Investigate how the speed of light differs in air and in Perspex.

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Gcse Physics coursework: Refractive Index

Planning

Aim: I am going to investigate how the speed of light differs in air and in Perspex.

Background info: The refractive index is a ratio for working out the speed of light. The ratio varies for different substances, it indicates the extent to how light refracts through different substances. On passing from a less dense medium to a more dense medium, light is refracted towards the normal, and thus the angle of incidence, i, is larger than the angle of refraction, r., Willebrord van Roijen Snell (1591-1626), came up with a law explaining the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for any given pair of medium.  so a simple statement of his law is:

sin i / sin r  =  a constant, n

So in this equation the constant n is equal to the refractive index.

N defines the speed of light in Perspex as a decimal of the speed of light in air

Example of refractive index values are: water (1.33); perspex (1.49); window glass (1.51); different glasses (between 1.46 and 1.69); and diamond (2.42). Diamond has a very high refractive index this is responsible for it having such a aparkle.

Apparatus

Lab pack

D-block

Ray Box

Glass lens

Collimator

Prediction

I Predict that the speed of light in Perspex multiplied by the refractive index of Perspex (1.49) will equal the speed of light in air.

I am able to draw up a triangle formula for my prediction. (fig 1)

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But in our experiment we are shining the ray onto the curved surface and seeing how light goes from Perspex into air so we will attempt to work out sin I over Sin R and multiply it by the speed of light in air to get the speed of light in Perspex.

fig.1

I predict that the graph of sin I over sin r will be a straight ...

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