The device that can detect optical signals is

Assertion: $A$ $P-N$ junction diode is used in reverse bias to detect the intensity of light.
Reason: In reverse bias condition,current is small but it is more sensitive to changes in intensity of incident light.

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 . . . . . . $\text{k}\Omega$.

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

Why are $Si$ and $GaAs$ preferred materials for solar cells?

Three photodiodes $D_1, D_2,$ and $D_3$ are made of semiconductors having band gaps of $2.5 \ eV, 2 \ eV,$ and $3 \ eV,$ respectively. Which ones will be able to detect light of wavelength $6000 \ \mathring{A}$?