Sodium Chloride Solution Effect On Refractive Index

Introduction

Refraction index is one of the most important intriguing concepts in physics. It demonstrates how light behaves in different mediums which is vital for thin film technology and fiber optics[footnoteRef:1]. As for this exploration, the change in refractive index of water due to salt concentration applies to archer fishers found in the South Pacific and the Indian Ocean fishing in saline coastal waters. Currently, the average salt concentration in the ocean is 3.5% (35 grams) of 1000ml (1 liter). Furthermore, before choosing a topic I was hoping to choose a topic that I am interested in and a topic that relates to hobbies I do. Therefore, I chose the refractive index of salt water as I go to the beach a lot during the summer which contains salt water.

Refraction occurs when a light goes through two different mediums. The light changes its wavelength and speed to cause a bend to the light. Depending on the speed, the light will bend[footnoteRef:3]. This physics concept is the cause of natural phenomenon such as the rainbow or mirages. In addition, it is used in glasses with the lenses adjusted to refract light from objects so that the light will converge to a focal point. [3: “The Cause of Refraction.” The Physics Classroom, www.physicsclassroom.com/class/refrn/Lesson-1/The-Cause-of-Refraction.]

Snell’s Law

Snell’s law is an equation that describes the relationship between two mediums in terms of refractive index. It uses the ratio of the velocity, wavelength and the angles of light in the two mediums to find the refractive index[footnoteRef:4]. In this investigation, the only measurement I will be able to do is the angle of incidence and refraction to find the refractive index. In the equation below, n1 and n2 are the refractive indexes of two different mediums. In addition, the unit of the angles are in degrees, the unit of velocity is meters/seconds and wavelength are measured in nanometers[image: ].

Salt Solution

One of the major steps in this investigation is dissolving the salt in water. How salt dissolves in water helps the understanding of how it affects the refractive index. Firstly, water's molecular formula is H20 and salt's molecular formula is NaCl. The forces that attracts these two molecules are dipole-dipole forces. Water has a partial negative charge element in the oxygen atom, while the hydrogen in water has a partial positive charge. The salt molecule has a partial negative charge in the chloride atom while the sodium atom has a partial positive charge. Because of this polarity, the negative part of the water (O-) will attract the positive end of salt (Na+) and vice-versa[footnoteRef:5]. In the end, when all the NaCl dissolves, the water molecules will surround the sodium and chloride ions. Through this, the particles become denser compared to pure water. The image below displays the attraction between salt and water. [5: “How Does Sodium Chloride (NaCl) Dissolve in Water?” Core Concepts in Chemistry, 26 Sept. 2018, www.masterconceptsinchemistry.com/index.php/2017/10/24/how-does-sodium-chloride-nacl-dissolve-in-water/]

Factors Affecting Refractive Index

When doing an experiment, external factors that can affect the results must be considered. In this case, there are two factors, the temperature of the liquid, and wavelength of light. A liquid at higher temperature has smaller refractive index as the medium is less dense and less viscous causing light to travel faster. At lower temperatures the refractive index is higher as the medium is denser and more viscous meaning light will travel slower. In addition, the refractive index varies with wavelength linearly because different wavelengths interfere to different extents with the atoms of the medium. Using a monochromatic light can make it simpler to measure the refracted light as monochromatic light only displays one color[footnoteRef:6].

Properties of Light

Light is electromagnetic radiation that has properties of waves. The electromagnetic spectrum can be divided into several bands based on the wavelength. The wavelength of visible light is between about 380 nm and 730 nm. From these wavelengths, our eyes interpret different colors from the spectrum. These colors from highest to lowest are red, orange, yellow, green, blue, indigo and violet (ROYGBIV). Furthermore, the established velocity of light is 2.99793 xcm/sec, the velocity of light will only change when it goes through a substance in which it will decrease. Through this knowledge, the equation shown below where the refractive index, n, of a material or substance as the ratio of the speed of light in a vacuum, C, to the speed of light in a material through which it passes[footnoteRef:7], . However, the value of refractive index will always be greater than 1.0, since the speed of light can never be lower than when light goes through a material. As previously explained, will depend on the density of the material (higher density – slower the speed of light).

