Using a Laser to Measure the Speed of Light in Gelatin
Introduction
The law of refraction, which is also known as Snell's law, actually applies to everyday life. For
example, before you answer the door and see your friend's face through the window, you see light that is
refracted through the glass. Snell's law compactly describes what happens to the trajectory of a beam of
light as it passes from one medium, such as air, to another, such as glass. As you apply Snell's law and
the definition of index of refraction in this science project, you will be able to measure the speed of light
in gelatin. The beauty of this science project also lies in how you can verify one of the most basic laws of
optics, experimentally, by using readily available and inexpensive materials.
Note that Snell's law not only applies to the case of the laser beam passing through air and
gelatin, but also to other examples of how the incident object changes direction as it passes from a
faster medium to a slower medium, and vice versa. For example, a marching band walks together in time
with the music and take the same-length steps. What if the band moves across a grassy football field at
an angle, and as each band member crosses the 50-yard line, he or she suddenly finds the field very
muddy and slippery? As a result, he or she steps in time but takes steps that are 20 percent shorter
because of the mud. What happens then? Answer: Those who have crossed the 50-yard line are
traveling at 80 percent the speed of those who have not, and the line of band members bends at the 50yard line, just like light in this experiment. With a little thought, one can even compute the angle at which
the line bends (actually the reverse of what you will be trying to do in this science project).
Terms and Concepts

Law of refraction (also called Snell's law)

Trajectory

Index of refraction

Incident object
Questions

What exactly is a laser?

What is the speed of light? What are some methods scientists have used to calculate it?

What are some applications of Snell’s law?
Resources
This resource provides more information about lasers:

Wikipedia Contributors. (2010, July 6.) Laser. Wikipedia: The Free Encyclopedia. Retrieved July
12, 2010, from http://en.wikipedia.org/w/index.php?title=Laser&oldid=371975776
To learn more about Snell’s law, try these links:

Wolfram Research. (n.d.). Snell's Law. Retrieved July 12, 2010, from
http://scienceworld.wolfram.com/physics/SnellsLaw.html

Kaiser, Peter K. (n.d.). Snell's Law. Retrieved July 12, 2010, from
http://www.yorku.ca/eye/snell.htm

Nave, R. (n.d.). Snell's Law. Retrieved July 12, 2010, from http://hyperphysics.phyastr.gsu.edu/hbase/geoopt/refr.html#c3

The Physics Classroom. (n.d.). The Mathematics of Refraction: Snell's Law. Retrieved July 12,
2010, from http://www.physicsclassroom.com/Class/refrn/u14l2b.cfm
The links below contain additional information about the index of refraction:

Reed, R. (n.d.). Refraction of light. Retrieved July 12, 2010, from
http://interactagram.com/physics/optics/refraction/

Wolfram Research. (n.d.). Index of Refraction. Retrieved July 12, 2010, from
http://scienceworld.wolfram.com/physics/IndexofRefraction.html

Nave, R. (n.d.). Index of Refraction. Retrieved July 12, 2010, from http://hyperphysics.phyastr.gsu.edu/hbase/geoopt/refr.html#c2
Materials and Equipment

A device that produces a visible laser beam, such as a laser pointer or a laser level: (for example,
search for "laser pointer" at Amazon.com)
Nowadays, many kinds of lasers are readily available on the market. Typically, lasers are
classified by their wavelength and maximum output power, which put them into one of several
classes e.g. class I, II, IIIa, IIIb, IV (please see Laser Safety Guide) . When handling lasers,
please keep in mind the safety measures that must be followed in order to prevent injuries.

A mounting device on which the laser device rests that can easily indicate where the beam is
pointed (it will be difficult to actually see the laser beam passing through air)

A protractor or a homemade protractor that can easily indicate the angle of refraction inside the
gelatin

Gelatin, a clear or a light/transparent color would generally work best

Plastic containers to mold the gelatin (of various shapes and sizes if pursuing one of the possible
variations below)
Experimental Procedure
Caution: Adult supervision is recommended. Even low-power lasers can cause permanent eye damage.
Please carefully review and follow the Laser Safety Guide.
Below is an outline of one of the ways of carrying out the experiment; again, there are multiple variations
and areas where you can insert your own creativity (see below):
1. First, come up with your own experimental setup. In addition to understanding the theory behind
the experiment, this project calls for much experimental design creativity and hand-on "playing
time." Please think about how one would measure the angle of incidence of a laser passing
through the gelatin, with reference to the normal (the line that is perpendicular to the surface of
the medium). Also, please consider how one could precisely direct the laser beam from the laser
pointer (or laser level) at a pre-determined "entry point." What type of mounting structure would
you come up with?
For example, the photo below (using a tub of liquid instead of gelatin) represents a clever setup.
The laser beam was originally along the line between the tiles on the countertop (you could also
use graph paper). You can see a bit of the beam exiting the laser in the center of the circle and
you can see the entry point of the laser in the plastic container. These two points show the
original path of the beam, and you can easily connect the points to create the original trajectory
(note that the normal, which is not shown, is the line that runs perpendicular to the side of the
plastic container that gets hit by the laser). Thus, we can accurately measure the angle here.
2. Make your gelatin according to the directions on the box. Remove the gelatin from the container
when it has set.
a. Note: Try to make the gelatin in a square container (like the container shown in the photo
above). If a square container is not available, use a large container and cut the gelatin in
a square or "box" shape with edges that are clean and vertical. Slanted and uneven
edges may cause disruption of the laser beam, so it is important that the form of the
gelatin be correct.
b. Note: The gelatin can be deformed if you are not careful when removing it from the
container—or deformed by the tools you use to remove it. One way to remove the gelatin
from the container is to set the container in hot water and let it float on the surface for a
bit. This softens the edges of the gelatin inside the container, allowing you to turn the
container upside-down to release the gelatin.
3. Mount the laser pointer on a pre-made device that will indicate where the beam is going and what
the angle of incidence is (recall that it will be difficult to actually see the laser beam passing
through air).
a. Note: It is important that the laser beam be perpendicular to the surface for proper
refraction results.
4. Fix the laser device and record the angle of incidence with respect to the normal.
Variations (Optional)
There are many areas of this project where one can use his/her own creativity:

