There are two main laws of refraction of light: 1. The refracted ray lies in the same plane as the incident ray and normal at the point of incidence. 2. (Snell’s law). The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for two given media. This constant is the refractive index (n). When referring to light, this is also known as the optical density and, as with refractive index in other cases, can also be calculated by dividing the velocity of light in one medium by its velocity in the second medium. The formula for calculating the refractive index is:
I will draw a graph of Sine I and Sine R. I will do this so the points become positively correlated and can have a line of best fit placed through them. I will use the line of best fit to check any anomalies and find the Sine R value of 1 (900). When I have this value I will be able to find the Sine I value and will then inverse Sine it. This will give me my angle of incidence, which causes the critical angle. I will take a range of results from 00 to 450. I will take this range as I predict that in-between 400 – 450 will be the critical angle.
On my graph I have decided to plot Sine I and Sine R. I predict the results will be close to the line of best fit. The reason chose to do this was because if I plotted I or R. I would have got a curve from my results. This would have made it difficult to predict the critical angle. By using Sine I against Sine R I can extrapolate the line of best fit (which has an intercept of 0) to predict the angle.
Apparatus:
- A3 paper
- Calculator
- Pencil
- PSU
- Ray Box/Slit
- Ruler
- Semi-circular plastic block
- Semi-circular protractor.
Diagram:
Method:
- Arrange the equipment as shown.
- Then in the middle of the paper, draw around the block of plastic and keep it placed there.
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Then around the curved part of the diagram mark out the normal line, then mark out the angles in 50 intervals.
- Connect the PSU to the mains socket and then the ray box to the PSU.
- Fit the slit on the ray box allowing only a small ray of light through.
- Then turn on the PSU.
- Then shine the light on the interval and make sure it comes out of the centre of the straight side.
- From the normal angle find the refracted angle by using a protractor.
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Record the angle in a table and repeat the process in 50 intervals up to 450.
Results:
Graph:
Conclusion: I found that the critical was approximately at 430 either way (+ or -) a degree. The refractive index was approximately 1.28. There was a trend in my results as an angle I was increased, angle R also increased. When I varied the angle I, angle R was also affected. The light bent as it passed through the plastic as it was travelling through a denser medium. The ray of light was slowed down as it was passing through the plastic.
Some mistakes were caused by human errors. For example, by me not using a blunt pencil, it would make my results less accurate by a slight difference. This is a marginal error but could have a grater effect on a larger scale. If the angle was out by 50, then the answer would be 20% incorrect. By showing that the critical angle is slightly out by a degree either side shows that these marginal errors have been taken into account.
Evaluation: My method of carrying out the experiment was good and very simple to follow. Before this version of the experiment, I performed a trial and error attempt of the experiment to get an approximate idea of what the angle would be. Some errors were caused by simple things like rounding an angle to certain decimal places and misjudging an angle. If I were to redo the experiment again then I would take these factors into account and make sure that they would not have a significant effect on the final results. These improvements would make my results more accurate.
My results were valid as they allowed me to get an accurate line of best fit which was important in calculating the critical angle. The results were all along the line of best fit, which was as I had predicted. My results did not have any anomalies which did not affect my results. This conclusion led me to a final conclusion of the critical angle was 430 and the refractive index of the critical angle was approximately 1.28.
