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The purpose of this experiment is to determine the refractive index of Perspex plastic. All of these variables are related through Snells Law.

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To determine the index of refraction of Perspex plastic

Background Information:

When light enters a transparent medium from another, the light appears to bend inside the medium it enters (refer to fig 1 below). This is known as refraction and is caused by the change in speed of light as it enters the medium (Reed, 2009). The angle of refraction depends on the medium’s refractive index. The purpose of this experiment is to determine the refractive index of Perspex plastic. All of these variables are related through Snell’s Law.

Fig 1. Diagram of light (from Britanica Inc)


Snell’s Law states (Weisstein, 2007):


Where:                image10.pngimage10.png = refractive index of the medium where light is leaving (no units)

image23.pngimage23.png = refractive index of the medium where light is entering (no units)

image18.pngimage18.png = angle of incident (measured in degrees o)

image19.pngimage19.png = angle of refraction (measured in degrees o)

For this experiment, light will be travelling from air to the Perspex plastic. The angle of incident is the independent variable and the angle of refraction is the dependent variable. The refractive index of air is 1.

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Sample Calculations for Processed Data:

Sample Calculations 1. Calculating average for the angle of refraction:


The average value of the angle of refraction cannot exceed the amount of decimal places of the uncertainty.

Example using results from the angle of incident of 10o



However, the uncertainty for this trial has no decimal places (refer to Sample Calculations 2)

The average angle of refraction is 6o for the angle of incident of 10o

Sample Calculation 2. Calculating uncertainty for angle of refraction using maximum deviation:

The maximum deviation will be used to calculate the uncertainty for the average angle of refraction. This maximum is determined by subtracting the highest and lowest value from the average angle of refraction and the magnitude of this result will be used as the uncertainty. However, if the instrumental uncertainty is greater than the maximum deviation, then the instrumental uncertainty will be used.

Example using results from the angle of incident of 10o

Uncertainty = Average values – Highest value

= 5.2 – 6

= |-0.8|

= ±0.8o

Uncertainty = Average value – Lowest value

= 5.2 – 5

= |-0.2|

= ±0.2o

Since both values of the maximum deviation is less than the instrumental value of 1o


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What effect it had on the experiment


Thickness of the light and pencil

As both the mark and the light rays were thick. This may have caused random error.

By using a thinner pencil and constantly mark in the middle of the light ray. More trials could be performed in order to

Imperfections of the Perspex plastic

The refractive index may change according to different manufacturers.

Obtaining the refractive index from the manufacturer

Sliding Perspex plastic

The Perspex plastic prism was constantly moved as the angle needed to be measured. The position of the prism creates random error for the angle of incident.

By performing more trials and marking more shape


A. L. (2007, September). Plastic Comparison Table. Retrieved April 22, 2012, from Machinist Materials: http://www.machinist-materials.com/comparison_table_for_plastics.htm

Reed, R. (2009). Refraction of Light. Retrieved April 23, 2012, from Interactagram: http://interactagram.com/physics/optics/refraction/

Weisstein, E. W. (2007). Snell's Law. Retrieved April 23, 2012, from Science World: http://scienceworld.wolfram.com/physics/SnellsLaw.html

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