On which principle does the $Vidicon$ work?

An $LED$ is constructed from a $p-n$ junction diode using $GaAsP$. The energy gap is $1.9\; eV$. The wavelength of the light emitted will be equal to

$A$ solar cell is to be fabricated for efficient conversion of solar radiation to emf using material $A$. The solar cell is to be mechanically protected with the help of a coating using material $B$. If the band gap energy of materials $A$ and $B$ are $E_{A}$ and $E_{B}$ respectively,then which of the following choices is optimum for better performance of the solar cell?

$A$ photo cell is a device used to:

The material used for solar cell should have a band gap:

Explain the construction and working of a photodiode.