Investigation:To find the refractive index of cooking oil.

INVESTIGATION: TO FIND THE REFRACTIVE INDEX OF COOKING OIL

PREDICTION AND SCIENTIFIC KNOWLEDGE

Light travels in straight lines. However, refraction is the change in direction of a wave passing from one medium to another. Light refracts at all angles except 0 degrees.

An example of refraction can be seen when you go swimming. If an object such as some swimming goggles is dropped in the water, they will appear to be in a different position to what they actually are. This is because the light changes direction at the surface, giving the illusion that the swimming goggles are closer than they actually are. The bending of the light, refraction, is caused by the light waves travelling slower. Either the density or the optical density of the object in which the light has to travel through causes this.

Density is the mass of the object divided by its volume. However, optical density is related to atoms. When the waves collide with atoms, which make up the material, the energy is absorbed. This therefore makes one of the electrons jump up to another orbit. The electron then descends to the shell it was originally in and re-emits the light wave. For every atom the light waves hit this happens, slowing the journey down. Some materials are faster, depending on how long it takes the electrons to re-emit the light.. If light does not travel through a material then the atom is not re-emitting the electron.

To find the refractive index the angles of incidence and refraction need to be calculated. Then the following is completed:

The angle of incidence is the angle at which the light enters the object. The angle of refraction is the angle at which the light leaves the object. The angle of incidence is always the same no matter what the object or material is.

The greater the optical density, the greater the refraction index will be. In order to find the refractive index of cooking oil I will calculate the refractive index of other materials to give me an idea of what the answer might be.

To ensure that the refractive index for cooking oil is as accurate as possible, ethanol and water will be tested during the pilot experiment. This should justify whether the method being used is correct because the results can be compared with the published values shown below.

I therefore predict that the refractive index of cooking oil will be between the refractive index of water and glass. I think that cooking oil could be less dense than water because when it is put with water it separates and floats on top. However, it does look more physically dense than water does, as it has a more syrupy texture, so I think that it has a greater refractive index than water, but less than glass.

I therefore predict that the refractive index of cooking oil will be approximately 1.40.

DEPENDENT VARIABLE (things to be kept the same)

In order to make this investigation a fair test, several pointers must be followed throughout the experiment.

These are:

The plastic tray, which is being used to hold the substances, must have be used throughout the whole experiment because other trays which look the same may have a varied density. This would ultimately mean that the angle of refraction would be different to the expected result. This is because the light would spend longer travelling through the plastic of the tray before reaching the liquid inside. Using the same container would ensure that the light travelled at the same speed through the plastic tray. This should prevent any possible inaccuracies.

The volume of the liquids being used in this experiment must be kept the same. This is because different volumes could have some effect on the angle of refraction. It is possible that the light ray may not actually travel through the liquid if the volume is different and too low.

Some of the plastic trays have bumps on the bottom. To compensate for this bump the ray box will be placed on a different surface to the container, but higher up. This would ensure that the light ray does not hit the bump, but instead travel over ...

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The volume of the liquids being used in this experiment must be kept the same. This is because different volumes could have some effect on the angle of refraction. It is possible that the light ray may not actually travel through the liquid if the volume is different and too low.

Some of the plastic trays have bumps on the bottom. To compensate for this bump the ray box will be placed on a different surface to the container, but higher up. This would ensure that the light ray does not hit the bump, but instead travel over it. If this is not compensated for, travelling through the bump could cause the light to change direction. Another method, which could be used to avoid hitting the bump, would be as follows. The tray could be drawn round and with the image on the paper it could be cut out. By placing it around the outside of the container, acting like a collar, when the light rays travel through the material they will not hit the bump and cause possible anomalies.

Using a stronger light bulb would ensure that the ray of light could be clearly seen on the other side of the material. This would make the angles easier to mark on because the line would be more visible.

The materials must be kept at a similar temperature. If this does not stay the same then the angle of refraction could possibly be affected. The reason for this is because when a liquid becomes hotter, the atoms inside the liquid begin to move faster. This would make a difference to the speed at which the light waves are re-omitted.

