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 sodium chloride
50g glucose powder
50g sucrose powder
200 ml corn oil
Weight balance
2 x retort stand
2 x power supply
2 x metal heating stick
Safety
- Laser pointer can cause blindness, remember to use a laser protection goggle when doing experiment. Do not directly looking at the source.
- Large portion of liquid, prevent spillage of liquid, which may cause slippery and fall. (Especially oil)
- Glucose and saline solution are safe, for safety reasons; prevent drinking since the apparatuses are sometimes contaminated.
- The heater can be very hot, prevent touching it after the testing
- Make sure that the power supply connected to the heater is off before taken out of the liquid after each testing
- Prevent heating up the corn oil; it will cause serious damage and burn.
- The container is made up by plastic, prevent heating the solution for up to 80 degree Celsius.
Variables
- Different types of solutions
- Different concentration of solutions
- Different temperature of the solution
- The laser point is always located in the same height and angle
- The horizontal distance between the laser pointer and the initial beam mark is always remaining constant
- Same container used, same ruler used and same person to take the reading.
Initial plan (for pilot test)
Use the rectangular shape container; the ideal choice will be a deep container with long length but a short width. Put the container on a stand so that I can mark on the A4 paper underneath.
Put the laser pointer up at a height of about 2 meters above the ground; use a clamp to hold it tight, since it will have to be at the same location for the rest of the experiment. Direct the laser pointer pointing towards the table.
Put the container on a rack, with an A4 paper underneath the container. Draw a straight line of symmetry on the paper. Then adjust the light beam to make it shone on the line.
Shone the beam and make a mark on the other side of the paper since I cannot move the container. Then measure the horizontal distance between the light source and this initial mark of the A4 paper. Then measure the vertical distance between the base of the container and the light source.
Use a 200ml beaker, measure 200ml of the solution that I am testing on. Pour the solution into the container and measure the depth of the solution in the container, but do not have to move the container. Make sure that the light beam is shone on the line. Then draw a final mark on the other side of the A4 paper.
I want to change the temperature of the solution. I can use an electric heating stick. Connect the power supply to the main supply. Turn the voltage to 13V in order to more it work in its fastest rate, use wires to connect the power supply to the plug of the heating stick. Use a retort stand to hold the stick and put it into the solution. Make sure that the stick is not blocking the light beam. Turn it on during the testing, put a thermometer into the solution and mark a reading at 30 degree Celsius, one at 40 and 50 degree Celsius.
There are two important right angle triangles involved in determining the refractive index. This is illustrated by the diagram below. Use the following formula to measure the refractive index.
Pilot test
The main purpose of the pilot test is to find out whether this plan is reliable and whether the apparatus can give out appropriate results. In this case I will use a substance with a known value of refractive index, which is pure water, with a refractive index of 1.333. I can measure the value of refractive index of pure water by these apparatus and measure the percentage difference with the actual value. In this case I have to use distilled water, but not tap water, because tap water will have impurity inside, which can change the refractive index.
Result for pilot test
I have measured the following (refer to the diagram in P.7):
- Height between the light source and the horizontal surface of the solution (h)
- Horizontal distance between the point, where the light beam enter the solution and the light source (x)
- Horizontal Distance between the light source and the initial mark (g)
- Distance between the initial mark, recorded at the bottom of the container before solution is added and the final mark, after the solution is added (L1)
- Distance between the normal line of refraction and the initial mark (L2)
- Depth of the solution in the container (d)
- The temperature of solution (t)
- The value of refractive index (μ), which have to be calculated by the formula:
Where h, x, L2 and d are already fixed, so that I do not have to measure it every time. The only variable is L1 and t.
Here are my results:
This result is very close to the standard refractive index of pure water, which is 1.3333. The percentage difference is about 1%. This can be due to either experimental error or difference in the wavelength of the light beam (red beam instead of white beam). In this case I would take 1% uncertainty as the uncertainty limit for this investigation. However the temperature difference does not change the refractive index of the pure water. I suspect that the temperature difference is too small to a detectable change of refractive index.
Method (review after the pilot test)
Base on the pilot test, I have modified the method as follows:
- The depth of the container that I used is too big, the angle of incidence is very small hence the angle of refraction, then L1 and L2 will be very small. So I change the container to a 12 cm (depth) x 15 cm x 12 cm size to ensure that the angle of incidence is bigger.
- 200 ml of solution is very little, the depth is only about 1mm, so I have to use a lot more liquid, to reach the depth of 8.6cm, which is about 3000 ml. If I use a smaller container, I need only 1400 ml to maintain a reasonable depth. Therefore I can use less liquid for a smaller container for saving of resources.
- The light beam will run out of batteries easily, however I cannot move the light source to change batteries once it is fixed. I have made a battery pack and connect it with the light source by electric wires, since the batteries are now AA batteries they can stay on for a lot longer time. The batteries are now external and I can change batteries easily. There is a switch at the battery pack, I can turn the power on and off without moving the light source (see photo 1 in the next page).
