Whenever a wire and a magnetic field move perpendicular to each other, a voltage is induced in the wire. If the wire is part of a complete electrical circuit, the voltage will cause a current to flow in the circuit. If a coil of wire is used instead of a single wire, the voltage obtained is the single-wire voltage multiplied by the number of turns in the coil.
In this Snack, every time the magnet stack moves through the coil of wire, the coil experiences a changing magnetic field, inducing a voltage in the coil. Since the coil is part of a complete circuit that includes the LED, current flows through the LED, and it lights.
When the magnet stack passes through the coil, two pulses of voltage of opposite polarity are produced. This happens because the direction of the voltage induced in a coil is related to both the orientation of the magnetic field lines relative to the coil and their direction of movement relative to the coil. When the north pole of the magnet stack, for example, is moving to the left through the coil, the magnetic field lines point away from the center of the coil, and the current flows in one direction in the coil. Then, when the south pole of the magnet stack passes through the coil, the magnetic field lines point toward the center of the coil, and the currents flows in the opposite direction (see diagram below).
The normal LED lights up once each time the magnet stack passes through the coil, regardless of which way the stack is traveling, even though two pulses of current are produced. There is only one flash because the LED is a normal diode, and a normal diode only allows current to flow in one direction. When the magnet stack passes by the coil in one direction, the first pulse of current lights the LED, and when it passes by in the other direction, the second pulse of current does the job.
The bipolar LED is actually two LEDs in one casing that allow current to flow in both directions. When current flows in one direction, one of the LEDs emits red light; when current flows in the opposite direction, the other LED emits green light. When you hook up this LED, you see two flashes of light each time the magnet passes through the coil—one when the north pole of the magnet stack passes through the coil and the other when the south pole passes through.