$A$ photo cell is a device used to:

For three circuit elements $A, B$ and $C$,following observations were made in lab:
$(a)$ element $A$ cannot conduct $d.c.$ but allows $a.c.$ through it.
$(b)$ element $B$ conducts both $AC$ and $DC$ but in one direction only.
$(c)$ Element $C$,when connected across $DC$,conducts only for one polarity and emits light whose intensity first increases and then decreases with increase in applied voltage.
The elements $A, B$ and $C$ are respectively identified as

In a photodiode,the value of the $emf$ produced by a monochromatic light beam is directly proportional to

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

What are the ideal materials for the fabrication of a solar cell? Give the criteria for their selection.

Select the correct statement.