Equation
na Sin θ a = nb Sin θ b
Prediction
My first prediction is that the angle of incidence will be 2/3 of the angle of refraction. This is because sin I divided sin r is the refractive index, which should be 0.66.
Also to back up my prediction I did a preliminary experiment, but instead we put the ray of light through the flat surface of the D-block.
These are my results
From the preliminary experiment I can predict that the line will definitely refract away from the normal this is opposite to what happens in the preliminary experiment. It will be refract because the speed of light in air is faster than the speed of light in perspex. When the ray hits the perspex one side of the ray hits it before the other side, so one part of the ray is travelling slower than the other, this causes the ray to refract. More particles are in the way, so it has to take a longer route around the particles so it refracts.
From my preliminary experiment I expect the speed of light in perspex to be: -300,000,000/1.49 = 201,342,281m/s
This is the speed of light going through the perspex on the flat side. This should be same for this the ray of light entering the curved side.
I expect the graph to be a straight line because Sin I over Sin R are directly proportional should be a constant.
Diagram
Apparatus
Perspex D-block (semi-circular)
Protractor sheet (A4)
Lens
Collimator
Optical pins
Power pack
Method
The first thing we did was shut all the blinds in the classroom so that no light could affect our results. Then we placed the ray box on a piece of paper so that the ray of light was clearer to follow and our results would be more accurate. Then we placed a collimator and a lens to make the ray more powerful and thinner, this helped increase accuracy. We then shone the ray at a D-block’s rounded edge at measured angles. To make it a fair test I will keep the protractor in the same place and keep the power of the ray the same. The light ray then refracted to produce a new angle, the angle of refraction, which we then recorded using a protractor to nearest degree. For safety reasons I would not touch the bulb directly after the experiment in case it is still hot.
Results
Analysis
As the light ray enters the D-block it slows down and refracts. It refracts because the speed of light in air is faster than the speed of light in perspex. When the ray hits the perspex, one side of the ray hits it before the other side, so one part of the ray is travelling slower than the other. When the light hits the perspex block it slows down because inside the perspex block there are more particles to avoid. So it has to take a longer route around the particles.
Also the ray splits into different colours of light, red light particles are the fastest so they refract away from the normal the least and blue/indigo light particles are the slowest so they refract the most.
From my results I can work out the speed of light in perspex.
Using the refractive index for perspex, (which is Sin I over Sin R) multiplied by the speed of light in air.
0.678 x 300,000,000 = 203,400,000m/s
This can be rounded down to 200,000,000, which we know is the speed of light in perspex.
The speed of light is as I predicted when the results are rounded. Also in my prediction I said that the ray would refract away from the normal and that Sin I/ Sin R are directly proportional.
On my graph it shows that Sin I/ Sin R is a constant, this is significant because it shows that my refracted index is 0.678 (when the results are averaged and the anomalous result is excluded). The actual refractive index for perspex is 0.66, this proves that my results are accurate and reliable. Although when I carried out the experiment I didn’t repeat the experiment twice, fortunately my results are close to the real refractive index.
