A Laser, or Light Amplification by Stimulated Emission of Radiation, has a high degree of spatial coherence. A high degree of spatial coherence means that the light that is being emitted from the laser is very concentrated making the visible distance that light is traveling much further than that of a standard light source. Another important property of lasers is that they only produce light from a single wavelength. The different wavelengths of light produced from the laser travel at different speeds making some colored lasers have a greater visible distance which they travel. Due to the laser's unique properties, it has gained significant popularity in various fields of science and the military. In the scientific applications, such as lunar laser ranging, the laser is required to travel from one medium to another, thus causing light refraction This light refraction can be accounted for if the index of refraction is known for both mediums and the incident angle. If a scientific experiment that used a laser, whose light was traveling through different mediums, was to switch their color of laser then their measurements would be different. This would happen because the different frequencies of the lasers which were used. In our experiment, we tested two different colored lasers, red and green, and the amount of refraction they have when their light travels from air to gelatin. In this experiment we hope to show how the different frequencies of lasers travel at different velocities.
Step 1: Research
Snell's law is able to calculate the refracted angle of the light given that you know the two medium's index of refraction and the incidence angle. From the website: http://hyperphysics.phy-astr.gsu.edu/hbase/tables/indrf.html we were able to find the index of refraction of air to be about 1 and the index of refraction for gelatin will vary based on how you make it and the index of refraction that we calculated for our gelatin recipe came out to be about 1.52. We calculated this by shining a laser through our sample and measuring the incidence angle and the refracted angle and then using the equation: (index of refraction of gelatin) = (index of refraction of air) (sine of refracted angle) / (sine of incident angle).
If you are have any other questions about Snell's law, take a look at this video:
Step 2: Design
Our sample will be shaped into a semi circle in order for their to only be one instance of refraction as we turn the table which the sample is sitting on. This shape allows multiple angles of refraction to be taken with only one sample. The data that we collect from each of the colored lasers will be compared to the predicted values based on the index of refraction for both mediums and the incidence angle. Our hypothesis for this experiment is that if we shine two different colored lasers (red and green) through a gelatin sample and measure the refracted angle of the light then the red laser will experience more refraction due to its smaller velocity in comparison to the green laser.
Step 3: Experiment Setup
1 Knox Gelatin packet
2 cups of boiling water
1 Plastic mold
1 PASCO Refracting light kit
1 Red laser
1 Green laser
Gelatin Sample Instructions
1. Put 1 cup of water into a pot and heat until boiling
2. Pour 1 packet of Knox Gelatin in the boiling water until dissolved
3. Remove pot from heat source and pour liquid into the mold
4. Place the mold in the refrigerator until firm
5. Use a knife to carefully separate the gelatin from the sides of the mold
6. Place sample centered on table
PASCO kit setup
1. Place magnetic rail in a secure location (preferably a table)
2. Attach the rotating protractor table to its swivel about 3/4 of the way from the beginning of the rail
3. Place the laser securely on the opposite side
4. Place some sort of wall at the end of the rail to block the laser from traveling further
Please refer to the picture for questions about setup of PASCO kit
Step 4: Results
The calculated index of refraction of gelatin was 1.52. The graph shows the comparison of the red laser and the green laser data that we collected. As the angle of incidence increased, the refracted angle increased faster for the red laser than the green laser. We were not able to calculate the angle for 80 because the angle of refraction was too high to calculate.
Sample Calculation of the Index of Refraction for Gelatin
(index of refraction of gelatin) = (index of refraction of air) (sine of refracted angle) / (sine of incident angle)
(index of refraction of gelatin) = (1) (sin 15.5) / (sin 10)
(index of refraction of gelatin) = 1.52
The uncertainty of this calculation is about +/- .5 because the protractor that we were using was only able to measure to the nearest degree, in addition when the light traveled through the gelatin sample, it became less concentrated making measuring the angle of refraction more difficult.
Step 5: Conclusion
The data that we collected was consistent with our hypothesis; the slower red laser refracted more than the faster green laser. However, there are changes that could be made to this project to further validate the data that we have. We only studied two laser frequencies: red and green. However, to see a trend, we could utilize the entire visible spectrum, using red, orange, yellow, green, blue, indigo, and violet lasers. From this, we would be able to see a pattern throughout more frequencies of visible light, possibly deriving a consistent formula that relates the wavelength and index of refraction with the angle of refraction. Another small error that we made was in the medium. We used Knox gelatin as our medium to refract through, since we figured that it would give us a significant angle of refraction while still being an easy experimental material to handle. However, when we molded the semicircle shape, we were unable to remove the Knox from the clear plastic mold, so instead used it while still inside it. This would have caused 2 instances of refraction before reading an angle, which would make our data inaccurate for the medium of purely Knox. Luckily, the purpose of our experiment was not to measure the index of refraction of Knox, but rather the change when a different frequency of light was passed through it. Since this plastic mold was present through both frequencies of lasers, its effect on the data we collected is irrelevant. If we were to perform this experiment again, though, we would like to collect data with pure gelatin if possible.
This experiment reinforced an idea central to the concept of refraction; light actually appears to slow down in different mediums. This can at first be hard to visualize, as many of us do not think about light on the subatomic level to where we can mentally simulate what is happening. This experiment enforces the idea that light’s speed is variable as well, dismissing the common misconception that there is one, set speed of light. This experiment is potentially both fun and educational; the content being covered is very important to understanding refraction, and the laser passing through the gelatin is a very cool sight to see.