- The initial method is not accurate since there can be refraction when light travel through the bottom of the container. To improve accuracy, instead of putting the container on the rack and mark on the A4 paper, I stick a ruler with silicon adhesive at the inside of the bottom of the container, before I put the solution into the container I set the light beam to a position where it shone exactly on the point ‘0’ on the ruler. I do not need the paper anymore since I can read the distance L1 from above the container (see photo 2).
- It is very difficult to measure the value of L2 and x, since the point where the light beam enters the solution is not shown on the solution surface. The method that I used is, use a ruler, short (about 15 cm), coloured, but not red (since the light beam is red). Put the ruler vertically without holding it, by using a clip (picture 1) to help it stand inside the solution. The ruler represents the normal line of the refraction. Then adjust the ruler towards or away from the light source until the light beam shone on the ruler at the surface of the solution. Since this vertical ruler is perpendicular to the horizontal ruler at the bottom. Record the value of L2, then g – L2 is the value of x. The depth, d can also be measured by this vertical ruler. Since there is a gap between the edge of the ruler and the point marked ‘0’, use a scissor to cut out this gap, so that the edge of the ruler is on the point 0 to minimize error. This side have to be at the bottom of the solution. Then when this ruler stands vertically on the solution, the point on the vertical ruler is the value of d (see photo 2).
- It is very difficult to measure the vertical distance from the light source to the surface of the solution (h) accurately. In order to improve this, I put a mirror in the place where I put the light source before, same height as the light source by a retort stand. Put the light source to a higher position, opposite to the mirror. The light should shine towards the mirror from an upper position, and produce a reflected beam to the bottom of the container. The point is, to measure the height from the red mark of the mirror to the table, then subtract this height by d to give h.
- Base on the pilot test, the reflective index remained unchanged with increasing temperature. This maybe due to small temperature increase form 30 to 50. I planned to increase the temperature from 30 degree to 80 degree Celsius. However, I cannot raise up the temperature to above 80 degree Celsius because the container is made by plastic.
- The area of the light beam emitted from the light source is having a big cross-section. I have difficulty in reading the exact L2 value on the horizontal ruler. That will make the value of L2 not precise enough. To minimize the error, I stick a piece of thick white sticker on the hole where the red light emitted, use a thin needle to make a tiny hole and therefore decrease the area of the emitted light beam.
- When I read the mark on the ruler, I have difficulty reading the exact point since he point usually covered about 2 or 3 mm. In order to make the result more precise, I took 1 reading from the left boundary and 1 reading from the right boundary and take the average as the final mark.
Analysis
Based on my investigation, the refractive index of pure water is 1.3207. When I compare the value with my physics book, the refractive index of pure water should be 1.3333. The difference is less than 1%. This can be due to experimental error. Another reason of this can be because the light source that I used is red; the wavelength of red light is longer than the wavelength of white light. Since I know that red light is less refracted than blue light, this is demonstrated when white light is shown on prisms. Therefore, the refractive index obtained by using red light should be smaller than the value obtained by using white light.
When I compare different types of solutions, the refraction index of pure water is the lowest (1.3207). Based on my result, the refractive index of salt solution at 5g/ 100ml (1.3766) is lower compare with sugar solutions at the same concentration. 100% corn oil has the highest refractive index (1.5130). The results proved that my prediction is right. The molecule of salt solution (NaCl) is smaller than sugar solution (C6H12O6) and oil (triglyceride, C57H110O6). Under the same concentration, the density of the solution is higher for sugar than salt. Therefore, the refractive index of sugar solutions are higher than salt solution.
The refractive index of glucose and sucrose solution at 2.5g/100ml (1.3563 for sucrose and 1.3613 for glucose) and 20g/100ml (1.4614 for sucrose and 1.4676 for glucose) does not show great difference, which are less than 1% (0.37% and 0.42% respectively). I think this is because the sizes of glucose and sucrose molecules are similar. They are isomers (same molecular formula but different structural formula, which is C6H12O6). Therefore their refractive index is similar.
From the result, as the concentration of the salt and sugar solution increase, the refractive index also increase, the relationship is linear. My prediction is correct, the more concentrate the solution is, and the higher the refractive index is. Take 1.25/100ml and 2.5/100ml as an example, if I subtract the refractive index in the concentration of that solution above with the value 1.3207, ‘my’ value of refractive index of water. Take salt solution as an example, the difference for 1.25g is 0.0162 and the difference for 2.5g is 0.0307. It shows that as the concentration is halved, the refractive index is halved. This is explained by the fact that as the concentration increase, the density also increases; therefore the speed of light travelled through the solution is decreased.
