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

What are optoelectronic devices? Give the names of such major devices.

On which principle does the $Vidicon$ work?

In an $LED$,when forward bias is increased (starting from zero),the intensity of emitted light will:

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Photodiodes are preferably operated in reverse bias condition for light intensity measurement.
Reason $R$: The current in the forward bias is more than the current in the reverse bias for a $p-n$ junction diode.
In the light of the above statements,choose the correct answer from the options given below:

The device that can detect optical signals is