Find the refractive index of a plastic block.

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Snell’s Law 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. Plot points on the paper to show the light rays. Connect the points to form lines.
4. Draw normal on the paper as shown. Measure angle i and angle r.
5. Use the following equation

6. Input  as 1 (refractive index of air),  as the angle of incidence and as angle of refraction. Rearrange the equation and calculate  to get refractive index of plastic.
7. Repeat step 3 to 6 at least 7 times to get sufficient readings. Use a different angle of incidence each time.
8. Compare the values obtained using the two methods.

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Data Collection:

Critical angle method:

Snell’s Law method:

Data Processing and Presentation:

Critical angle method:

Refractive of plastic using the critical angle method: 1.360 ± 0.014

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Snell’s Law method:

Since the medium of incidence is air and it has a refractive index of 1.003, therefore it has been assumed that  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.