The following graph represents:

$(i)$ Name the type of diode whose characteristics are shown in figure $(a)$ and figure $(b)$.
$(ii)$ What does the point $P$ in figure $(a)$ represent?
$(iii)$ What do the points $P$ and $Q$ in figure $(b)$ represent?

$A$ $p-n$ photodiode is fabricated from a semiconductor with a band gap of $1.4 \ eV$. It can detect a signal of wavelength:

The $I-V$ characteristics of a photodiode for different illumination intensities $I_1, I_2, I_3$ and $I_4$ are shown in the figure. Which of the following represents the maximum intensity?

Statement $-1$: $A$ photodiode operates in reverse bias.
Statement $-2$: The fractional change due to the photo-effects on the minority carrier dominated reverse bias current is more easily measurable than the fractional change in the forward bias current.

$A$ photo cell is a device used to: