Start of by placing a block with redstone on it. Then place another block diagonally (left one block, up one block/right one block, up one block) and place redstone on it. Finally, place a piston on top of the first redstone you just placed. Power the bottom redstone and you will see that the top redstone does not get powered. Unpower the bottom redstone, power the top redstone and you you will see that the bottom redstone does not get powered. From this test, you will find out that pistons are a non-solid but also a non-transparent block.