Name___________________________
Score____________
Date _______________
Lab Partner____________________________
AP Physics – Lab 9
Refraction
Objectives:
• To test the law of refraction, Snell’s
law.
• To measure the index of refraction.
Physical Principles:
Light always travels in a straight line through
any uniform medium. However, when light
crosses from one material to another, its path
will bend by an amount determined by both
materials. This “bending” of light is called
refraction. The interface between two materials
also causes reflection of light. See Fig. 1. The
law of reflection states that the incident angle θi
is equal to the reflected angle θ2:
θi = θ2
(1)
Figure 1 Reflection and refraction of light when crossing
from one medium into another
The law of refraction, referred to as Snell’s Law,
states the following relationship between the
incident angle θi and the refracted angle θr:
n1 sin(θ1 ) = n 2 sin(θ 2 ) (2)
The parameter n represents the index of
refraction, its value being an inherent property of
the material. For example, the index of refraction
for air is 1.00, for pure water 1.33 and for crown
glass it is 1.52.
Procedure:
1. Index of Refraction of Glass
Take a sheet of 8.5 × 11 inch paper and draw a
set of perpendicular lines through its center.
Tape this sheet to the plotting board. Place the
square glass plate so that the clear face is
centered along the 8.5 inch line.
Place a pin at point P on the back side of the
Figure 2 Refraction of light crossing from air to glass
glass plate. Direct the laser through the glass
plate so that it hits pin P, mark the path of the
laser beam. Warning: Do not shine the laser beam at anything (or anyone) other than the
experiment. Failure to abide by this rule will result in loss of credit. Repeat with the laser
beam directed at a second angle, as shown in Fig. 2. Mark the points necessary to be able to
draw the lines in Fig. 2. Measure and record the dimensions shown in the figure. The distance Da
is called the apparent depth.
D = _______________.
θi = _______________.
Da = _______________.
θr = _______________.
X = _______________.
Compute the value of the index of refraction for class according to the formula:
n2 =
n1 sin(θ i )
sin(θ r )
(3)
Record this index of refraction and compare it with the theoretical value nt = 1.52.
Calculated index of refraction = _________________
%diff =
n t − n calc
×100 = __________________
nt
2. Index of Refraction of Water
Take a sheet of 8.5 × 11 inch paper and draw a set of
perpendicular lines through its center. Place this sheet on the
plotting board and secure it with masking tape. Draw five lines
on the paper, representing incident rays with angles 10°, 25°,
40°, 50°, and 60° so that they all meet at the intersection of the
perpendicular lines. See Fig. 3.
Fill the plastic semi-disk container with water and place it so
that the flat side is centered along the 8.5 inch line. Trace the
curved edge on the paper. Direct the laser beam toward the
Figure 3 Index of refraction of water
flat side of the semi-cylinder along the line that makes a 10°
experiment
angle from the normal to this flat side. Mark the point where
the light exits the semi-cylinder on the curved side. Repeat this process for each incident angle.
You will have five dots on the curved side of the semi-cylinder so make sure you mark the
corresponding incident angle for each one. Remove the container and draw lines through these
points and the center of the paper (where the perpendicular lines intersect). These are the
refracted light beams. Measure the angles of refraction (θr) and complete Table 1 below. Using
Graphical Analysis software plot a graph of sin(θi) on the vertical axis versus sin(θr) on the
horizontal axis and find the slope of the line. Compare this slope with the accepted value of the
index of refraction of water na = 1.33. Print out the graph and attach to this report.
Table 1: Refraction in water.
Incident angle θi
Refracted angle θr
sin(θi)
sin(θr)
10°
Slope of the line ncalc = _______________,
%diff =
n a − n calc
×100 = _________________.
na
Conclusions:
Staple the two drawn sheets and one Graphical Analysis graph to this report. Explain in two or
three sentences what you learned from this lab.