The equation I will be using to work this out will be:
Refractive Index of substance = speed of light in air
Speed of light substance
This experiment will be carried out with 3 different liquids- ethanol, sunflower oil and water. The angles I will be using to measure the Refractive Index will be 10o, 20o, 300, 40o, 50o and 60o. I have decided to use these angles because they are at regular intervals and they are easier to work out. The experiments will be repeated as many times as possible to assure an accurate set of results.
Prediction
When light moves from one medium to another, the direction of light is altered. This is called refraction. The quantity of refraction depends on the properties of the two media, and the difference in their optical densities. The greater the difference, the greater the refraction and so the more the light would bend towards the normal. Therefore, a greater optical density results in a greater refractive index. Consequently, I predict that the refractive index of cooking oil is going to be higher than the refractive index of water and Ethanol based on the theory of optical density, which causes refraction, refers to the transparency of the medium. I think that the refractive index of cooking oil will be lower than the refractive index of glass. This is because glass is a solid and so its molecular structure is more compact.
Safety
In this experiment I will be using liquids, electricity and alcohol together. Therefore I have to make sure that the liquids do not come into contact with the electricity. I will also be using flammable alcohol. I will have to be cautious and keep this away from electricity as it could cause a fire.
Fair test
The intensity of the light source will also be kept the same. I will try to keep the beam as narrow as possible. This is because if the beam is too wide, the emergent angle will be harder to determine compared with a narrow angle. In this experiment I will be keeping a number of things the same. I will be keeping the amount of liquid the same despite making no difference to the refractive index.
Results
Water
Oil
Ethanol
Conclusion
The lines on all three graphs are straight, and go through the origin. This shows that sin I is proportional to sin r. For instance, if sin I increases then sin R will increase.
The gradient of each of the three lines was taken. It is known from scientific knowledge that:
- The gradient of the line (the refractive index) of water is 1.33.
- The gradient of the line (the refractive index) of ethanol is 1.34.
- The gradient of the line (the refractive index) of oil is 1.52.
By looking at the steepness of the graphs it is clear as to which one has the greatest refractive index. The graph that has the steepest line is the liquid with the biggest refractive index and the graph with the least steep line is the liquid with smallest refractive index. Oil has the steepest graph, then ethanol and then water therefore implying that oil has the biggest refractive index, followed by ethanol and water.
Therefore, from these results it can be seen that my prediction was correct.
I also predicted that the substances with a higher optical density would refract light more, and from my results we can see that this is correct. Oil is the most optically dense and in the experiment this bent towards the normal the most, unlike with water, had the least optical density.
Evaluation
My experiment went quite well and therefore I was able to work out
water, ethanol and oil’s refractive indices. My results allow me to see whether oil has the greatest refractive index so I could see whether my prediction was correct or not.
My results are reasonably accurate. There were not any outstanding inaccuracies but anomalies can be seen on the graphs, circled. Most of the errors were when I was investigating the refractive index of water and I think this is probably because the angle of refraction was not measured carefully enough with the protractor.
When testing ethanol, two beams of light could be seen on the paper because some light went over the shutter in the ray box. So it was not necessarily clear as to which one was the correct angle of refraction. Despite this, accurate results seem to have been obtained.
The experiment was reliable enough to complete the investigation safely, fairly and accurately. It was an easy way of succeeding in finding out what was needed to be investigated.
However, although in the plan it said that each experiment was going to be repeated to check for errors and therefore make the results more accurate, there was not enough time to do so. So if there was an opportunity to do this investigation again, each experiment for each of the three liquids would be repeated. This would mean that an average could be taken and so the results would be even more accurate.
To take the investigation even further, it could be done in slightly different circumstances. The liquids could be poured into a different shaped tray, or a tray made out of thicker plastic, or the liquids could be a different temperature. This would then give a larger range of results, by showing how the refractive index changes under different circumstances.
Mr Strange Physics Coursework
