Understanding Double Slits, Polarization and Interference

Double Slit Property 1:  The position and spacing of the double slit interference pattern is not altered by the polarization of light. 

Double Slit Property 2:  Perpendicularly oriented waves (horizontal and vertical) do not interfere with one-another.  This is a property of waves, including electromagnetic waves (light).  This is demonstrated by placing a horizontal linear polarizer before one slit and a vertical linear polarizer before the other slit.  A single slit interference pattern is formed.

Double Slit Property 3:  A halfwave plate retards electromagnetic radiation, rotating the axis of polarization 90⁰.  Begin with perpendicular linear polarizers on the front face of the slits as in ‘Property 2’ and add a halfwave plate on the back face of one slit.  IF the light is pre-polarized +or- 45⁰ relative to the front face polarizers THEN the double slit interference pattern will be present.  IF the light is pre-polarized parallel or perpendicular to the front face polarizers THEN the single slit interference pattern will be present.