MYIB / Honors Physics
Refraction of Light (Snell’s Law) Lab
Objective: To measure the index of refraction of acrylic in air.
Background:
When a light wave crosses a boundary between two media, it bends (or “refracts”) according to Snell’s
law:
𝑛1 sin 𝜃1 = 𝑛2 sin 𝜃2
The two angles are the angle of incidence 𝜃1 and the angle of refraction 𝜃2 (see the inset in Figure 4.1),
and the two coefficients 𝑛1 and 𝑛2 are the indices of refraction of the two media. For example, the index
of refraction of air is 1.00. In this lab, you will measure the angle of refraction for light hitting an acrylic
lens at various angles of incidence, then use Snell’s law to calculate the index of refraction of acrylic.
Materials:
 Optics Bench
 Light Source
 Ray Table and Base
 Component Holder
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Slit Plate
Slit Mask
Cylindrical Lens
Procedure:
1. Set up the equipment as shown in Figure 4.1. First, place the Light Source on the Optics Bench,
leaving the aperture uncovered. Attach the Slit Plate and the Slit Mask to the two sides of a
Component Holder and insert them between the Light Source and the Ray Table. The Ray Table
should be centered on the Ray Table Base so that it can be rotated freely.
2. Adjust the components so a single ray of light passes directly through the center of the Ray Table
Degree Scale. Align the flat surface of the Cylindrical Lens with the line labeled “Component” and
rotate the ray table so that the incoming ray is incident on the flat surface of the lens at a 0° angle.
With the lens properly aligned, the beam will pass directly through the lens and out the other 0° mark.
3. Record the initial angle of incidence (0°) and angle of refraction (0°) for this initial state in your data
table for light traveling from air into acrylic.
4. Without disturbing the alignment of the lens, rotate the Ray Table in 10° increments and record the
angle of refraction for each angle of incidence between 10° and 80°.
5. Now, rotate the Ray Table so that the incident light enters perpendicular to the curved surface of the
lens, as shown in Figure 5-2. The light will be refracted as it passes from acrylic back to air at the flat
surface. Start a new data table and record the angle of incidence and angle of refraction for light
traveling from acrylic into air, starting at 0° and increasing the angle of incidence in 10° increments.
6. At some point, the light will no longer reflect. The incident angle for which light no longer travels
from light into air is called the critical angle of acrylic. Whenever the angle of incidence exceeds this
amount, the light will be completely reflected by the flat surface. This is known as total internal
reflection. Record the approximate value of the critical angle below your second data table.
Data Analysis:
1. For each angle of incidence 𝜃1 and angle of refraction 𝜃2 in table one (air into acrylic), calculate and
record the index of refraction 𝑛2 of acrylic using Snell’s law. (Use 𝑛1 = 1.00 as the index of
refraction for air.) Average your values together to determine the experimental index of refraction of
acrylic when light travels from air into acrylic.
2. Repeat the calculations for each angle of incidence 𝜃1 and angle of refraction 𝜃2 in table two (acrylic
into air), calculating the index of refraction 𝑛1 of acrylic using Snell’s law. (This time, the index of
refraction for air is represented by 𝑛2 since the light is traveling from acrylic back into air.) Average
your values to determine the experimental index of refraction of acrylic when light travels from
acrylic into air.
3. Using the average index of refraction you calculated in step two (acrylic into air), determine the
theoretical critical angle for acrylic in air: 𝜃𝑐 = sin−1(1/𝑛).
Conclusion:
Your conclusion should address the following points.
 According to your data, what is the index of refraction of acrylic?
 Do your two calculated values of index of refraction (air into acrylic, acrylic into air) match?
 Based on observations during the lab, what was the approximate critical angle for acrylic in air?
 Does the observed critical angle match the theoretical critical angle you calculated above?
Your conclusion may additionally address the following points.
 Any observations you make regarding the reflected rays at any point during the experiment.
 Any other observations you deem relevant to the experiment.