Thursday, 7 May 2015

Measuring the refractive index of a rectangular prism


Rectangular prism (glass or perspex)
Raybox with single slit fitted and power supply
Plain paper
Sharp pencil


Place the block on the paper and draw around it.
Mark on a normal line perpendicular to the block outline midway along the long side of the block so that it extends to within the block outline drawn.
Use the protractor to measure and mark out lines every 10 degrees from the normal, up to 60 degrees.

Shine the ray from the raybox so that it enters the block at an angle of 10 degrees from the normal.
Carefully mark 2 or three dots in the centre of the ray leaving the block on the opposite side.
Remove the block and use a ruler to join these dots to show the path of the exiting ray up to the block outline.
Use the ruler join up this point with where the normal intersects the other side.

Measure the angle between the refracted ray drawn inside the block and the normal and record this.
Repeat this drawing on of dots and measuring the angle of refraction for each angle of incidence.

Calculate the refractive index for each pair of readings by finding sin i / sin r - then calculate a mean value from the five values of refractive index calculated.


Rayboxes can and do get hot whilst in use so care should be taken to allow it to cool before packing away. Glass or perspex prisms could shatter if dropped onto a hard surface - if so any broken material should be cleaned away using a dustpan and brush, not fingers.