Name
AP Physics B - Adkins (ML@ETHS)
Refraction Lab
Purpose
To investigate Snell's Law.
Materials
Semi-circular tank
Ray box
Plastic Semi-circle
Polar graph paper
Liquid
Plastic Rectangle
Laser (blue or green)
Procedure
1. Make sure that your tank is 3/4-full with water and set it on the polar graph paper. The flat side of the tank
should be parallel to the short side of the paper with the center of the flat side of the tank on at the center of
the polar graph paper. (as shown below)
2. Plug in the ray box and adjust the end piece so that a single ray shines out from the one side.
3. Direct the ray along line perpendicular to the center of the flat side of the tank (an incident angle of zero
degrees). Record the incident angle. Record the angle the beam makes in the water (with respect to the
normal to the flat side).
4. Now move the ray box so that it strikes the flat side of the tank at an angle of 20° with the perpendicular to
the side. Make sure that the ray enters at the center of the flat side. Measure the angle the refracted beam
in the water makes with the normal. Since the beam hits perpendicular to the curved side, it does not
bend. Thus the angle on the paper is the same as the angle in the water.
5. Repeat step 4 each incident angle and record the refraction angle for each of the above incident angles.
6. Now aim the ray from the ray box into the circular side of the tank so
that it passes through the water and strikes the middle of the flat side.
Vary the angle until you find the largest angle (as measured from the
middle of the curved side) for which the light emerges from the flat
side out onto the paper again. This is called the critical angle for the
water/air interface. Record the numerical value for this critical angle.
tank
7. Repeat steps 1-6 for the plastic material.
ray
box
polar graph paper
Data
Water
Incident
Angle
i
Refracted
Angle
r
Plastic
Incident
Angle
i
0
0
20
20
40
40
60
60
80
80
Critical Angles: Water
Refracted
Angle
r
Plastic
Position the rectangular plastic shape on the rectangle below. Positioning a laser (either blue or green) over the
‘laser’ point shine the laser into the rectangle at position A. You should have (3) separate rays leaving the
rectangle. Trace each one of them onto your paper.
Position laser
along this line.
Calculations:
Answer each of the following questions for each of the two different materials used.
1. Make a new table which includes the sin i and sinr.
Water
sin i
Plastic
sin r
sin i
sin r
2. Graph sin i vs. sin r on attached graph paper or using a computer. Either by hand or by computer, add a
best fit line to your data, and show the equation of the line. Create one graph for each set of data.
3. Use the equation of the best fit line to calculate the experimental value for the index of refraction of the
water and the plastic. Show or explain calculation.
nexp (Water) =
nexp(plastic)=
4. Use the experimental index of refraction to calculate the theoretical value for the critical angle of each
material. Compare this to your experimental determination from step 6.
c(theoretical,water)=
c(theoretical, plastic) =
What is the percent difference for each of these waters? Utilize the measured value from step 6 as "Your
result" and the values from question #4 as "Accepted Value".
%𝐸𝑟𝑟𝑜𝑟 = (
% error =
|(𝑌𝑜𝑢𝑟 𝑟𝑒𝑠𝑢𝑙𝑡) − (𝐴𝑐𝑐𝑒𝑝𝑡𝑒𝑑 𝑉𝑎𝑙𝑢𝑒)|
) (100)
(𝐴𝑐𝑐𝑒𝑝𝑡𝑒𝑑 𝑉𝑎𝑙𝑢𝑒)
% error=
5. Calculate the speed of light in each of the materials.
vwater=_______________________
vplastic =_______________
6. Explain why it is important that we used a semi-circular tank for our experiment.
7. Label the three rays that you measured using the laser and the rectangular block. Explain why there are
three different rays exiting the block when only one ray has entered.
EXTRA CREDIT:
The rectangular block and the semicircle block are made of the same plastic. Measure the angles formed by
each of the rays. Using the Law of Reflection and Snell's Law justify mathematically the angles created by
each of the three rays that you traced onto your diagram.