INDEPENDENT VARIBLE (Things to be changed)

During the experiment the following things will be changed in order to obtain results:

The substance in the plastic tray will be changed during the pilot experiment to ensure that the correct method is being used to test the substance ethanol and water. Six angles of refraction will be found. During the final experiment, this will not be changed. Instead of this, repeats will be performed for the same substance, which is cooking oil.

In order to obtain six different sets of results, six different angle of incidence need to be used.

PILOT EXPERIMENT APPARATUS

Water
Ethanol
Measuring cylinder
Protractor
Calculator
Paper
Cardboard box
Scissors
Plastic tray
Light ray box and cardboard slit
Pencil
Power supply

PILOT EXPERIMENT

The purpose of the pilot experiment is to test the refractive index of other materials, to see if they comply with the given refractive indexes. If the guideline for the refractive index meets the results shown then the experiment is appropriate for the final experiment. These results may not be exact, but a rough guide, which will help to predict the refractive index for oil. I will use the following method for the pilot experiment.

Prepare the required apparatus
On the piece of paper place the empty plastic tray and draw round the base of the tray. Draw a horizontal line or 0-degree line on the paper. Once this has been drawn, a protractor can be used to measure every ten degrees up to sixty degrees.
The tray will be placed partly in a box to stop the light travelling through the bump. To do this, the plastic tray will be drawn round (onto the cardboard) and then cut out. This will leave an area for the plastic tray to slot into.
The refractive index of the plastic tray will be calculated first. This will allow the refractive index of water and ethanol to be worked out without having to allow for the plastic tray. However, if the refractive index of water and ethanol are near the published results then the refractive index of the plastic tray does not necessarily need to be calculated.
The light ray box will then be switched on, ensuring that the cardboard disk has been inserted. The ray of light will then be placed on the 0 degree line to prove that light does not refract at this angle. On the ray of light that can be seen on the other side of the tray, a line will be drawn. This will be joined to the line on which the light entered. The angle of incidence and refraction will be measured. (The angle entering the plastic tray and the angle leaving the plastic tray).
The refractive index will be calculated for each angle by doing Sin I divided by sin r.
I will repeat the same steps for ethanol and water. Fill the plastic tray with a suitable amount of ethanol, which will be equal to the amount of water, which will be tested later. The tray can then be placed on the outline, which has been drawn on the paper.
As with the plastic tray, each angle of incidence and refraction will be calculated. This will allow the refractive index to be worked out; by doing sin I divided by sin r.

Note: The refractive index of the plastic container is not large enough to make a difference to the refractive index of both liquids.

PILOT EXPERIMENT RESULTS

The following table shows the results for the refractive index of water

AVERAGE INTERNAL REFRACTION = 1.33

The gradient of the graph for this set for this set of results is calculated by choosing two points and dividing the sin I by sin r, or up over across. The gradient for the graph is also the same as the average worked out from the table of results.

Gradient = 0.766

0.574

Therefore the gradient = 1.33449 or 1.33 to 3 significant figures.

CONCLUSION

I therefore conclude that the method for this part of the pilot experiment was accurate, as the refractive index for water is the same as the published set of results. Both the results and the graph show that the results were fairly accurate as there is only one anomalous result.

The following table shows the results for the refractive index of ethanol

AVERAGE INTERNAL REFRACTIVE = 1.36

Gradient = 0.500

0.367

Therefore the gradient = 1.3623 or 1.36 to 3 significant figures

ANOMALOUS RESULTS

The results were very accurate, but there was one anomalous result. This was out by quite a lot when compared to the other results. This could have been for a number of reasons, including that the ray of light may have been getting less visible, so it was harder to see where the line was. The percentage of error for this result is 40 %, which is under half. This result would not have affected the average refractive index for ethanol, as the gradient excludes the anomalous results.

CONCLUSION

I therefore conclude that the method for this part of the pilot experiment was accurate, as the refractive index for ethanol is the same as the published set of results. Both the results and the graph show that the results were fairly accurate as there is only one anomalous result.

Both the ethanol and water results show that the method being used is on the whole; very accurate as the results have been the same as those published. This is reassuring when it comes to completing the remainder of this investigation, as now I know that I will not have to change anything other than what is already stated in the final method procedure.

