the sin, and can be re-written
as:
q
2 = arcsin ( n1
sin q
1 / n2
)
Refraction
of white light in a prism
Fig
1.5
Fig 1.5. This diagram show how
a glass prism separates white light into is component colours,
taking the pervious details about refraction into account, we
know that light waves entering a higher - index material is
slowed down (refracted), also, different wavelengths travel
at different speeds within a given medium. As we know white
light is made up of different colours or wavelengths, with all
this in mind, we can implement these theories to the above diagram,
and these form the conclusion as to why a glass prism will split
white light into its component colours.
Refractive Index
Light travels more slowly in
matter, air at one atmosphere will cause light to slow down.
The ratio of the
speed of light
in a vacuum (c)
to the speed of light in a material (v)
is the Refractive Index:
Refractive Index =
c /
v
Because the speed of light is
faster in a vacuum than in material, the refractive index is
greater than one.
Refractive
Index of common materials
Material
|
Index
|
Dry Air
(1 atmosphere) |
1.000278 |
Ice (-8oC) |
1.309 |
Water
(0oC) |
1.33346 |
Methyl
Alcohol |
1.3284 |
Ethyl
Alcohol |
1.3614 |
Ethylene
Glycol |
1.4318 |
Glass,
Fused Silica |
1.459 |
Glass,
Pyrex |
1.474 |
Pespex |
1.488 |
Glass,
Crown |
1.52 |
Optical
glass |
1.51 ~ 1.81 |
Crystal
quartz |
1.544 ~ 1.553 |
Sapphire |
1.757 ~ 1.779 |
Ruby |
1.757 ~ 1.779 |
Glass,
Flint 71% pb |
1.805 |
Nd:YAG
@1064nm |
1.818 |
Cubic
Zirconia |
2.173 ~ 2.21 |
Diamond |
2.419 |
|
Table
1.0