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Determining the Refractive Index of Glass
Method · January 2016
DOI: 10.13140/RG.2.2.27749.63206
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Determining the Refractive Index of Glass
Snell's law allows us to investigate the relationship between angles of incidence and refraction for a
wave changing speed on an interface between two media with different indexes of refraction.
Aim: To measure the refractive index of glass by recording a set of readings of different angles of
incidence and refraction.
Hypothesis: Due to Snell’s law, the refraction (bending) of the light occurs because the light slows down
when it enters the glass, so the index of refraction is the ratio of the speed of light in air to the speed of
light in the glass and therefore refraction of light will be seen.
Variables: - Dependent variable: angle of incidence
- Independent variable: angle of refraction
- Control variables: type of glass, intensity of light, frequency of light, surface (table) where
the experiment is conducted and temperature
Equipment: Rectangular glass, protractor, light box, power box, ruler and plain A4 paper
Diagrams:
FOR EDUCATIONAL PURPOSES ONLY.
Introduction: The refractive index of a material is also known as the ratio of the sine of the incident
angle for light in a vacuum (air) to the sine of the refracted angle in that material. Snell’s law allows us to
predict the amount of refraction. The equation for this is:
sin  i
n
sin  r
© Nicolette Lunsingh Tonckens
1. Place the glass box in the middle of the paper and use a pencil to draw around the edges
2. Shine a single ray on to any point on the left side of the glass
3. Mark where the light enters and exits (also mark and follow the paths of light on the paper so
that you can draw a straight line with your ruler where it comes from, do the same on the right
side)
4. Repeat steps two and three for different angles and points on the block
5. Remove the block and connect the refracted lines (rays) from left to right
6. Using a protractor, draw the normal line (make sure it is perpendicular to the edge of the block)
and from the normal line, measure and record all the incident and refraction angles and record
them onto a table
Results:
Angle of incidence (º) ± 1º
34.0
35.0
36.0
38.5
59.0
Angle of refraction (º) ± 1º
21.0
23.0
23.5
24.0
37.0
Processed Data:
Angle of incidence (º) ± 1º
Angle of refraction (º) ± 1º
34.0
21.0
35.0
23.0
36.0
23.5
38.5
24.0
59.0
37.0
Average = (1.6 + 1.5 +1.5 + 1.5 + 1.4)/5
= 1.5
Refractive index ± 1
1.6
1.5
1.5
1.5
1.4
FOR EDUCATIONAL PURPOSES ONLY.
Method:
Conclusion and Evaluation: The graph shows a positive correlation; the greater the angle of incidence,
the greater the angle of refraction. The refractive indexes of all angles were more or less the same, yet
unfortunately I would have liked to try more angles to see if this would have affected the refractive
index. Nevertheless, the aim was obtained because I was able to measure the refractive index from this
experiment by applying Snell’s Law to my results. The values are rounded to a refractive index of 1.5 for
all angles which is a constant according to Snell’s Law. The graph is not valid enough to draw
conclusions but if it results in a curve then the gradient cannot be calculated and this proves Snell’ Law
correct. The experiment has also proven that light bends/refracts when it enters a different medium.
Measuring the accuracy of angles was a bit hard because if the mark was between one degree, it could
have ranged anywhere from 0.1-0.9 but I had to round it to 0.5. I also need to plot error bars the next
time I draw a graph. I chose the thinnest beam but it was still relatively thick, making it hard to mark the
exact point at which the light enters the medium. The uncertainties lie within marking the exact point of
the light rays, parallax erros and estimating where to draw the point. However, the results were quite
accurate because I compared it to others’ results and we all had similar results.
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FOR EDUCATIONAL PURPOSES ONLY.
© Nicolette Lunsingh Tonckens