Answer:
When the ignition is turned on, battery voltage passes down the wire to the alternator to 'excite' the alternator when the engine starts. When the engine starts and the alternator generates a voltage, the light is extinguised by the charging voltage cancelling out the battery voltage coming down the wire. If and when the alternator completetly fails the the light will come back on as there is no sancelling voltage. if the alternator produces a voltage lower than the voltage from the ignition switch then the battery light will glow.