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An Experiment to investigate the change of speed of light through perspex.

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Josh Mills 11X

An Experiment to investigate the change of speed of light through perspex


 To measure the change of speed of light in perspex and the in the air and record the lateral displacement of the ray.

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 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 θ a and refraction θ b

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From my preliminary experiment I expect the speed of light in perspex to be: -300,000,000/1.49 = 201,342,281m/s  

This is the speed of light going through the perspex on the flat side. This should be same for this the ray of light entering the curved side.

I expect the graph to be a straight line because Sin I over Sin R are directly proportional should be a constant.









Perspex D-block (semi-circular)

Protractor sheet (A4)



Optical pins

Power pack


The first thing we did was shut all the blinds in the classroom so that no light could affect our results. Then we placed the ray box on a piece of paper so that the ray of light was clearer to follow and our results would be more accurate. Then we placed a collimator and a lens to make the ray more powerful and thinner, this helped increase accuracy. We then shone the ray at a D-block’s rounded edge at measured angles.

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From my results I can work out the speed of light in perspex.

Using the refractive index for perspex, (which is Sin I over Sin R) multiplied by the speed of light in air.

0.678 x 300,000,000 = 203,400,000m/s

This can be rounded down to 200,000,000, which we know is the speed of light in perspex.

The speed of light is as I predicted when the results are rounded. Also in my prediction I said that the ray would refract away from the normal and that Sin I/ Sin R are directly proportional.

On my graph it shows that Sin I/ Sin R is a constant, this is significant because it shows that my refracted index is 0.678 (when the results are averaged and the anomalous result is excluded). The actual refractive index for perspex is 0.66, this proves that my results are accurate and reliable. Although when I carried out the experiment I didn’t repeat the experiment twice, fortunately my results are close to the real refractive index.

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