The double-slit experiment shows fringes on a screen. Closing one of the slits there is still an interference pattern on the screen behind the slit. Making the slit wider we still see fringes between the shadow and the exposed area. Even with single photons or electrons. On the over hand, using a polarisation filter it is possible to let through about 50% of the light. In our case the electric field of the photons may be equally distributed. Using a second filter, 90° rotated to the first, no light (suitable wave length) goes through. The amazing fact is that using a third filter between the other two - best under 45° - some light goes through. That means that there is influence of the material of the edges. What is the realtionship between material of the edges and the dimensions of the fringes on a screen?