Diode

A diode allows current in only one direction. An LED, or light-emitting diode, has the diode symbol with two arrows pointing outwards to represent emitted light. Compare these with the arrows pointing inwards on an LDR: the LDR responds to incoming light, while the LED produces light.

A diode behaves differently in the two directions. In the forward direction, little current flows at small potential differences; once the potential difference is sufficient, the current rises rapidly. In the reverse direction, its resistance is very high and the current is approximately zero over the normal testing range.