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Find the refractive index of a plastic block.

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Name        Jonathan Tam                Class: 13A

Yew Chung International School                Physics

Aim:        Find the refractive index of a plastic block


I will use two methods to find the refractive index of plastic. The first one is to use the critical angle, and the other one is to use Snell’s Law.

Critical Angle method:


  1. Set up the apparatus as shown. Put a piece of white paper under the apparatus. Turn off lights from the surroundings.
  2. Put a black slid into the light box so that only a thin ray will be shone.
  3. Turn on the light box. Adjust the angle of incidence so that there are no refracted rays.
...read more.


image10.png to get refractive index of plastic.Repeat step 3 to 6 at least 7 times to get sufficient readings. Use a different angle of incidence each time.Compare the values obtained using the two methods.

Data Collection:

Critical angle method:

Critical Angle (± 0.5°)




Snell’s Law method:

Angle of Incidence (± 0.5°)

Angle of Refraction (± 0.5°)















...read more.


image03.png is 1.

















The refractive index of plastic using Snell’s Law is: 1.344 ± 0.06

Conclusion and Evaluation:

From the two experiments, the refractive index of plastic is coherent to be between 1.344 to 1.36.

There are many systematic errors in these experiments, which came from the apparatus itself. These include:

  • protractor
  • light box

The protractor has a very high uncertainty of ± 0.5°. This has generated a high percentage error in our results (from 0.6% up to 3.3%). When we take sin for both angles and add the two percentage errors (or uncertainties) together, it has generated a whole range of refractive indexes.

However these results are acceptable because they both fall within the range of uncertainties (i.e. no contradictions). Therefore these experiments, although carries a high percentage error, can still be considered as valid.

...read more.

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