Index of refraction of liquids versus amount of additive

Introduction: (Initial Observation)

In many chemical and food manufacturing plants, products at some stage are in the form of a solution. For example sugar is a sugar solution and salt is a salt solution prior to crystallization.
It is important for production staff to know the concentration of such solutions in different stages of production.

One method of testing the concentration, is getting a sample and evaporate it quickly in the laboratory and weight the remaining solids; however, this process may take up to one hour each time. If the refraction index of a solution be a function of its solute, then we may quickly use a refractometer to find out the concentration of a solution.

Information Gathering:

Find out about light and how it changes while passing through a liquid or solid. Read books, magazines or ask professionals who might know in order to learn about the use of light in identifying substances or learning about their physical and chemical properties. Study about refractometer and find out how you measure the refraction index. Keep track of where you got your information from.
Following are some sample information that you may use.

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.

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.

Glossary:

Refraction:
1. The bending of light when it passes from one transparent substance to another.2. Bending of waves due to a change in a medium
Reflection:
1. The bouncing back of light from a surface. 2. Waves that strike and bounce off an object. 3. When light bounces off a surface we say it is reflected. Everything reflects some light.

color spectrum: the rainbow of colors resulting from white light passing through a prism (red, orange, yellow, green, blue, indigo, and violet).

laser: 1. Light amplification by stimulated emissions of radiation..2.A machine that can produce a narrow, powerful beam of light of one particular wavelength. The word “laser” stands for Light Amplification by Stimulated Emission of Radiation.

light: A form of energy that stimulates the eye and makes it possible to see things.

prism: 1. A solid transparent shape usually made of glass, which can be used to separate the colors in visible light. 2. A block of glass or plastic that separates white light into the colors of the visible spectrum.

wave: 1. A disturbance that travels through matter or space. 2. A way in which energy moves from one place to another.

wavelength – 1.A property of a wave that gives the length between two peaks of the wave. 2.The length between identical points on two waves next to each other. Each type of electromagnetic radiation has a different wavelength. 3. Distance between consecutive crests or troughs. 3. Each type of radiation has a different wavelength.

wave speed: Wavelength times wave frequency.

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. Following are sample questions/ Purpose.

1. Can we determine the concentration of salt in a salt water solution by knowing the refraction index of salt water?
2. Can we determine the density of a sugar water solution by knowing it’s refraction index?

The purpose of this project is to determine the index of refraction of a specific liquid such as water versus the amount of additive such as salt.

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. Following is an example for question number 2.

The independent variable (also known as manipulated variable) is the concentration of sugar (as an additive) in a sugar water solution.

The dependent variable (also known as the responding variable) is the refraction index of the sugar water solution.

Controlled variables are temperature and the light color.

If you want a general variable definition that covers all liquids, use the following example, however you will not be able to generalize the results of your experiments.

The independent variable is the concentration of the additive in a solution.

The dependent variable is the refraction index of the solution.

Controlled variables are temperature and the light color.

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.

We can use the refraction index of sugar-water solution to estimate it’s density.

If you have generalized your variables, this is an example of a hypothesis

We can use the refraction index of a solution to estimate the concentration of solute (any specific additive).

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: You may not have access to a refractometer for your experiments. In this case you need to make your own refractometer. A laser light can be used to make a high precision 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 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, add some water to the prism so the laser light goes trough the water. Make necessary adjustments so the light will hit the screen. Mark the screen with a dot where the light spot is and write 0% additive above that. You refractometer is now ready for test and graduation for water based solutions.

Experiment 2:

Introduction: In this experiment you don’t actually measure the refraction index of a liquid, Instead you observe and record the changes of refraction by the changes in the concentration of an additive. Use water as the base for liquid and sugar as an additive.

