Ysolde's two stacked lenses dimmed the view to nearly black at a quarter turn. Plain tinted glass never does that.
So what is a polarizing filter actually doing to the light? The answer starts with what the light looks like before any filter touches it.
Light is an electromagnetic wave, and its oscillation is perpendicular to its travel direction. Picture the wave coming straight at you, and the oscillation can point up and down, side to side, or along any other direction in the plane facing you.
Ordinary light, from the Sun or a lamp, arrives as a jumble: it oscillates in every plane perpendicular to its direction of travel, all mixed together.
Light like that is called unpolarized.
Everything oscillating along the transmission axis passes through. The rest does not.
So the light that emerges from a polarizing filter oscillates in a single plane. Light left oscillating in a single plane is called polarized.
Now the two-filter pattern explains itself. The light arriving at the second filter is already polarized by the first: it oscillates in one plane. Second filter lined up with that plane: the light passes, and the view is bright. Second filter a quarter turn from that plane: the light's one oscillation direction is exactly the direction the filter does not transmit, and the view goes dark.
In between, the single plane and the transmission axis are partly lined up, so some light gets through and the view is dimmer. That is the steady dimming Ysolde watched.
Bard shines a flashlight beam through one polarizing filter. Describe the light on each side of the filter.
Flashlight light arrives unpolarized: it oscillates in every plane perpendicular to its travel direction.
The filter transmits only the oscillations along its transmission axis.
So the light that emerges oscillates in a single plane.
Unpolarized light goes in; polarized light, oscillating in a single plane, comes out.
Light that is already polarized, oscillating up and down, reaches a filter whose transmission axis points up and down. What happens?
The light's single oscillation plane lies along the filter's transmission axis.
The filter transmits oscillations along that axis.
The light passes through, still polarized up and down.
Lamp light crosses a room toward Zeno, with no filter in the way. How is that light oscillating?
Bard says: 'A polarizing filter transmits only the oscillations along one direction.' Is Bard correct?
Light leaves a filter oscillating in a single plane. What is light like this called?
Sunlight reaches a window with nothing in its path. Which statement describes its oscillation?
Ysolde's two filters sit with the second a quarter turn from the first, and the view through the pair is nearly black. Which statement explains the darkness?
Polarized light oscillating up and down reaches a filter whose transmission axis also points up and down. What emerges on the far side?