Polarization

Light is a transverse wave: its electric field oscillates perpendicular to the direction the wave travels, and it can wiggle in any of those perpendicular directions at once. A polarizing filter is built from long molecules lined up in one direction, which absorb the electric field component parallel to them and let through only the component perpendicular to them. Ordinary light hits the filter with its field pointing every which way, so the filter only lets through the piece of each wave that lines up with its transmission axis. Rotate a second filter relative to the first, and the fraction of light that gets through shrinks as the angle between the axes grows, hitting zero when the axes are exactly 90 degrees apart, crossed.

That projection idea explains the surprising third-polarizer trick. Light leaving the first filter is polarized along axis one. A second filter crossed at 90 degrees blocks it completely, because there is zero component along that axis. But slide a filter in at 45 degrees before the crossed one, and it grabs a slice of the light along its own axis, so light now exits at 45 degrees instead of the original direction. That tilted light does have a component along the final, crossed axis, so some light gets through. Each filter projects, it does not simply subtract.

Sound never does this because it has no transverse field to restrict. Air molecules push and pull along the same line the sound travels, so there is no side-to-side plane for a filter to select. Polarization is a signature of transverse waves only, and it sits alongside interference and diffraction in this unit as more proof light behaves as a wave, not a stream of particles.