PhysicsLAB Lab
Index of Refraction: Glass

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Purpose: To use ray sightings to calculate the index of refraction of glass.

Equipment:
 
  • 7 cm x 7 xm glass square
  • ruler
  • cardboard
  • protractor
  • yellow data paper
  • 2 straight pins
 
Set-up:
 
 
Procedure:
 
  1. Place the yellow paper on the cardboard.
  2. Place the glass plate in the center of the yellow paper and trace its outline in pencil.
  3. Place the straight pins into the paper along a slanted line between 2 to 5 cm from the top of the glass.
  4. Sight the base of the pins through the glass until the edge of the ruler "appears" to line up with the pins.
  5. Using a ruler, sketch this line on your paper.
  6. Remove the glass and connect this sight line to the lower edge of the glass outline.
  7. Next draw the line that connects the two stright lins to the top edge of the glass outline.
  8. Connect the point where the line from the pins on the top edge of the glass outline to the point where the line draw along the edge of the ruler hits the bottom of the glass outline.
 
Measurements:

 
Using your protractor, measure all four angles: the angle of incidence and the angle of refraction at the top interface and the angle of incidence and the angle of refraction at the bottom interface. Label your diagram and then place your answers in the data table provided. Next use Snell's Law to calculate the experimental index of refraction for glass based on the angle data for each interface.
 
nglass sin(θglass ) = nair sin(θair )
 
since nair = 1.0
 
nglass =  sin(θair )/sin(θglass )
 
 
Data Table
 
top
interface
bottom
interface
air
glass
experimental index
Measure in cm the length of the path followed by the light through the glass plate. 

 
 
Analysis and Conclusions
 
What is the average of your two experimental values for the index of refraction of glass? 

What was the percent difference between your two experimental values for the index of glass? 

Calculate the average speed of light (in m/sec) through glass using your average experimental index of refraction. 

Calculate the time required for the light to pass through the glass plate. 



On your papers, in addition to labelling your angles, color the ray from the pins in color #1 and the normal and glass plate in color #2. Remember to place arrows on each ray showing that the light originated at each pin and traveled through the glass to your eye.

 
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