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KATHMANDU UNIVERSITY

SCHOOL OF SCIENCE DEPARTMENT OF NATURAL SCIENCE (PHYSICS)

Study of concentration dependence of

Refractive index of Sugar and Salt

Solution

A 2nd Year 1st Semester Project Report

Submitted By:

Akur Khadka

Uttam Bhandari

February 2012

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Abstract

Refraction is a major phenomenon in the day to day life. Different gas, liquids and solids have

different refractive index. This project is focused to measure the refractive index of sugar

solution which can be used in various works in laboratory and industries.

The project consists of LASER as its source of monochromatic light which is passed thorough

sugar solutions having different concentration to measure its refractive index. Using a

horizontal beam of light, the deviation of the refracted beam striking a horizontal scale placed

at a known distance from the prism can be known.

The advantage of using laser light, in addition to its monochromaticity (single wavelength), is

that it is so intense that it can easily penetrate strongly colored liquids.

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Acknowledgement

While working in this project we received help from many people. Without the support and

cooperation from these people our project would not have succeeded in this way.

We would like to express our gratitude Department of Physics for their support and continuous

guidance

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1 Table of Contents

1. Introduction.................................................................................................................... 5

1.1. Background ............................................................................................................. 5

1.1.1 Objectives ................................................................................................................................................ 5

To study concentration dependence of refractive index of sugar solution and salt

solution ............................................................................................................................. 5

1.2 Literature review ..................................................................................................... 5

1.3 Methods .................................................................................................................. 6

1.3.1 Apparatus................................................................................................................................................. 6

1.3.2 Working ................................................................................................................................................... 8

1.3.3 Calculation ............................................................................................................................................... 8

2 Results and discussion .................................................................................................... 9

1.2. Result of sugar solution............................................................................................ 9

1.3. Result of salt solution ............................................................................................ 11

1.4. Reasoning for decrease in refractive index ............................................................. 12

1.5. Application of the result ........................................................................................ 12

1.6. Financial report ..................................................................................................... 13

1.7. Work schedule ....................................................................................................... 13

1.8. Complications ........................................................................................................ 13

2. Conclusion & Recommendation ..................................................................................... 14

4 Reference ..................................................................................................................... 15

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1. Introduction

1.1. Background

Refractive index is one of the most important optical properties of a medium. It plays vital role

in many areas of material science with special reference to thin film technology and fiber

optics. Similarly, measurement of refractive index is widely used in analytical chemistry to

determine the concentration of solutions. Recent studies [Schwartz 1999, Olesberg 2000,

Shlichta 1986] provide more detailed discussion on the concentration mapping by the

measurement of refractive index of liquids

The absolute refractive index of a medium is the ratio of the speed of electromagnetic radiation in

free space to the speed of the radiation in that medium. The relative refractive index is the ratio of

the speed of light in one medium to that in the adjacent medium. Refraction occurs with all types of

waves but is most familiar with light waves. The refractive index of a medium differs with

frequency. This effect, known as dispersion, lets a prism divide white light into its constituent

spectral colors. For a given color, the refractive index of a medium depends on the density of the

medium and density of a solution varies with the concentration of solute. Thus refractive index of a

solution depends on its concentration.

By calculating the minimum deviation of light through a prism we can calculate the refractive index

of the material inside the prism by using,

1.1.1 Objectives

To study concentration dependence of refractive index of sugar solution and salt

solution

1.2 Literature review

The method we used in the experiment to measure the refractive index of sugar solution and

salt solution consists of intense monochromatic source of light, LASER. For a given color, the

refractive index of a medium depends on the density of the medium. In any type of solution, the

density of the solution increase with increase in concentration of solute.

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1.3 Methods

1.3.1 Apparatus

Following apparatus were used in the experiment

Source

We use LASER as source of light in our experiment because it is intense and

monochromatic.

Circular base

This base can be rotated about a fixed vertical axis.

Hollow prism

This hollow prism of circular base is easy to fix on the rotatable base.

Scale

1.3.1.1 Apparatus setup

At first the circular base is placed such that, its center coincide with a fixed vertical axis about

which it is rotatable. Than the source is placed at left side of the base such that, LASER from the

source passes through the vertical axis and falls perpendicularly on the horizontally placed scale

at fixed distance from the base of the axis on right side.

Apparatus setup figure are shown below.

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Figure: 1.1 Light falling directly on the scale

Figure:1.2 light suffering refraction through prism before falling on scale

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1.3.2 Working

At first we prepared highly concentrated solution, sugar solution with 60% concentration and

salt solution with 24% concentration (concentration by weight). We measured refractive index,

lowered concentration, by 3 in sugar solution and by 2 in salt solution, again measured

refractive index. We continuously repeated the process, lowering concentration then

measuring refractive index till we obtained refractive index of sugar solution at 3%

concentration and salt solution at 2%.

