Are sunglasses lenses linearly polarized?
Well, normally they aren't. Only if the label specifies it and even so, always check (if they are too cheap, probably they aren't).
You can check by taking two sunglasses and rotate one lens of the first with respect to one lens of the other. If the light disappears (you see dark) it means they are polarized (or Polaroid lenses).
Sunglasses lenses are simply darkened lenses or coloured while to make a polarizer the process is a little bit more complex (you have to heat your plastic film and stretch the polymer fibres to form a kind of grid).
If you go to the cinema you can impress your friends by showing them the same effect of the figure with the glasses they give you to watch 3-D movies (which use polarization to create the 3-D effect)...probably you'll impress them...