= Sin(i)/Sin(r).
Apparatus
Perspex block (semi-circular)
Protractor sheet (A4)
Lens
Collimator
Optical pins
Power pack
Method
- First we gathered all the required equipment.
- Then we set it up by placing the perspex d-block on the A4 protractor paper and plugged in the ray box into the mains.
- We then shone the ray of light at numerous different angles through the curved surface of the block.
- At every angle from 5-40 in 5° intervals we recorded the angle of refraction and then repeated the measurement once more to ensure accuracy. This is reflected in the two tables I produced.
- Upon completion of the experiment I created a table of results that shows the angle of incidence, angle of refraction, the sine of both these angles and the sine of i over the sine of r.
- I then averaged the results as there was a wide spread of them.
- I did this so that I could calculate the refractive index.
- This enabled me to calculate the speed of light through perspex.
Diagram of experimental setup
Prediction
I believe that the speed of light in perspex will be less than in air because the speed of light in air is close to that of a vacuum, which is where light travels fastest. Also solids are denser than air, and so light will have to travel through more particles.
The ray of light will travel directly towards the normal inside the perspex D-block. Then on leaving it will refract away from the normal. The effect of this is that the emergent ray is parallel to the incident ray, but is laterally displaced from it.
Prediction information
It is known that the speed of light in air is 300,000,000 m/s, so firstly work out what sine I over sine r is and you multiply 300,000,000 by what ever you work out sine I over sine r to be. E.g. if the refractive index = 0.65 you would do 300,000,000 x 0.60 = 180,000,000. If the refractive index was actually 0.65 then the speed of light in perspex would be 180,000,000 m/s.
Prediction Graph
Results (first set of results)
From the table I am able to calculate the speed of the light in perspex.
Speed of light in Perspex
= 299,792,458 x 0.68 = 203,858,871.4 m/s
Analysis
The speed of light through perspex (203,858,871.4 m/s) is less than that of air (300,000,000m/s). This clearly proves my prediction to be correct. The graph shows that Sin(i)/Sin(r) is constant and proportional, and can be used as the refractive index, as does the results table. There is a positive correlation between Sin (i) and Sin (r). The results have therefore proved my prediction to be correct, the speed of light through perspex is slower than the speed of light through air. However this experiment alone does not prove that this is due to the density of the medium. This proves that light slows down from air into perspex, and in doing so refracts.
Evaluation
From conducting this experiment I can conclude that my prediction has been satisfactorily proved and my results are reliable to a certain extent. This is true despite the fact that not all of them fell on the line displayed in the graph. These results can be called anomalous results as they may have been certain factors which affected them in order to make them inaccurate. There are a number of factors that may be responsible for this effect. Firstly the pencil line drawn on the A4 protractor paper was quite thick as it was drawn with a blunt pencil. This obviously could have an effect upon the measuring process. This could have provided me with results which are infact entirely inaccurate. The effect of the aforementioned blunt pencil may have increased as the experimentation process went on. This is because the pencil may have become blunter and blunter as the experiment went on. A thick line produced by a blunt pencil makes the accurate reading of a drawn line almost impossible. However trivial this may seem a blunt pencil can have an adverse effect upon the accuracy of the readings taken in the experiment. Another factor may be using the collimator to produce the light beam. The collimator had a small slit in to allow light to travel through; I feel that the slit used in the experiment could have been a bit narrower to have produced a much more slender ray so we could have drawn it a bit more accurately. The D-block was occasionally knocked and moved slightly off the point it was supposed to be on. This meant that we had to place it back a number of times. This unfortunately may have resulted in readings being taken with the d-block at a different position than the previous readings. The results would therefore be inaccurate.
If implemented these changes would provide a repeat experiment with a higher degree of accuracy.
Further Experimentation
I could also find out the speed of the various colours of light through perspex to gain a wider range of results. This would mean using a wider slit on the collimator to get a wider scatter of the 7 colours, and so that each is distinguishable by some margin. Once the data has been collected, averaged, graphed and analysed hopefully it would be possible to get an answer such as mine, but increasingly accurate. This would provide a figure that would be most definitely closer to the actual speed of light through perspex.