Which of the following semiconductor devices requires a reverse bias to operate?

In an $LED$,visible light is produced by:

The $V-I$ characteristics of an $LED$ are correctly shown by which graph?

The semiconductor used for the fabrication of visible LEDs must at least have a band gap of (in $eV$)

For detecting light intensity,we use:

$A$ red $LED$ emits light at $0.1 \ W$ uniformly in all directions. The amplitude of the electric field of the light at a distance of $1 \ m$ from the diode is.... $V \ m^{-1}$