Experiment 1: The Index of Refraction and Snells Law - Ray, Optics & Optical Instruments Video Lecture - Class 12

refraction is the bending of light rays
as they pass from one medium into
another this common phenomenon is
illustrated throughout the physical and
natural world such as in the apparent
bending of a pencil in a glass of water
or the displaced image of a fish in
water although you can point to a fish
underneath the water you are pointing at
an image not the fish unless you are
directly over the fish refraction is
also accompanied by a change in the
velocity of the light wave the ratio of
the speed of light in a vacuum sea to
the speed of light in that medium V is
called the index of refraction and is
given by n equals C divided by V where C
is the vacuum speed of light or 3.0
times 10 to the 8th meters per second
the speed of light in the medium is also
referred to as the phase velocity one
complication with respect to the
refractive index is that it is
wavelength dependent this effect leads
to chromatic dispersion or pulse
spreading and it is critical in
understanding the performance of fiber
optic systems the refractive indices of
some common media are given in table 1
note that the change of velocity in air
is negligible and that most common
glasses are between one point four five
and one point five one this means that
the light slows down to about two thirds
its vacuum speed in glass optical fiber
semiconductors used in
telecommunications usually have indices
greater than three
in 1621 Willow Birds Snell used the
properties of various transparent
materials to describe refraction
mathematically he observed the
relationship between incident and
refracted rays and assigned an index of
refraction to each material he
experimented with this graphical
representation shows refraction at the
interface of two media given by n1 and
n2 and then the relationship as
determined by Snell's law is given by n1
sine theta 1 equals n 2 sine theta 2
where theta 1 is the incident angle as
measured from the normal to the
interface and theta 2 is the angle of
the refracted ray as shown a closer
examination of Snell's law applied at
the interface of 2 media reveals some
important concepts regarding the
refraction and reflection of light the
results for light traveling from a more
dense into a less dense material and one
greater than n2 are significantly
different than the opposite case and 2
greater than n1 let us determine what
happens at the interface for the 2
possible conditions according to Snell's
law n2 greater than n 1 note that as the
light enters a denser medium as shown in
Figure 1 1 it tends to slow down and
bend toward the normal light is
transmitted and then refracted with an
incidence angle from 0 degrees to 90
degrees N 1 greater than n2 here light
bends away from the normal and speeds up
at some incident critical angle the
refracted ray is at 90 degrees and for
incident angles greater than this
critical angle the light is entirely
reflected
no light is transmitted into the second
material this result is quite different
than the simple refraction case and
forms the basis for propagation in
optical fibers
the purpose of this experiment is to
determine the refractive index of an
acrylic plate using Snell's law
measurements of the refractive angle for
several different incident angles will
be recorded and the material refractive
index will be calculated an average
refractive index will be determined and
the velocity of light in the material
will also be calculated
for this experiment you will need an
acrylic block a laser mount a
helium-neon laser several sheets of
white paper a protractor a ruler in a
meter stick first you must adjust the
height of the laser to about one inch
above the table and mount the laser
place a sheet of white paper on the
table approximately centered below the
beam path of the helium neon laser place
the acrylic plate on the sheet of paper
at an arbitrary angle around 45 degrees
to the laser beam and align the height
of the laser such that the beam passes
through the center of the acrylic block
without moving the paper or the block
trace the outline of the acrylic block
you
mark two points along both input and
output beams
you
turn off the laser and remove the
acrylic plate
your paper should look similar to the
work shown
use the ruler to draw straight input and
output lines
you
use a protractor to construct a normal
at the air acrylic interface and measure
and Mark incident and refracted angles
your paper should now look like this
you
determine the refractive index of the
acrylic using Snell's law and enter it
in Table two
repeat steps one through seven for two
additional incident angles
determine the speed of light in the
medium an average speed and average
refractive index and add this data to
Table two
as our experiment shows refraction and
Snell's law can be demonstrated using
relatively simple equipment but what
about that fish is that really just an
image mother nature and Snell's law both
tell us yes the fish we see is not
really there but it is actually closer
to us than it seems we hope that through
this experiment you have gained a better
understanding of just how refraction
works
you