PhysicsLAB Lab
Index of Refraction: Water

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

  • semi-circular water trough (D-cell) filled up two-thirds of the way with water
  • ruler
  • cardboard
  • protractor
  • green data paper
  • 2 straight pins
  1. Place the green paper on the cardboard.
  2. Place the semi-circular water trough in the center of the green paper and trace its outline in pencil.
  3. Asymmetrically, place the straight pins into the paper between 5 to 10 cm from the top of the trough's position.
  4. Sight the base of the left pin through the water until the edge of the ruler "appears" to line up the pin with the scratch mark in the center of the flat side of the water trough.
  5. Using a ruler, sketch this line on your paper - connecting it to the scratch mark on the flat side of the trough.
  6. Repeat the above process with the right pin.
  7. Remove the water trough and connect the line of sight for each pin to the central scratch mark on the flat side of the trough.

Using your protractor, measure the angle of incidence and the angle of refraction for each pin. 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 water based on the angle data for each pin.
nwater sin(θwater ) = nair sin(θair )
since nair = 1.0
nwater =  sin(θair )/sin(θwater )
Data Table
left pin right pin
experimental index
Analysis and Conclusions
Why should the rays not "bend" when they first enter the D-cell? 

What is the average of your two experimental values for the index of refraction of water? 

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

The accepted value for the index of water is 1.33. Calculate the percent error for your average experimental index of refraction of water. 

Calculate the average speed of light (in m/sec) through water using the accepted index of refraction. 

Using your previous value for the average speed of light, calculate the time (in sec) required for the light to travel through the water in the D-cell. 

On your papers, in addition to labelling your angles, color the rays from the left pin in color #1, the rays from the right pin in color #2, and the normal and water trough in color #3. Remember to place arrows on each ray showing that the light originated at each pin and traveled through the water to your eye.

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