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Find out the speed of light through Perspex by passing a narrow ray of light through a D-Block of Perspex, by using the same concepts and ideas as Snell's Law.

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Physics Investigation

Investigation to find out the Speed of Light through Perspex


To find out the speed of light through Perspex by passing a narrow ray of light through a D-Block of Perspex, by using the same concepts and ideas as Snell’s Law.


Light is an electromagnetic wave. The speed of light depends on the medium through which it propagates: it goes fastest in a vacuum, almost as fast in air but considerably slower in glass. Because of the special role it plays in many parts of physics, the speed of light in a vacuum has been given its own symbol: c. The speed of light in any other material we denote with v. The ratio of the two is defined as the refractive index, symbol: n.



  =a constant =  image11.pngimage09.pngimage10.pngimage08.png

I could also use my graph to calculate the refractive index


  • Ray Box
  • Perspex D-Block
  • Protractor paper
  • Pen/ Pencil
  • Ruler






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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:



  • 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.
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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.

Faraz Auckbarally 10Z        -  -

...read more.

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