- Rectangular Perspex block
- Several sheets of blank white A4 paper
- Pin board
- 2 tall pins
- Black paper
- Range of sugar concentrations
- Sharp pencil
- Protractor
In my apparatus I have made steps to improve the sensitivity and accuracy of this investigation. For example I will be using black paper to make visibility of the disappearance easier to spot. I must ensure that I use a sharp pencil so that my measurements are precise. By using the pins and pin board system I can take time to check my measurements are correct before drawing in the lines.
Method
- Place a clean sheet of paper on top of the pin board, and then the Perspex block on top.
- Draw accurately around the block and put a mark at half way down the long side of the block.
- Cut a strip of black paper exactly the half the length of the block and the same thickness of the block.
- Soak the piece of black paper in a sugar solution and place on the side of block as shown in the diagram (this forms a very thin layer of solution between the block and the paper).
- Using your eye at the level of the block on the desk move your eye from left to right until you determine the exact point the black paper disappears from sight.
- Without moving your eye, place a pin next to block in line with your sight of disappearance.
- Repeat but with the pin further away from the block.
- You can now remove the pins and join the pin holes in a straight line to the block, and another line from where this reaches the block to where the edge of the black paper is.
- You should remember to make a note on this piece of paper what concentration was used.
- After thoroughly washing the block to remove any traces of the sugar solution, repeat using a clean sheet of white paper and a clean strip of black paper for the rest of the solutions including the unknown concentrations.
Note that when the piece of paper has been soaked with sugar solution the rest of the steps in the method should be taken rather swiftly. This is to prevent the evaporation of the sugar solution which could affect the accuracy and reliability of results.
Safety:
This experiment is relatively hazard free, however the following points should be taken into consideration for the sake of the safety of people working in the laboratory:
- All bags and unnecessary stool or obstructions on the floor should be put in a suitable place to reduce ripping which doing active experiments.
- Any spilt sugar solution should be cleaned up immediately to prevent tripping by people in the laboratory.
- Whenever using glass wear care should be taken to minimise the chance of breakage, e.g. keep it well on the bench.
Variables:
My input variable will be the concentration of the sugar solution. I will use a fairly broad and evenly spaced range of concentrations to ensure my graph is accurately drawn and leaves little space to miss important changes in correlation.
This should give me an output variable of refractive index which I will calculate using the equation I have stated previously.
There will be a few control variables that I will have to ensure are kept constant to maintain the credibility of my experiment:
- The same Perspex block should be used so that the refractive index that I am measuring is the same throughout the experiment.
- I will use the same equipment for the whole experiment to reduce variation in accuracy and sensitivity, for example different types of black paper may alter the accuracy of my experiment.
- An especially important part of apparatus to keep constant is the measuring equipment (ruler and protractor), as they may have variations in the accuracy of their scale. The scale at which I am able to read the protractor will affect the sensitivity of my investigation.
- If I chose to do a repeat experiment, I should make sure I use exactly the same materials to protect the reliability of my experiment.
Changes to the method:
When I started my experiment I found that there was not a precise position at which the black paper went out of sight, therefore I used the position at which part of the black paper finally disappeared.
Results:
The following table shows the set of results which I collected:
It is obvious from these results that my experiment was not accurate and the refractive index increases and decreases etc.
Therefore I will use results from another source whose results were reliable to use for my analysis:
Graph: Attached at the back
Analysis and Evaluation:
From looking at my graph I can confirm my prediction. The higher the concentration, the more a ray is refracted. This is due to the higher density in a higher concentration, the ray of light has a greater change in speed and therefore it refracts at a larger angle. In conclusion I have worked out from my graph that:
Concentration of A ~ 11.0%
Concentration of B ~ 26.5%
Looking at my graph I could say that there were no anomalies in this set of results as I have decided to use the first and last concentration in my line of best fit, however they could be excluded as they are not quite as in line with the others.
As I could not use my results because of their inaccuracy, there must have been considerable accuracy/sensitivity problems in my experiment. I think that probably the most considerable error was human error in distinguishing with our eyes the disappearance of the black paper. If I did this experiment again I would take a range of readings with the pins; one when the black paper first starts to disappear and one when the paper has completely disappeared. I would then take a mid-point of this range as my value.
I also think that the sensitivity in my protractor was limited as I could only read it accurately to ±0.5º, and as the angles were so close it would have been better to have used a more accurate measuring tool.
In conclusion, I am disappointed that my results did not work as I controlled all independent variables etc to ensure the reliability of the experiment. This shows that the sensitivity and accuracy of this experiment is poor, however with a good set of results I feel I came to a good conclusion of concentrations
