Introduction: (Initial Observation)
As light passes from one transparent medium to another, it changes speed, and bends. This has lead to defining refraction index as one of the physical properties for transparent material. The index of refraction is defined as the speed of light in vacuum divided by the speed of light in the medium.For example the refraction index for water is 1.33. It simply means that the speed of light in vacuum is 1.33 times the speed of light in water.
As with many other physical properties, refraction index may be used in identifying certain chemical compounds or determining the concentration of certain solutions. For example while a sugar solution in a sugar factory is passing through a clear glass pipe, technicians can use the refraction index and determine the concentration of sugar in the solution. This allows quick analysis and automation of production line.
The question is how does the temperature affect the refraction index? Does it matter if the passing sugar solution is cold or hot? Or if the transparent substance is a known substance, can we use the refraction to determine its temperature?
In this project you will study the effect of temperature on the refraction index.
Information Gathering:
Find out about refraction of light and other forms of waves. Read books, magazines or ask professionals who might know in order to learn about the effect of temperature on refraction. Keep track of where you got your information from.
Following are samples of information that you may find:
Speed of Light
Experimental measurements of the speed of light have been refined in progressively more accurate experiments since the seventeenth century. Recent experiments give a speed of
c = 299,792,458 ą 1.2 m/s
but the uncertainties in this value are chiefly those of comparisons to previous standards for the length of the meter. Therefore the above speed of light has been adopted as a standard value and the length of the meter is redefined to be consistent with this value.
c = 299,792,458 m/s
Source…
Snell’s law states that a light is traveling low index to a high index (e.g. air is a low index and glass is a high index), the light ray will be bent toward the normal. On the other hand if light is traveling from a high index to a low index (e.g. glass to air), the light will bend away from the normal. Here is a diagram of Snell’s law.
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The purple line represents a light ray coming hitting a glass object and you can see that according to Snell’s law that the light would bend towards the normal in this case (the red line).
Mathematically speaking Snell’s law is stated as:
n1 * sin(a) = n2 * sin(b) where n1 and n2 are the index of refraction for each medium.
The refractive index sensor has a built in LED source. Light refracted in a liquid is analyzed and the refractive index is calculated as a difference.
Source…
Question/ Purpose:
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to determine if the temperature affect the refraction index of a liquid.
Identify Variables:
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
- Independent variable (also known as manipulated variable) is the temperature of the liquid.
- Dependent variable (also known as responding variable) is the refraction index (observe the refraction angle to determine if the refraction index is changed)
- Constants are the liquid (type and concentration), experiment method and procedure.
- Controlled variable is the outside air temperature.
Hypothesis:
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
This is a sample hypothesis:
The temperature does affect the refraction index. My hypothesis is based on my observation of a refraction index table indicating that the refraction index of water is 1.33 at 20šC. This simply means that the refraction index of water may vary in other temperatures.
This is another sample hypothesis:
The temperature does not affect the refraction index of a liquid. My hypothesis is based on the existence of refractometer devices that measure the refraction index at any temperature and to use them, the liquid does not have to have a specific temperature.
Experiment Design:
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment 1: Build a refractometer
Introduction: Changes in the refraction index of a prism, will change the amount a light beam bends while passing through the prism. Since liquids cannot be formed like a prism, we make a hollow prism and fill it up with the liquid we are testing. The source of light beam can be a laser pointer. These two together will form a basic refractometer.
Procedure: In this experiment you will make a high precession laser beam refractometer. This refractometer consists of a laser pointer producing a beam of light that passes trough a prism and hits a screen or wall about 5 to 10 feet away. The prism that you will use is a special hollow prism that you can fill it up with different temperature liquids to observe and measure their refraction rate. Here are the details:
Make a clear glass or Plexiglas container in the form of a triangular prism. You may use silicon glue to connect all the pieces together.
1 mm tick sheet of clear Plexiglas can easily be cut to small pieces. To do that you will first use a sharp object like a utility knife to create grove lines. Then it can easily break from the grove lines with some pressure. Cut 3 pieces of 2″ x 1.5″ to make the walls and one piece of 3″ x 3″ to make the base.
You can then use silicon glue to connect them together. Silicon glue needs about a day to dry and you must be careful with that and read and follow the safety instruction on the glue container. Silicon glue can be used both for glass and plastic. When your container is ready, test it with clean water to make sure that it does not leak. Now is the time to place the laser pointer and the prism in a way that the horizontal beam of laser pointer hit the prism about 1/3rd of inch above the base.
After I made the above hollow prism, I learned that high quality ready made hollow prisms are available at MiniScience.com. Click here to see a sample.
Laser pointer can be mounted on a box, but if possible, secure it with more stable tools such as wood, clamps, metal rods, pipes or any thing else that you have access to. In the picture below, the laser pointer is mounted on a laboratory stand.
You will also need to make a fixture for the base of your prism, so it does not move around. To do that you can tape or glue a few extra pieces of Plexiglas to the table or box, around the base of the prism. In this way if you need to remove the prism, wash it and put it back, it will sit at exact place that it was before.
When your setup is ready, use a siring or pipette to fill up the prism with ice cold water. Make necessary adjustments so the light beam will pass through the prism and hit the screen. Mark the screen with a dot where the light spot is and write 0šC.
Use a pipette or syringe to empty the hollow prism without moving it. Then fill up the hollow prism again with 20šC. You may use a hair dryer to cause slight changes on the temperature if needed. Mark the screen again where the light spot is and write 20šC.
Repeat this step with 40šC and 60š waters as well.
I do not suggest using very hot water because it may break a glass hollow prism.
Without changing the position of the laser pointer and prism, measure the angles the laser light bent in each temperature. You may need to use some strings in place of the light beam and some rulers and protractors to do such measurements.
Write your results in a table like this:
| Water temperature | Refraction angle | Deviation caused by warmer water |
| 0šC | ||
| 20šC | ||
| 40šC | ||
| 60šC |
Deviation caused by warmer water is the distance of their light spot to the light spot of the 0šC water. This is helpful for drawing graph especially if you have a hard time to measure the angles.
Draw a bar graph:
Draw a bar graph to visually present your results. Use one vertical bar for each temperature that you test (0, 20, 40, 60).
The height of each bar will represent the refraction angle that you have measured. For example for a 35 degrees refraction angle you may make a bar that is 35 centimeters tall.
Materials and Equipment:
Material used in a sample experiment are:
- Laser pointer
- Glass alcohol thermometer
- Glass hollow prism
- Ruler
- Protractor
- Masking tapes
- Cold and hot water
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Calculations:
If you do any calculations in this project, write your calculations in this section of your report.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Conclusion:
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
Possible Errors:
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
References:
Visit your local library and find books related to optics, prisms, light and refraction. Such books are generally found in the physics section of the library. Remember that refraction of light is what makes the lenses change the direction of the light and do what they do. Also find information about refractometers and their uses in science and industry. Following are some web resources:
