$A$ $p-n$ junction photodiode is fabricated from a semiconductor with a band gap of $2.5 eV$. It can detect a signal of wavelength: (Given: Planck's constant $h = 6.6 \times 10^{-34} Js$,speed of light $c = 3 \times 10^8 m/s$,elementary charge $e = 1.6 \times 10^{-19} C$)

Statement $-I$: $A$ photodiode operates in reverse bias.
Statement $-II$: 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.

$A$ 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?

$A$ photodiode is a device

Which of the following statements is true about LEDs?

The maximum rated power of the $LED$ is $2 \text{ mW}$ and it is used in the circuit with an input voltage of $5 \text{ V}$ as shown in the figure below. The current through resistance $R$ is $0.5 \text{ mA}$. The minimum value of the resistance $R_S$ to ensure that the $LED$ is not damaged is . . . . . . $\text{k}\Omega$.