Rectangular glass block
Raybox with single slit
Draw round the glass lock and shine the ray from the raybox so that it enters one long side of the block and exits the other.
Mark the centre line of the two rays using the pencil.
Remove the block and join the dots to the outline of the block. Connect up the entry and exit points within the block outline.
Mark on a normal where the ray enters the block outline.
Measure the angles of incidence (outside block) and the refracted angle (inside block)
Record the pair of angles.
Repeat for a number of different angles of incidence.
Rayboxes can get hot whilst in use, leave to cool down before putting away, take care when moving the box between tests, switch off between measurements.