Light wiggles side to side as it moves forward. When I say it
wiggles side to side I mean it could be left and right or it could be up
and down or it could be anywhere in between. Think about it like
pointing at something with your hand out flat. If the light shoots out
your finger tips then the wiggling is flat like your hand. You can
rotate your hand.
Polarizers like your sunglasses only let light through that is
polarized the right way just like you could stick your flat in a slot
only if you rotate your hand right. If the light isn't quite polarized
the right way it cuts out some of the energy and lets through only the
part that is the right way. If the light is turned 90 degrees from the
way the polarizer lets through then no light gets through. Polarizers
work by having something like little slots in them. Something inside is
rotated all one way. Often it is crystals that are long and thin and
all rotated the same way.
The glass on car windows is a special kind of glass made to be very
hard. The way they make it very hard is called tempering and it means
heating and cooling it so it forms the right crystals. The pattern that
they heat the glass means that in a dot shape the crystals in the
tempered glass are all pointing the same way so they become polarizers.
The glass then only lets through light that is polarized in one way
but the way which way that is depends on the pattern it was heated. In
some places the light is polarized by the window to be the same way as
your glasses so you see it as bright. In some spots the light gets
polarized the wrong way, 90 degrees from the way your glasses let in so
it gets blocked by your glasses and looks dark.