Hypothesis

If salt were to be dissolved into water and a laser was to shine through it, then the laser would increase its refraction compared to pure water. When salt is added to water, the density of the water will increase as density is measured by the mass of a material per unit volume and adding salt will increase the mass thus increase the density. Furthermore, the cause of refraction is due to light traveling slower in higher densities as the frequency of the first medium is higher than the second medium; therefore, the laser in this experiment will refract more.

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Evaluation

From the results derived from the experiment, we can see there is a direct correlation between the salt concentration and refractive index. The refractive index increases with more salt dissolved in the water, proving my hypothesis. My hypothesis looks to be valid through the 0-20 grams of salt concentration but once the salt intake is above 20 grams, the index of refraction is the same. The effect of concentration is strong up to 20% concentration. However, after this value the dependence of the salt becomes weak. Even though there was not a way of measuring the density of the salt water, I have to assume that it increased due to the increase in the refractive index further proving my hypothesis. There is a small average increase in refractive index of 0.0017. After using Excel to graph the experimental data that was collected, a line of best fit was inserted (the graph is already linearized). The line of best fit displays a linear relationship between the salt concentration and the refractive index. The slope derived from the graph is 0.0089 showing a constant increase in the refractive index.

Though my hypothesis seems to be mainly valid, there were some inconsistencies when gathering data which can be atoned to sources of error. To begin with, the first source of error I suspect was that the salt was not fully dissolved in the water when measuring. Due to error, the increment of salt I was supposed to dissolve could be lower than suggested. In addition, the undissolved salts affect the lasers as it going through the grain of salt means going through a different medium. The cause of this error was the time it took to dissolve the salt. Using a stirring rod was very inefficient as higher salt concentration took up to 10-15 minutes to fully dissolve therefore, I had to rush some to get my measurements on time. This displays a limitation as the inability to know when the salt is dissolved was a major issue.

The plastic of the refraction dish had a certain yet very small effect on the results. When the laser gets to the salt water, it must first go through the outline of the refraction dish. The outline has a small thickness but in that small thickness, the laser definitely refracted. According to the laws of refraction, light bends or refracts when going from one medium to another different medium slowing down the light. Using a refraction dish means the laser by definition went through 5 mediums (air to plastic to salt water to plastic to air). Although this influences the result by a minimal margin, it is worth mentioning it as it serves as another limitation to the experiment.

Additionally, another medium that the laser possibly went through was the dust or stains on the outside of the refractive dish. Furthermore, the placement of the protractor paper affects the end result. Since the results increases by very small increments, the protractor paper must be near perfect to be within experimental uncertainty. Having the protractor paper slightly off to the right or left can have an effect on the measurement readings.

To gain data that is more accurate and more detailed, improvements to the lab could be made. The main improvement I suggest that minimizes the number of materials needed is using a refractometer. A refractometer is a device used for refractive index measurements. It measures the extent of how much light is refracted from the liquid placed. The liquid or solid or any transparent substances can be used in a refractometer. Then this is used in order to identify a liquid sample, analyze the sample's purity and determine the amount or concentration of dissolved substances within the sample. This can improve the lab as it can further conferment that there is a correct amount of salt concentration when measuring the refracted angles.

Another improve which would help use the time taken to do the experiment more efficiently is a magnetic stirrer. A magnetic stirrer is a laboratory device that employs magnet that is spinning due to another magnet, thus stirring it. The device also has a setting on how fast to stir the solution. In this case, this device would have been used to dissolve the salt in the water. The stirrer, in general, will decrease the time taken for the experiment, therefore, my time can be used for other parts of the experiment such as calculating the refractive indexes.

Moreover, there are still some minor improvements that can be made to this experiment. Firstly, using a laminate protractor paper instead of paper that can get wet from salt water. Secondly, replacing the refraction dish with a cleaner version for the reasons mentioned above.

Bibliography

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