Verification of Snell's law of refraction

When light rays pass from one medium to another (different density), they change direction (are refracted). When they enter a more dense medium they bend towards a normal line drawn at the point of incidence.

In this experiment we investigate the relationship between the angles of incidence and of refraction for light rays travelling into perspex and into glass. Using these angles, or a suitable graph, we can then find the refractive index of perspex and of glass.

Procedure:

  1. Click the "Measure i" button to measure the angle of incidence. Record the value
  2. Click the "Measure r" button to measure the angle of refraction. Record the value
  3. Place the mouse pointer over the light source and click and drag the source to a new position
  4. Measure and record the angles of incidence and refraction
  5. Repeat until you have at least 6 sets of results
  6. Click the "Glass block" button and repeat steps 1 to 4 to find a set of readings for glass
  7. Divide the sine of any angle of incidence by the sine of the corresponding angle of refraction to find the refractive index of the medium

Results: Record the results in a table as shown here.

i
r
Sin i
Sin r
       

Graph: Draw a graph of Sin i (y-axis) against Sin r (start both axes at zero). A straight line graph (best fit) through the origin shows that Sin i is proportional to Sin r. Select two suitable points on the graph (far apart) to find the slope of the graph. This is an average value for the refractive index of the medium.

Precautions:

  • Carefully draw a normal line at the point of incidence to help you to place the protractor properly
  • Avoid using small angles of incidence as errors in reading the angles would then be relatively large
  • Place two dots far apart on the incident and refracted light beams to accurately locate the beams