Jump to main content

Using a Laser to Measure the Speed of Light in Gelatin

1
2
3
4
5
2,322 reviews

Abstract

Think it takes expensive, sophisticated equipment to measure the speed of light? Think again! Outfit yourself with a simple handheld laser pointer, a protractor, and gelatin, and you're ready to get started.

Summary

Areas of Science
Difficulty
 
Time Required
Average (6-10 days)
Prerequisites
Basic understanding of optics and trigonometry
Material Availability
Readily available
Cost
Low ($20 - $50)
Safety
Adult supervision is recommended. Even low-power lasers can cause permanent eye damage. Please carefully review and follow the Laser Safety Guide.
Credits

Shijun Liu, Science Buddies

Harvey Lynch, Stanford Linear Accelerator Center (SLAC)

Objective

The objective of this science project is to measure the speed of light in gelatin by using an inexpensive laser such as a laser pointer or a laser level.

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.

Snell's law is expressed as the following equation (see Figure 1 below for an illustration of the variables):

Equation 1:


Diagram shows light changing directions as it passes through mediums that have a different index of refraction
Figure 1. Definitions of the variables used in Snell's law. An incident light beam approaches the interface between two materials at angle θ1 to the normal. It passes through the interface and is refracted to angle θ1 to the normal. The first material has an index of refraction n1 and the speed of light in the material is v1. The second material has an index of refraction n2 and the speed of light in the material is v2 (image credit Wikimedia Commons user Oleg Alexandrov).

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 50-yard 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

Questions

Bibliography

This resource provides more information about lasers:

To learn more about Snell’s law, try these links:

The links below contain additional information about the index of refraction:

Materials and Equipment

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.

    A red beam of light is visible as it passes through a plastic container filled with a clear liquid
    Figure 2. One possible setup for the experiment, using a plastic container of clear liquid instead of gelatin.
  2. Make your gelatin according to the directions on the box. Remove the gelatin from the container when it has set.
    1. 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.
    2. 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. 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.
  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).
    1. 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.
  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.
    1. Note: 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.
  6. Find the speed of light in gelatin: first use Snell's law (refer to the Background tab) 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.
icon scientific method

Ask an Expert

Do you have specific questions about your science project? Our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

Variations

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?

MaterialIndex
Vacuum 1.00000
Air at STP 1.00029
Ice 1.31
Water at 20 °C 1.33
Acetone 1.36
Ethyl alcohol 1.36
Sugar solution (30%) 1.38
Fluorite 1.433
Fused quartz 1.46
Glycerine 1.473
Sugar solution (80%) 1.49
Plexiglas 1.51
Crown glasses 1.52-1.62
Sodium chloride 1.54
Polystyrene 1.55-1.59
Carbon disulfide 1.63
Flint glasses 1.57-1.75
Methylene iodide 1.74
Sapphire 1.77
Rare earth flint 1.7-1.84
Lanthanum flint 1.82-1.98
Arsenic trisulfide glass 2.04
Diamond 2.417
Gallium phosphide 3.50
Table 1. Indices of refraction for commonly found materials.
  • 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

Careers

If you like this project, you might enjoy exploring these related careers:

Career Profile
You can send emails and text messages around the world in a matter of seconds. Surgeons routinely perform laser surgery that results in less bleeding and faster healing time. Our Air Force defends our skies using electronics-based aircraft that can almost fly themselves! What is one of the connections among these advances? Photonics technology! Photonics is the science of using light and electronic devices to create tools that improve our lives. Isn't it amazing that light can solve so many… Read more
Career Profile
Do you enjoy watching cable television, texting on your phone, and surfing the Internet? Do you know anyone who has had eye surgery and been back to normal the next day? Many of the advances in telecommunications and medicine are due to laser and fiber-optic technology. This technology has led to devices that provide faster and richer communication, advanced surgeries, and faster healing times, as well as amazing robotics for manufacturing. But, as with all equipment, someone has to install and… Read more
Career Profile
Physicists have a big goal in mind—to understand the nature of the entire universe and everything in it! To reach that goal, they observe and measure natural events seen on Earth and in the universe, and then develop theories, using mathematics, to explain why those phenomena occur. Physicists take on the challenge of explaining events that happen on the grandest scale imaginable to those that happen at the level of the smallest atomic particles. Their theories are then applied to… Read more
Career Profile
Our universe is full of matter and energy, and how that matter and energy moves and interacts in space and time is the subject of physics. Physics teachers spend their days showing and explaining the marvels of physics, which underlies all the other science subjects, including biology, chemistry, Earth and space science. Their work serves to develop the next generation of scientists and engineers, including all healthcare professionals. They also help all students better understand their… Read more

News Feed on This Topic

 
, ,

Cite This Page

General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

Science Buddies Staff. "Using a Laser to Measure the Speed of Light in Gelatin." Science Buddies, 20 Nov. 2020, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Phys_p009/physics/using-a-laser-to-measure-the-speed-of-light-in-gelatin. Accessed 19 Mar. 2024.

APA Style

Science Buddies Staff. (2020, November 20). Using a Laser to Measure the Speed of Light in Gelatin. Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Phys_p009/physics/using-a-laser-to-measure-the-speed-of-light-in-gelatin


Last edit date: 2020-11-20
Top
We use cookies and those of third party providers to deliver the best possible web experience and to compile statistics.
By continuing and using the site, including the landing page, you agree to our Privacy Policy and Terms of Use.
OK, got it
Free science fair projects.