Activity: Refraction
Purpose
To use Snell’s Law to determine the index of refraction of the acrylic Prism.
Equipment
Ray box, Prism, Protractor, White paper
Theory
According to Snell’s Law, n1sinq1= n2sinq2, where q1 is the angle of incidence, q2 is the angle of refraction, and n1 and n2 are the respective indices of refraction of the materials. The angle of refraction depends on the angle of incidence and the index of refraction of the material. See Figure 2.1. Because the index of refraction for light varies with the frequency of the light, white light which enters the material at a given angle of incidence will separate out into its component colors as each frequency is bent a different amount.
The Prism is made of Acrylic, which has an index of refraction of 1.497 for light of wavelength 486 nm in a vacuum, 1.491 for wavelength 589 nm, and 1.489 for wavelength 651 nm (red). Notice that in general for visible light, the index of refraction for Acrylic increases with increasing frequency.
Procedure
Snell’s Law
1. Place the Prism on a sheet of paper and draw a sharp pencil line around it. Make a dark dot somewhere on the square edge. Use a protractor to measure 90° from the surface at that point; this will be your normal to the incident ray.
2. Use the protractor to measure angles in 10° increments from the normal down to the block’s surface. These will be your incident rays.
3. Place the ray box, label side up, on a white sheet of paper on the table. Adjust the light box so that only one beam of light exits the box. Line this up along the 10˚ line.
4. Place the Prism on the table and position it so the ray passes through the parallel sides as shown in Figure 4.2.
5. Make 1 dark mark on the light beam exactly where it exits the far side of the glass. Make a second mark somewhere else on this light beam. Use a ruler to connect these marks.
6. Remove the Prism and on the paper draw a line connecting the points where the ray entered and left the Prism.
7. Measure the angle of incidence (q1) and the angle of refraction (q2) with a protractor. Both these angles should be measured from the normal.
8. Repeat the above for the remaining angles.
DATA TABLE A:
(air)
(°) / Angle of Refraction (Acrylic)
(°) / Sin qi / Sin qr /
nr
from graph
/% error
1020
30
40
50
60
70
80
Calculations
· Make a graph of sin θi vs sin θr.
· Use your graph of sin θi vs sin θr to find the equation of the line. Record this equation and the correlation coefficient.
· Show why the slope of the line is the index of refraction of acrylic. Start with Snell’s Law, rearranging it to get it in the form of sin θi vs sin θr. (Remember that graph titles are y vs. x.)
· If the index of refraction of acrylic is actually 1.50, what is the percent error? (If it is greater than 10%, you need to redo your data collection!)
Analysis Questions
1) How does the angle of incidence compare to the angle of refraction when light travels from a medium of low optical density (air) to a medium of high optical density (acrylic)?
a. Choose one: always smaller, always bigger, or always constant.
b. Provide evidence from the lab.
2) How does the angle of incidence compare to the angle of refraction when light travels from a medium of high optical density (acrylic) to a medium of lower optical density (air)?
a. Choose one: always smaller, always bigger, or always constant.
b. Provide evidence from the lab.
3) What would happen to a light that entered the acrylic along the normal?
a. The light refracted towards the normal.
b. The light refracted away from the normal.
c. The light only reflected off the plate.
d. The light did not refract, but went straight.
4) Discuss how the angle of refraction changed with the angle of incidence. As the angle of incidence increased, the angle of refraction
a. Increased.
b. Decreased.
c. Remained the same.
ActivityRefraction.doc