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# Investigation to find the change in the speed of light through perspex.

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Introduction

Investigation to find the change in the speed of light through perspex Aim I intend to investigate the change in the speed of light in perspex. I intend to do this by passing a narrow ray of light through a D-block of Perspex, by using the same concepts and ideas as Snell's Law. Background Information Light is an electromagnetic wave. The speed of light depends on the medium through which it propagates: it goes fastest in vacuum, almost as fast in air but considerably slower in glass. The speed of light in a vacuum has been given its own symbol: c. The speed of light in any other material is denoted with v. The ratio of the two is known as the refractive index with the symbol n. When light goes from one medium to another, it doesn't only change its speed. Part of the incident light is reflected, and the remainder is transmitted. The transmitted light is generally also deviated or refracted. It was found experimentally by Descartes and by Snell, some 400 years ago that: � The incident, reflected and refracted ray all lie in the same plane � The angles of incidence ? ...read more.

Middle

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) No. i (�) r(�) Sin i(�) Sin r(�) Sin i/Sin r 1 5 7.5 0.087 0.13 0.67 2 10 15 0.17 0.25 0.67 3 15 24 0.25 0.40 0.64 4 20 31 0.34 0.51 0.66 5 25 38 0.42 0.61 0.69 6 30 45 0.5 0.70 0.71 7 35 57.5 0.57 0.84 0.68 8 40 69 0.64 0.93 0.69 Average 0.68 No. i (�) r(�) Sin i(�) Sin r(�) Sin i/Sin r 1 5 7.3 0.087 0.12 0.75 2 10 16 0.17 0.27 0.63 3 15 21 0.25 0.35 0.71 4 20 31 0.34 0.52 0.65 5 25 41 0.42 0.66 0.63 6 30 47 0.5 0.73 0.68 7 35 54 0.57 0.80 0.71 8 40 71 0.64 0.95 0.67 Average 0.68 From the table I am able to calculate the speed of the light in perspex. ...read more.

Conclusion

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. ...read more.

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This is a well structured and well written result.
1. The sources of information need to be referenced.
2. The word 'proved' is used many times and needs to be removed.
3. The conclusion needs to explain the results using researched information.
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Marked by teacher Luke Smithen 13/08/2013

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