FINAL EXPERIMENT METHOD

The final experiment has a very similar method to the pilot experiment except only one substance is being tested, which is cooking oil.

The same plastic tray will be used in the final experiment, as this will ensure there is more accuracy in the results. This tray will be filled to the top with cooking oil.
As with the pilot experiment the cardboard box will be used; this will help to prevent the light from travelling through the bump in the plastic tray.
The angles of incidence can be marked on the paper, after a horizontal line has been drawn. This line represents the normal. The angles of incidence are spaced at ten degrees and the highest value is sixty degrees. To draw these lines a protractor needs to be used.
The light ray box will then be switched on, ensuring that the cardboard disk has been inserted. The ray of light will then be placed on the o degree line to prove that light does not refract at this angle. On the ray of light that can be seen on the other side of the tray, a line will be drawn. This will be joined to the line on which the light entered. The angle of incidence and refraction will be measured. (The angle entering the plastic tray and the angle leaving the plastic tray).
The refractive index will be calculated for each angle by doing Sin I divided by sin r.

FINAL EXPERIMENT RESULTS

The following table shows the results for the refractive index of cooking oil.

AVERAGE INTERNAL REFRACTION = 1.45

The gradient of the graph for this set for this set of results is calculated by choosing two points and dividing the sin I by sin r, or up over across. The gradient for the graph is different to the average refractive index calculated from using the table.

Gradient = 0.766

0.500

Therefore the gradient = 1.532 or 1.53 to 3 significant figures.

ACCURACY

As this was the main experiment, it was harder to decide if the results were accurate or not because there were no published results to compare the final average refractive index of cooking oil. It is hard to tell from the results table whether the results are accurate or not, as the data in the table is not consistent, so the graph was a good thing to have as it visually represented the data. The line of best fit shows that were a few anomalous results. However, when it comes to choosing the correct average refractive index I would be inclined to choose the one derived from calculating the gradient of the graph, as this is a more accurate method.

CONCLUSION

In my prediction I said that optical density is related to how quickly light can pass through a substance. When the waves collide with atoms, which make up the material, the energy is absorbed. This therefore makes one of the electrons jump up to another orbit. The electron then descends to the shell it was originally in and re-emits the light wave. For every atom the light waves hit this happens, slowing the journey down. I predicted that some materials are faster, depending on how long it takes the electrons to re-emit the light. If light does not travel through a material then the atom is not re-emitting the electron.

Before I completed this investigation I predicted that the refractive index of cooking oil would be between the refractive index of water and glass. I thought that cooking oil was less dense than water because when it is placed with water, it separates and floats on top. However, the prediction I made was base mainly on the physical density because cooking oil looks denser because of its syrupy texture. I also thought this because ethanol looks denser than water and also has a higher refractive index than it. I predicted that the cooking oil would have a refractive index of approximately.

The density did have an affect on the results gained, but a point that I noticed was that the more transparent the object was the lower the refractive index. Water is very transparent and had a low refractive index, ethanol was less transparent and had a higher refractive index and this pattern continued.

I therefore predict that the refractive index of cooking oil will be approximately 1.40.

The prediction that I made was correct in some aspects as I said that the refractive index of cooking oil would be higher than that of both ethanol and water, which it was. The refractive index for cooking oil is 1.50.

In conclusion, the light wave photons raised the energy level, when absorbed by the atom. Light is made up of photons, some are immediately released and others take a longer duration of time to be released. Not every photon is absorbed by the atom. The natural qualities of each individual substance, causes the time taken for the electron to descend to its original shell to differ. The light is remitted more slowly in cooking oil than when compared to water and this is why the refractive index is higher.

All three graphs that were produced showed that the results were not random, but directly proportional to each other. Using the graph was a better way to gain the results than by using the table, as when the gradient is calculated, the anomalous results were not included in the average. This therefore means that the anomalous results will not affect the refractive index for each substance.

Teacher Reviews

Here's what a teacher thought of this essay

Luke Smithen

5th of Jul 2013

This is a well detailed report that has a good structure. 1. The background information section is well researched. 2. The preliminary test is performed well. 3. The table of results is well presented. 4. The conclusion is very detailed. 5. The report is missing an evaluation. **** (4 stars)

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