Lab 8: Index of Refraction/ Snell’s Law
Background: When light rays pass obliquely (at an angle) from one medium to
another, they are refracted or bent at the boundary between the two media.
When a ray enters a more optically dense medium it is bent toward the normal;
when a ray enters a less optically dense medium, it is bent away from the
normal.
The index of refraction of a substance is the ratio of the speed of light in a
vacuum to the speed of light in the substance, or
n=c/vsubstance
The index of refraction is therefore always greater than one (why?). The index
of refraction is a component of Snell’s Law, which states that
nsinQ=n’sinQ’
where n and Q are the index of refraction and the angle of incidence of the first
medium, and n’ and Q’ are the index of refraction and the angle of refraction
for the second medium. If n=1 (which it does for air), then
n’=sinQ/sinQ’
Purpose: to measure the angles of incidence and refraction of light rays
incident on a glass plate., and to use Snell’s Law to calculate the index of
refraction of glass.
Procedure:
Fig 1
Fig 2
1. Make a data table that will contain the angle of incidence, angle of refraction
and the sin of each of those angles as well as your calculated index of
refraction.
2. Place the glass plate in the center of a sheet of white paper. Trace the
outline of the glass in pencil.
3. Remove the plate and construct a normal NB at the top left of the
outline (Fig 2).
4. Using a ruler and protractor, draw a heavy line AB at an angle of 30° to
the normal. Angle ABN is the angle of incidence (Q).
5. Replace the glass plate over the outline on the paper. With your eyes on
a level with the glass plate, sight through the glass plate until you can
seethe line AB through the glass. Sight your ruler along the line so that it
appears to be a continuation of the line. Draw line CD with the ruler (fig1).
6. Remove the glass plate and draw a line connecting lines CD and AB
under where the plate was. Extend the normal NB to the other side of the
line.
7. Measure the angle CBN. This is the angle of refraction (Q’). Record this
angle in your data table with the angle of incidence (Q=30°). Use your
calculator to get the sine of Q and Q’, and record in your table. Calculate
the index of refraction using the ratio of sinQ/sinQ’, and record this index
of refraction.
8. Turn the paper over and repeat steps 2-7 for an angle of incidence of
45°.
Lab Report Notes:
Fasten your diagrams into your lab book; make sure each partner has a
copy. Also make copies to put in your lab report. Your report should
include the usual sections. Your Results section should have your data
table as well as the diagrams. Calculate your percent error based on the
accepted value of the index of refraction of glass (given in class). In
addition to your discussion, answer the following conclusion questions in
your report.
Conclusion Questions
1. How well do your two values of n’ agree?
2. Do your diagrams match the expected direction of refraction
discussed in the background? Explain.
3. Using one of your n’ values, calculate the speed of light in glass.
How does it compare to the speed of light in air?
4. Is the path of the “ray” you have drawn on your diagram
reversible? Justify your answer.
5. If the faces of the glass were not parallel, would your calculations
provide the same index of refraction or not? Discuss.