This investigation does not show any significant effect of temperature on the refractive index. The refractive index remained unchanged with increased temperature from 20 degree Celsius to 80 degree Celsius. This is different from what I predicted. When I did more research on this, I found that at liquid state, the molecule moved around randomly. Even if I raised the temperature, the molecule moved faster but they are still moving around randomly. The speed of light only change very little with increasing temperature. The design of the experiment should be more precise in order to detect that little change. I found from the Internet, that unless the state of the solution is changed from liquid to solid. At solid state, the molecules are bounded to a position and do not flow around freely. The density is increased, therefore the refractive index increased.
Evaluation
This investigation proves that my first, second and third hypothesises are correct, it does prove that (1) the relationship between concentration of salt and sugar solutions and refractive index are linear. As the concentration of the solution increase, the refractive index increase. (2) The refractive index of salt solution is lower than the refractive index of sugar solution of the same concentration. (3) The refractive index of glucose solution and sucrose solution are similar.
In order to increase the accuracy, what I have done is: (1) make up an external power supply to change battery for the light source. (2) Stick a ruler at the inside of the bottom of the container. (3) Use a mirror to reflect the light source for easier measurement of the value of h. (4) Make the light beam as narrow as possible by covering the hole of emit, and make a pinhole on top of it. (5) I have taken the average of the 2 edges of the mark.
I think the results of refractive index of pure water are accurate; the refractive index of pure water calculated from this experiment is only less than 1% different from the standard refractive index of water.
However, the results of the refractive index of sugar solution are not accurate. I read from the book ‘AS advanced Physics- P.257’ 2, it shows a table on refractive index with concentration of sugar solution at 20 degree Celsius. From my investigation, the refractive index in 5g/100ml solution is 1.3871; from the book it is 1.340 for same concentration. (Assume that the sugar mentioned is sucrose). The difference is about 3.4%.
There are two anomalous results, both are due to sugar solutions, the first is 10g/100ml sucrose solution, the refractive index is 0.04 away from the best fit straight line. The percentage uncertainty is 2.9%. The second anomalous result is 5g/100ml glucose solution; the refractive index is 0.035 away from the best-fit line. The percentage uncertainty is about 2.6%.
There are several limitations, which can account for the inaccurate and anomalous result. (1) The sugar may not be 100% dissolved in water. During the experiment I have to stir the solution for over 10 minutes, until no more visible crystals of sugars can be seen. I think some sugar crystal may be left or not dissolved. (2) There might be some solutions left over in droplets inside the previous container so the concentration may not be accurate. (3) When the mark is on the point ‘0’ before pouring any solutions into it, the horizontal direction of the light beam may not be exactly parallel to the line on the horizontal ruler. So when solution is poured into the container, the beam is not exactly located on the line, it can form an angle with the line on the ruler. (4), the beakers that I used are 1500ml and they are not very accurate, because it is very wide and short in height, the marks between hundreds are very close to each other. Which may cause error when diluting solutions.
In this case I think 1% uncertainty in a bit too strict, since the uncertainty set before is only taking account of pure water so that the first and second limitation can be omitted. As a result, I think the limit of uncertainty can be allowed to increase to 5%, 1% allowance for each problem. If the two anomalous results is compare to the 5% uncertainty they are still acceptable.
However, this investigation does not prove my fourth hypothesis that the refractive index decrease as the temperature increase. From the research from the Internet, there are some changes for the refractive index with the temperature, but very small 4. For example, the difference in refractive index of water at 50 degree Celsius and 60 degree celsius is only 0.001. It may require a scientific refractometer like an Abbe refractometer 2 to detect a tiny change. Instead of increasing the temperature, I may need to decrease the temperature to below the melting point, so that the solution changed from liquid state to solid state and see whether the refractive index increase.
One drawback of using oil is, the concentration of oil is 100%, I cannot compare it with the other results, since it cannot be diluted by water and I have difficulty looking for other solvents to dilute oil. Besides using oil I can use some other solutions such as ethanol, sulphuric acid, acetic acid, etc.
Further investigation can be done using different wavelength of light. I have tried to look for a light source providing blue monochromatic light beam. I predicted that the reflective index would be different if I use another light source of different colour since they are having different wavelength. However blue monochromatic light source is no commercially available.
Conclusion
This investigation proves that various factors affect the refractive index of liquid. They include different concentrations and different types of liquid. The temperature of the solution does not change the refractive index in same state. My results are fairly accurate; all the results are within the uncertainty limit. Although this experiment looks simple and easy to conduct, actually it requires very precise skills, since one millimetre difference can change the refractive index to about 0.05, which is a great change. The principle of this investigation is very useful in food industry like Abbe Refractometer. It can be used for determining the sucrose content of food. I found this investigation very challenging and interesting.
Bibliography
[1].
[2]. Salter Horner AS Advanced Physics P. 257
[3]. Eric Webster- AS/A-level Physics
[4]. CRC handbook for Chemistry and Physics (from: http://www.protein-solutions.com/psi_books/light_scattering/static/is_the_solvent_refractive_index_o_wavelength_dependent_.htm)
[5]. Letts Study Guide A level Physics