SUN GLASSES

P7 has the best answer...

But Let's step back to facts, shall we?

Polarized lenses literally block light, unless the light-waves are
oriented at a specific angle to the lens.

Almost all LCDs (including PDAs, cell-phones, iPads, and even simple
displays like Cambridge *-NAV devices or digial variometers) have 1 or
2 polarized screens, as part of how they work. See he
http://en.wikipedia.org/wiki/Liquid-crystal_display

Stacking multiple polarizing filters in the proper orientation lets
light pass through. But stacking them in the wrong orientation causes
the light passing through the first filter to be BLOCKED by the
second.

Your polarized sunglasses _are_ a "second filter", sitting between
your eyeballs and the LCD screens in your cockpit. Ergo, if the
orientation of your sunglasses' polarized surface is not near the same
angle of your electronic displays, the visibility of your instruments
will be negatively impacted. Here are two video demonstrations of
this:
http://www.youtube.com/watch?v=GkLBMAY406Q
http://www.youtube.com/watch?v=HcQWAqL2JUw

Polarized sunglasses are not all manufactured the same; and even
different batches of the same sunglasses may not always have the
orientation to the polarized lenses - don't assume that because your
glasses work for you, that you can recommend them to others or that
you can simply buy another pair of the same, and have them work
identically. Additionally, just because you find glasses that may work
with _your_ cockpit instruments and canopy, do _not_ assume that it
will work with _all_ instruments or canopies. Finally, don't assume
that all of your instrument displays have the same polarization on
their screens. I have polarized lenses that work fine when viewing
some of my instruments, but not with others (unless you rotate the
glasses 90-degrees). Also, these polarized sunglasses worked fine
with my DG-300 canopy, but created horrible moire & checkerboard
patterns on a couple of Diamond DA-20 rental aircraft canopies.

If you're going to fly with a lot of cockpit electronics, I strongly
recommend against polarized lenses because you don't want to be
distracted or fumbling with your instruments because of this effect.
There are plenty of effective non-polarized sunglasses out there that
give you good visibility and protection.

Lastly, if you're going to be flying in a club or rental environment,
where you switch ships with any regularity, do NOT assume that
polarized sunglasses which work in one cockpit will work in another.

--Noel