Total Internal Reflection with a Plastic Prism

PIRA 6A44.18

Set a laser up to shine straight down into the large triangular plastic prism from the blackboard optics as shown in the picture at near right.  The prism rests partially in the small cubic glass tank with the longest side horizontal.  The tank is partially filled with water with a little bit of powdered milk to make the laser beam visible.  A little bit of Pine Sol works very well also.  Adjust the laser so that the beam experiences total internal reflection. 

Slowly pour more of the water with the powdered milk into the cubic tank until the spot where the laser beam was internally reflected becomes submerged, as shown in the picture on the far right above.  With the raising of the index of refraction of the media outside the plastic prism, the condition for total internal reflection no longer holds, and the beam refracts into the water.

This requires a dark room for visibility.  Unfortunately, the pictures above do not show the laser beams very well.