Investigating Refractive Index

This practical measures the refractive index of glass using a rectangular glass block and a ray box.Method:
1. Place the glass block on a sheet of white paper and draw round it.
2. Draw a normal at 90° to one long side, near the middle.
3. Use a ray box with a single narrow slit to aim a thin ray of light at the point where the normal meets the block, at a measured angle of incidence i.
4. Mark the incident ray and the ray leaving the far side with small crosses. Remove the block and join the crosses. Draw the refracted ray inside the block by joining the entry and exit points.
5. Measure the angle of refraction r from the normal with a protractor.
6. Repeat for a range of i, for example 10° to 70° in 10° steps.Analysis: from Snell's law, 1.00 × sin i = n sin r. Plot a graph of sin i (y-axis) against sin r (x-axis). It should be a straight line through the origin, and the gradient equals the refractive index n of the glass.Worked example: one reading gives sin i = 0.60 and sin r = 0.40, so n = 0.60 / 0.40 = 1.5. The graph gradient uses all the readings and is more reliable than one pair.Improving accuracy: use a narrow ray and a darkened room to reduce uncertainty in locating the ray; mark the crosses far apart; use a sharp pencil. The ray box gets hot, so let it cool before handling.