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# An Experiment to Measure the Speed of Light in Glass.

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Introduction

An Experiment to Measure the Speed of Light in Glass Plan Method Apparatus - Glass D-Block Optical pins Cork board Protractor printed on paper To find the refractive index you must first find the angle of incidence and reflection. You use the method of no parallax, I will use ray tracing with the method of no parallax. This involves placing the D-Block on the protractor paper, so the normal is at 0o, the center of the D-Block. I will place two optical pins pointing at the normal, lining up the two pins, I will place the third lined up with these two, looking through the D-Block. I will measure the angle between the normal and the line that connects the optical pins in front of the D-Block to the D-Blocks front center (i), and the angle between the normal and the line that connects the optical pin behind the D-Block to the D-Blocks front center (r) and record them. To be safe I will make sure I am careful with the apparatus. Theory I know that when light goes through a different medium from which it is already in (a vacuum for example) ...read more.

Middle

Snell's law states that "the refractive index of the medium light is passing into/ the refractive index of the medium light is passing out of = Sini/Sinr". As air has a refractive index of 1, it simplifies to - the refractive index of the medium light is passing into = Sini/Sinr * Prediction I think that the speed of light in glass will be less of that than in air because the speed of light in air is so close to that of a vacuum, which is where light travels fastest. It also stands to reason that a solid is denser that air, and so light will have to travel through more particles. Variables The independent variable is the angle of incidence; I will vary this from 10o till 80o at 10o intervals. At 0o there will be no refraction as explained, and at 90o the D-Block will be missed. The dependant variable is the angle of refraction; I will repeat the results at least twice and take a wide range of readings so that the experiment will be fair, and reliable. ...read more.

Conclusion

* This proves that light slows down from air into glass, and in doing so refracts. This refraction stays constant, as expected and the level of refraction-(which is constant, so works) can be obtained using Snell's law. This refractive index can be used to obtain the speed of light in that particular medium, by using the speed of light in air/a vacuum as a basis. Evaluation * The results are firm enough to draw accurate conclusions, and test the prediction because the points on the graph stick very close to the line of best fit. * The second set of results are almost all above the first set by 0.5o suggesting that one variable was overlooked, but didn't change, possibly the D-Block was off the normal slightly. * Also saying that the refractive index of air is 1; when it is 1.0003, may render the results slightly out if repeating the experiment more accurately, though here it does not make a difference. * You could do the experiment backwards, from glass into air, to prove that Snell's law still works. If the refractive index turned out the same then it should be proven. * Extracts from web-site: http://www.geo.utexas.edu/courses/347k/redesign/PDF_Handouts/snells.pdf (Preliminary work) Sean Fabri ...read more.

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