The methods of calculation are mentioned below,

1.3.3 Calculation

Direct position of light is noted on the scale. Then, Solution whose refractive index is to be

known is filled in the hollow prism. When the light passes through the prism it suffers refraction

and its position on the scale is displaced. Minimum displacement on the scale is found by

slightly rotating the prism. After knowing the minimum displacement (d), Dmin (angle of

minimum deviation) can calculated using (1),

(

)……………. (1)

Figure 2.1 relation: between minimum displacement and minimum deviation

Then refractive index (n) is calculated using (2),

(

)

……………… (2)

Here A is angle of prism, we used equilateral triangular prism so value of A is 60,

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2 Results and discussion After the calculations with above mentioned ways, we got following results,

1.2. Result of sugar solution

As predicted we found decrease in refractive index with decrease in concentration,

experimental data are in the table below,

S.no Concentration (%) by weight

position of mean deviation

Dmin (angle)

Refractive index

1 60 43.7 32.91 1.44978 2 57 37.9 29.31 1.40571 3 54 37 28.72 1.39844

4 51 36.4 28.33 1.39353 5 48 35.7 27.87 1.38773 6 45 35 27.4 1.38186 7 42 34.4 27 1.37676

8 39 33.7 26.53 1.37076 9 36 33.1 26.12 1.36555

10 33 32.5 25.709 1.36029

11 30 32.1 25.43 1.35675 12 27 31.6 25.08 1.35229 13 24 31 24.66 1.34689 14 21 30.5 24.31 1.34235

15 18 30 23.96 1.33777 16 15 29.6 23.67 1.33408 17 12 29 23.24 1.3285 18 9 28.6 22.96 1.32475

19 6 27.8 22.38 1.31717 20 3 27.3 22.02 1.31238 21 0 27 21.8 1.3095

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The graph, concentration verses refractive index for sugar solution is shown below,

We see linear dependence of concentration with refractive index from the graph above. The

refractive index of water is 1.33 but here in the graph we see 1.33 value of refractive index

between concentrations 12 and 15 and that of water is below 1.32, this is because of error in

our apparatus.

1.3

1.32

1.34

1.36

1.38

1.4

1.42

1.44

1.46

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63

Refractive index vs concentration

Refractiveindex

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1.3. Result of salt solution

As predicted we found decrease in refractive index with decrease in concentration,

experimental data are in the table below,

S.no Concentration (%) by weight

Position of mean deviation

Dmin

(angle)

Refractive index

1 24 100.8 26.5 1.37337

2 22 99.3 26.23 1.36698

3 20 98 25.9 1.36318

4 18 96.8 25.65 1.35964

5 16 95.5 25.35 1.35578

6 14 94.4 25.1 1.35249

7 12 93.2 24.8 1.34888

8 10 91.9 24.51 1.34495

9 8 91.9 24.32 1.34251

10 6 90 24.06 1.33914

11 4 88.8 23.78 1.33544

12 2 88 23.59 1.33296

13 0 86.1 23.1 1.327

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The graph, concentration verses refractive index for salt solution is shown above. We see linear

dependence of concentration with refractive index from the graph above. The refractive index of water

is 1.33 and we have nearly got it.

(Refractive index of sugar solution and salt solution were calculated

with different apparatus setup)

1.4. Reasoning for decrease in refractive index

As the density of the medium increases the refractive index of that medium also increases. A

solution with high concentration of solute means it is denser than the solution with low

concentration of solute. Thus, solution with high concentration of solute has high refractive

index than the solution with low concentration.

1.5. Application of the result

From the concentration verses refractive index graph of any solution, refractive index of

solution at any concentration can be known and the solution can be used in different optical

devices, in experiments of light.

From the graph we can also know concentration of any solution by measuring its refractive

index and use the solution in different chemical processes.

1.32

1.33

1.34

1.35

1.36

1.37

1.38

0 2 4 6 8 10 12 14 16 18 20 22 24 26

Refractive index vs concentration

Refractive index

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1.6. Financial report

As all the materials required for the project were available in the laboratory we only needed

to purchase few things.

S.no Material quantity cost

1 Hollow prism 1 200

1.7. Work schedule

September October November December January February

Finalize Project

Proposal

Presentation

Literature

Survey

Apparatus Setup

Experimentation

and Data

Collection

Final

Presentation

1.8. Complications

The complications aroused during the project were as follows:

We had problem during apparatus setup, we required no change in position of

apparatus during the experiment, which didn’t happen.

We didn’t have a scale long enough to take reading at large distance from the prism, so

we could have got more accurate data.

We didn’t get pure salt for our experiment, so we used salt with iodine.

We didn’t get pure sugar. There were some insoluble impurities in our sugar solution.

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2. Conclusion & Recommendation The method of calculating refractive index of any solution with any concentration by the

use of LASER makes easy to understand the concentration dependence of refractive index

of that solution. In our experiment we found some error while comparing refractive index

of water, that we measured with the standard one. This is due to small errors in apparatus

arrangement, which could be recovered in another similar project with following

recommendations.

Fixing a long horizontal straight thick bar, above which source, circular base and

scale stand could be fixed. This will minimize errors from accidental displacement of

apparatus.

Using measuring tape instead of scale. This will give us high resolution data as

measuring tape can be placed far from prism.

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4 Reference

www.wikipedia.com

KATHMANDU UNIVERSITY JOURNAL OF SCIENCE, ENGINEERING AND TECHNOLOGY VOL.II, No.1, FEBRUARY, 2006.