Zeno can dim light to darkness with two turned filters. He wonders whether any filter could do the same to sound.
The answer is no, and the reason is worth more than the trick. The two-filter experiment quietly proves something deep about what kind of wave light is.
Remember the two wave shapes. A transverse wave oscillates perpendicular to its direction of travel. A longitudinal wave oscillates along its direction of travel.
A polarizing filter works by selecting an oscillation direction: it transmits the oscillations along one direction and blocks the rest.
Here is the picture for a mechanical wave. Send a wave along a rope through a narrow slot in a fence. An up-and-down rope wave fits an upright slot and passes. A side-to-side rope wave meets the same upright slot crossways and is blocked. The slot can select, because the rope's oscillation is perpendicular to the wave's travel.
Now try the same idea on sound. A sound wave's air oscillates back and forth along the travel direction. There is no up-and-down or side-to-side to choose between. A slot, a screen, a filter: nothing has a crossways oscillation to block.
So sound, a longitudinal wave, cannot be polarized. No filter trick will ever dim sound the way Zeno's filters dimmed light.
Light CAN be polarized. The filters really did select an oscillation direction, so that direction must exist, perpendicular to the light's travel.
So the two-filter experiment does more than a trick: it shows what kind of wave light is. Light is transverse.
Amity's filter pair dims a flashlight beam to nearly black at a quarter turn. What does the dimming prove about the flashlight's light?
The second filter selectively blocked an oscillation direction.
A wave only has a blockable oscillation direction if it oscillates perpendicular to its travel.
Oscillation perpendicular to travel is the transverse shape.
The light is a transverse wave.
Could a filter polarize the sound of a school bell?
The bell's sound oscillates along its travel direction: it is longitudinal.
A filter can only select between oscillation directions perpendicular to travel.
A longitudinal wave has no such direction to select.
No. Sound cannot be polarized.
Which kind of wave can a polarizing filter selectively block?
Why can sound not be polarized?
Amity says: 'Light can be polarized, so light must be a transverse wave.' Is Amity correct?
Light can be polarized, and sound cannot. What does this difference show?
In a class demo, a flashlight beam passes two stacked filters, and turning the second filter to a quarter turn dims the spot on the wall to nearly black. What does the dimming show about the beam?
Zeno wonders whether some clever filter could do to sound what polarizing filters do to light. Why is that impossible?