Index Of Refraction Of Glass
The index of refraction of a substance is defined as the ratio of
the speed of light in a vacuum to its speed in that substance.
Since the speed of light in air is only slightly different from the
speed in a vacuum, a negligible error is introduced when we
measure the index of refraction by permitting light to travel from
air into another medium.
Wiliebrord Snell provided a simple, direct method of measuring
the index of refraction by defining it in terms of functions of the
angle of incidence and the angle of refraction. The
mathematical relationship, known as Snell’s law, is
sin i
n
sin r
where n is the index of refraction, i the angle of incidence, and r
the angle of refraction
AC
DB
sin i 
and sin r 
AO
OB
Since AO and OB are radii of the same circle, they are equal. Therefore,
sin i AC
n

sin r DB
The index of refraction of glass varies with its composition and with the wavelength of the light
incident on the glass. Ordinary crown glass, when illuminated by white light, has a refractive
index of 1.52; medium flint glass has an index of 1.63. Your experimental results should be
precise enough to enable you to identify the glass specimens used in this experiment.
Objective
After completing this experiment, you should be able to determine the index of refraction of a
refractive material such as glass by means of refracted light rays.
Materials
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5-cm glass cube
metric ruler
drawing compass
pencil type protractor
pins
drawing paper
Procedure
1. Place the glass plate or cube on the center of a sheet of unlined paper and outline it
with a sharp-pointed pencil. About 1 cm from the lower left-hand corner of the plate,
place a pin, F as close to the glass as possible.
2. At a point about 1 cm from the right-hand corner of the plate, place a second pin 0 as
close to the glass as possible.
3. At a point not less than 7 cm from pin O, place a third pin G so that pins F, O and G are
perfectly aligned as seen through the plate. This is done by looking through point F at
an angle toward point O, with the sighting eye near the level of the table top while
positioning pin G. It is important, to avoid aligning the three pins as seen above the
glass plate. Suggestion: Try locating the best final position of pin G by holding it upright
on the paper (not embedded) the appropriate distance beyond pin 0 and approximately
G
6.
7.
8.
9.
aligned with pins F and 0 as viewed
above the glass, then looking through
the glass plate along the sightline of
pins F and 0, slowly move pin G in a
clockwise arc until it is observed to
pass across the FO sightline.
Determine the final alignment point for
pin G by moving it slowly back and forth
across the FO sightline.
4. Remove the glass plate and join the
points F, 0, and G to represent the path
of the light ray traveling from pin G
through air to pin 0 and through the
glass to pin F. Label the incident ray
and refracted ray.
5. Construct the normals NO and ON’.
Using as large a radius as possible, describe a circle with point 0 as the center that
intersects OF, ON’, OG, and ON.
From the point of intersection of the circle with the incident ray 0G. draw a line x perpendicular to ON. Measure the length of x to the nearest 0.01 cm and record it in the
data table.
From the point of intersection of the circle with the refracted ray OF, draw a line y
perpendicular to the normal ON’. Measure the length of y to the nearest 0.01 cm and
record its value.
Using a protractor, measure the angles of incidence i and refraction r to the nearest
0.5. Record in the data table.
Data Table
Trial
X
(cm)
Y
(cm)
i
()
r
()
Calculations
1. Compute the index of refraction of your glass plate from the measured values of x and y
and again from the measured values of angles i and r. Record both results in the
calculations table. Identify the kind of glass.
2. Calculate the absolute and relative error.
Trial
Index of
Refraction
(x/y)
Index of
Refraction
sin i/sin r
Kind of glass
Absolute
Error
Relative
Error
(%)