To find out the factors affecting the refractive index of liquid by using different temperature and concentration of salt solutions, glucose solutions, sucrose solutions and corn oil.

Authors Avatar

13/4/02        Physics Investigation        Henry Chow 13J      

 

Factors affecting the refractive index of liquid

Aim

To find out the factors affecting the refractive index of liquid by using different temperature and concentration of salt solutions, glucose solutions, sucrose solutions and corn oil.

Research and rationale

Before doing this investigation, I have a few choices of topics, including properties of condom, centripetal force of a four wheel drive toy car, and refractive index.  I started researching through the Internet.  I have come across a web site called ‘Brother Gregory investigate’, 1 concerning measurement of refractive index.  I found this experiment very interesting, using simple and easily available instruments and appropriate for A-level standard.  I found that refractometry is very useful in measuring sweetness in food industry.2   The experiment in the web site is using glass, in order to make more variations, I choose to use salt and sugar solutions.  Moreover, the refractive index of some liquids is already known, so that I can have a basic idea of how accurate are my results.  Here are some of the basic principles about refractive index found from some web sites and books.      

Physics principle

Refraction is the process that takes place when light travels from one medium to another with a different optical density and its path bends. Refractive index of a medium means the magnitude of refraction as light travels through the medium. The higher the value of refractive index means the light bends more as it travels through the medium. The amount the light bends is dependent upon the velocity of light in the medium. The speed of light decreases in the denser medium.3  If the refractive index is 1, which means it travels through the medium in a straight line, the angle of incidence (i) is equal to the angle of refraction (r). Based on Snell’s law, the basic formula for measuring the refractive index is:

Where μ is the refractive index of the medium.


Refraction occurs due to different densities between the two mediums.  For example, if light travels from air to glass.  The density of air is obviously less than glass, so when light travels through air the velocity of the light will be faster than the velocity when travelling through the glass.  There is another formula showing how does velocity related to the refractive index:

V1 is the velocity of light travelling through air; V2 is the velocity of light travelling through glass.  The value of the refractive index should be larger than 1 because V1 is larger than V2.

Refractive index of pure distilled water is about 1.333.  I read from Physics textbook, 2 it mentions that the refractive index of sugar solution will increase from 1.333 as the concentration of sugar solution (measured in %) increases, in a linear relationship. The “Brother Gregory Investigation” website also mentioned that the refractive index of a solution is linear with weight % composition.1 As the concentration increase, the number of molecules increases; the density of the solution is higher.  Therefore the velocity of light through the solution is decreased, refractive index is higher.  

Since different types of solutions such as salt solutions, glucose and sucrose solutions, and oil have different sizes of molecules, even if the concentrations are same, the density of the solutions is different and the refractive index will be different.  For a solution with larger molecule size, the density is higher; the velocity of light travel through the solution decreases and the refractive index is higher.

The refractive index of sugar solution quoted from the Physics textbook 2 stated the temperature of the solution. The CRC Handbook of Chemistry and Physics 4 also mentioned that temperature of the solution can affects the refractive index. As the temperature of the solvent increases, the density decreases which leads to increase in velocity of light travel through. Therefore, the refractive index decreased with increasing temperature.

The website 1 mentioned that the amount of bending depends also on the wavelength of the light being used. The wavelength of the light beam used can affect the refractive index of the medium since the velocity of the light beams travel through the medium are different. This is shown when white light passes through a prism, the beam is split into the colours of the spectrum. This effect occurs because the refractive index (of the prism) is greater for blue light (shorter wavelength) than for red light (longer wavelength). 4,5

 


Hypothesis

Firstly I predicted that the relationship between concentration of salt and sugar solutions and refractive index should be linear.  

Secondly since the size of a sugar molecule is larger than a sodium chloride molecule.  I predict that within the same concentration, the refractive index of sugar solutions will be higher than the refractive index of salt solutions.  Since the density of oil (100% concentration) is the highest, I predict that the refractive index of oil is greater than sugar and salt solution.

Join now!

Thirdly when I compare the same concentrations of sucrose and glucose solution, the refractive index should be similar; since they are both hexose and they are isomers (molecules with same molecular formula but different structural and display formula).  Their size and shape are similar.  

Fourthly, the temperature of the solution can also affect the refractive index, as the temperature increase, the refractive index will be lower.

Equipment list

1 x red colour laser beam (commercially available and cheap)

1 x (21cm(depth) x 21cm x 14cm) container

1 x 15cm ruler

2 x 1.5L beaker

50g ...

This is a preview of the whole essay