Determination of the refractive index of a medium
Watch this video for a practical demonstration of how to measure the refractive index of light through glass.
- Draw round the semi-circular block and draw in the dashed normal line at 900 to the midpoint of the straight edge.
- Set up the raybox with an incident ray at 10掳 to the normal line and mark in the refracted ray with two pencil crosses at shown.
- Remove the raybox and semi-circular block then measure angles of incidence and refraction from the normal using a protractor.
- Repeat for angles of incidence from 150 鈥 400.
Plot a graph of sine of angle of incidence (y axis) against sine of angle of refraction.
The gradient of this graph is the refractive index of the material of the semicircular block.
Question
Why is the block semicircular in shape?
So the ray emerging undergoes no change in direction as it leaves as the emerging ray leaves at 90掳 to the boundary at any point on the curved surface. (The radius of a circle is always perpendicular to the circumference) If the line drawn back from the two crosses does not pass through the centre of the semi-circle the ray path should be repeated.
Question
Why must the incident ray be directed to the centre of the straight edge?
To ensure the refracted ray is at the centre of the semi-circle and hence undergoes no change in direction on leaving the block. If the line drawn back from the two crosses does not pass through the centre of the semi-circle the ray path should be repeated.
Question
What is the expected reading uncertainly of the angles?
Most readily available protractors have a smallest division of 10 so a reading uncertainty of 卤10 is a reasonable reading uncertainty. This will introduce an uncertainly of just over 1% to the final result.