Complex sentence: Since it is rotated a small amount each time it interacts with or passes through the atomic and sub-atomic particles that make up an optically active molecule this assumes that it will pass through an equal number of dextro and levo molecules.
Split into simpler sentences: Since it is rotated a small amount each time it interacts with or passes through the atoms and particles that make up an optically active molecule. This assumes that it will pass through an equal number of dextro and levo molecules.