Laser mounting device. Make your own device to ensure that the laser beam precisely enters
the gelatin at the pre-determined entry point.

Measuring scheme for the angle of refraction. Design your own scheme to measure the angle
of refraction inside the gelatin. For example, where would you place a custom-made protractor to
indicate the angle of refraction with respect to the normal?

Faster or slower gelatin. Attempt to change the index of refraction of the gelatin and create
"faster" or "slower" media. For example, what happens when one dissolves various amounts of
sugar in a well-mixed gelatin solution? Does the index of refraction of the gelatin change?
Consequently, how does the speed of light in gelatin change? Note that one can represent the
concentration of the sugar-gelatin solution by doing a one-step percent composition by mass
calculation, which is basically the mass of the solute divided by the mass of the solution (mass of
solute plus mass of solvent), multiplied by 100.
For example: Determine the percent composition by mass of a 100 g salt solution which contains
20 g salt
Solution: 20 g NaCl / 100 g solution x 100 = 20% NaCl solution
From the table shown below, we see that the index of refraction for a 30% sugar solution is 1.38
while for an 80% solution, is 1.49. Can you verify a positively-correlated trend for sugar?

Other readily available materials. Calculate and verify the indices of refraction of other readily
available materials like household liquids or ice. For example, ethyl alcohol has an index of
refraction of 1.36, while ice's is 1.31. Can you verify this? A table of indices of refraction for
commonly found materials is shown below. Note that extra caution must be taken if you are using
a plastic container to hold the liquid as the container itself serves as an interface between the
laser and the media. Please think about ways to minimize this distortion. For example, can you try
to aim the laser at the surface of the liquid (at an angle with respect to the normal) from above?
How would you measure the angle?
Indices of refraction for commonly found materials:
Material
Vacuum
Air at STP
Ice
Water at 20°C
Acetone
Ethyl alcohol
Sugar solution (30%)
Fluorite
Fused quartz
Glycerine
Sugar solution (80%)
Plexiglas
Crown glasses
Sodium chloride
Polystyrene
Carbon disulfide
Flint glasses
Methylene iodide
Sapphire
Rare earth flint
Lanthanum flint
Arsenic trisulfide glass
Diamond
Gallium phosphide

Index
1.00000
1.00029
1.31
1.33
1.36
1.36
1.38
1.433
1.46
1.473
1.49
1.51
1.52-1.62
1.54
1.55-1.59
1.63
1.57-1.75
1.74
1.77
1.7-1.84
1.82-1.98
2.04
2.417
3.50
Making lens cross-sections out of gelatin, then "ray trace" with the laser. Basically, ray
tracing involves establishing the position and orientation of an object's image by tracing strategic
rays of light from the object passing through the lens, using knowledge of the focal length of the
lens and the position of the object. Here is an introduction:
http://boson.physics.sc.edu/~rjones/phys153/raytrace.html
5. Shine the laser through the gelatin (you may need another person to help out by holding down
the button if you use a simple laser pointer) and measure the angle of refraction inside the
gelatin.
6. Find the speed of light in gelatin: first use Snell's law to calculate the index of refraction of the
gelatin and then apply the definition of index of refraction to find the speed of light in the medium.
Notes:

Measuring the angle from the normal can be tricky. One must remember that if one looks directly
into a refracting medium, i.e. perpendicular to the surface, the angles are accurate, but if one
looks off the normal, the angles are distorted. For example, a straight stick thrust into water looks
broken at the surface. Therefore, it is important to set up the angle measuring device properly so
accurate readings can be obtained.

Having the gelatin out of the container gives a clean optical interface, but gelatin may not stay
straight once it is out of the mold. If the sides bulge significantly, the angles will be distorted.

Note: Gelatin may start to run if set in room temperature for too long!
Final Report
Your final report will include these sections:

Title page.

Abstract. An abstract is an abbreviated version of your final report (N.B. Write the abstract section
last, even though it will be one of the first sections of your final report.

Table of contents.

Question, variables, and hypothesis.

Background research (i.e. theory section or literature review)

Materials list.

Experimental procedure.

Data analysis and discussion. This section is a summary of what you found out in your experiment,
focusing on your observations, data table, and graph(s), which should be included at this location in
the report.

Conclusion(s).

Bibliography. This section should include any references used, including any from the “Resources”
section and any additional references. Use a recognized bibliographical formatting style (i.e. MLA,
APA, ApJ)