Procedure:

1. Make a 10% sugar water. To do that, weight 10 grams of sugar in a 250 mL beaker and then add 90 grams of water (that is 90 mL). Stir it to get a clear sugar-water solution. Label the beaker ” 10% sugar “.
2. Make a 20% sugar water. To do that, weight 20 grams of sugar in a 250 mL beaker and then add 80 grams of water (that is 80 mL). Stir it to get a clear sugar-water solution. Label the beaker ” 20% sugar “.
3. Make a 30% sugar water. To do that, weight 30 grams of sugar in a 250 mL beaker and then add 70 grams of water (that is 70 mL). Stir it to get a clear sugar-water solution. Label the beaker ” 30% sugar “.
4. Repeat the above procedure to make 40%, 50%, 60%, 70% and 80% sugar. You may need to use some heat to dissolve the sugar.
5. Setup your laser refractometer, so the laser pointer and the prism container can not move during your experiment.
6. Fill up the prism with water and make the final adjustments to see the light spot on the screen or wall. Mark the spot and label it 0%.
7. Use a pipette to extract water and refill the prism with a 10% sugar solution. Locate the light spot. Mark it and label it 10%.
8. Use a pipette to extract the 10% sugar-water solution and refill the prism with a 20% sugar solution. Locate the light spot. Mark it and label it 20%.
9. Continue to mark the light spot with 30%, 40%, 70% and 80% solutions.
10. Now you should be able to identify the concentration of an unknown sugar water solution. Have someone else to make a solution and test it to see if you can determine it’s concentration by it’s light spot. How accurate is your result?

In one experiment I managed to adjust the distance between the prism and screen in a way that each 10% variation in sugar concentration displaced the light exactly 10 centimeter. In this way every 1 cm was an indication of 1% sugar.

Experiment 3:

Introduction: In this experiment you will measure and record the refraction index of water with different amounts of sugar in that. If you have access to a refractometer, just test the refraction index of sugar water solutions of previous experiment and record them in a table like this:

Then you use this table to make a graph to see if changes to the refraction index are linear. If you don’t have access to a refractometer, you will make one as described in the following procedure.

Procedure:

Make a small container in the shape of a rectangular prism using small pieces of glass such as microscope slide or clear plastic (Plexiglas). You can use silicon glue to connect these pieces together. Instead of glass, you may use Plexiglas or any similar clear plastic. Your prism can have 3, 4 or 5 sides. One base of the prism can be a larger sheet of hard plastic and the other base can remain open. That opening will be the top of your container.

Place two piece of wood on two sides of the prism so you can align the light beam on their surface. Fill half of the clear prism with cloudy water. One drop of milk can make your water cloudy.

Pass the light beam of laser pointer through the prism from different angles and observe the light beam that exists the prism.

Take some pictures from the top so you can later use a protractor to measure and compare the angles of light beam in water and outside water.

Can you find a relation between the angle of light beam that enters and the angle of light beam that exits the prism?

Try different light angles including vertical.

Does the light bend when the light beam hits the container in a 90º angle?

Use a protractor to measure the angle that the light beam makes with a vertical line at the point of entering to the water. This angle in the air will be different from the angle in the water.

Divide the angle of light beam in the air by the angle if light beam in the water and record the ratio.

Use your experiment setup as a part of your display.

n1 * Sin(a) = n2 * Sin(b) where n1 and n2 are the index of refraction for the air and solution.

If we use 1 as the refraction index of air, then the refraction index of solution is:

n2 = Sin(a) / Sin(b)

Measurements and calculations:

Fill up the prism with liquid that you want to test its refraction index.

Measure the angle of incidence (a) and the angle of refraction (b).

Use a calculator to calculate the Sin(a) and Sin(b).

Divide the Sin(a) by Sin(b) to calculate the refraction index of the liquid.

You may take a picture of light beams from the top and use your printed picture to measure the angles.

You will need to repeat the measurement and calculations for 10% to 80% sugar solutions and enter the results in the table described in the introduction section of this experiment.

Materials and Equipment:

List of material can be extracted from the experiment section.
Hollow prism may be purchased from MiniScience.com; however, I have provided detail instructions in case you want to make your own.

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:

In the experiment 3, you may need to calculate the refraction index of liquids.

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 see some books about the light physics.
Also search the Internet with keywords such as light and refraction index for more references.

http://www.minerali.it/rifrenglish.html

If you don’t have a scale for your measurements, you may make your own. Click here to find out how you can make a basic balance scale.