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.

- Click on the radio button for Glass or Perspex to select the material of the block (the default material is glass).
- Using the slider, move the ray box a little to the right.
- Press "Measure i" and record its value.
- Press "Measure r" and record its value.
- Use the formula: n = sin i / sin r to calculate n.
- Repeat steps 1 to 5 until you have at least six sets of readings. Find the average value of the refractive index of the material of the block.
- Repeat the procedure for the other medium to find the value of the refractive index for that medium.

Perspex

**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.

- Avoid using small angles of incidence as errors in reading the angles would then be relatively large.
- When doing this experiment in the lab. place two dots far apart on the incident and refracted light beams to accurately locate the beams.