PURPOSE: To demonstrate how light is polarized when it reflects from dielectric surfaces, and remains unpolarized after reflecting from conducting surfaces.

DESCRIPTION: When sunlight reflects off a horizontal dielectric surface such as water in a lake, wet roads, or even dry smooth roads, the reflected light is largely horizontally polarized. Polaroid sunglasses are oriented vertically so they remove "glare," which is horizontally polarized specular reflection from such surfaces.

Position the point source so that it reflects from the lecture table onto the front white screen at about the Brewster angle. Rotating a polaroid in the light from the point source before reflection shows clearly that the reflected light is polarized. Individually viewing the reflected light directly using a polarizing filter demonstrates the value of polaroid sunglasses in removing glare. Placing a piece of aluminum foil where the light hits the table demonstrates that reflection from a conducting surface is not polarized.

The photographs above show the polarization axis (a) parallel and (b) perpendicular to a dielectric surface, and (c) parallel and (d) perpendicular to a conducting surface (a sheet of aluminum foil).

SUGGESTIONS: See Question of the Week #233 and #234 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.) See demonstration M7-18 POLAROID SUNGLASSES

EQUIPMENT: Bright point source with condenser and iris on tilting optical rail table, lecture table top, aluminum foil, and polarizing sheet.

SETUP TIME: 5 min.