Info for prediction
We know the speed of light in air which is 300,000,000 m/p/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 refractive index = 0.7 you would do 300,000,000 x 0.7 = 210,000,000, so speed of light in Perspex is 210,000,000.
This is a sketch of what I expect my final Graph to look like:
Method
- First we gathered all our equipment
- Then we set it up: We placed the Perspex d-block on the protractor paper and plugged in the ray box.
- We then shone the ray of light and 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.
- Once we had completed the experiment we created a table of results that showed the angle of incidence, angle of refraction, the sine of both these angles and the sine of i over the sine of r.
- But because we had a wide spread of figures we had to then average them.
- We did this so that we could calculate the refractive index and therefore calculated the speed of light through Perspex.
Fair testing and Safety
To ensure a fair test I will:
- Use the same Perspex block for every experiment
- Use the same ray box for every experiment
- Repeat the experiment a second time to ensure any anomalous results can be scraped or justified.
For safety there are no extreme cautions to be made except to not look directly into the ray of light
Further work
For further work I propose to then separate the white light into its various different colours. So then I can work out the speed of various different colours and use this to support my primary experiment.
Results
I have worked out the speed of light in Perspex to be:
Speed of light in Perspex = 299,792,458 x 0.68=203858871.4 m/s
Analysis
First of all because the line on the graph is a straight line through the origin this means that sine I and sine r are directly proportional.
From my original prediction I can say that my results gave me exactly what I expected. I first work out the speed of light through Perspex to be 210000000 m/s
And the results I gained from my experiment via the curved edge was 203858871.4 m/s. so the results were very close together showing only maybe minor inaccuracies. It is also good because I did two sets of results for accuracy and that gave me the same value for n. and their dispersions on the graph were very narrow with no clear anomalous results. In fact the range of the figures in the second set of results is wider but it averages out as the same because there are extremely high results and really low ones so they average out in the middle which was 0.68.
At first I would guess that my results were correct because Perspex is a denser medium than air so the speed of light would be less.
Evaluating
I am very convinced that my prediction has been satisfactorily been proved, and is very reliable. This is because of the closeness of my final answer to my initial prediction. Also because there was a narrow spread on my graph it was obvious that the results were reliable, whereas if I gained a wide spread of points there would have been some cause for concern over the reliability of the results.
If I was to do the experiment again I would definitely use a much thinner slip of plastic to narrow the beam or ray of the light. This would provide us with more accurate results to base our final answer on. As stated before I would also find out the speed of the various colours through Perspex to gain a wider range of results.
This would mean using a wider slit to get a wider scatter of the 7 colours, and so that each is distinguishable by some margin. So then once the data has been collected averaged, graphed and analysed hopefully we would gain an answer close to the one I have recorded, but more accurate.
