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

If a semiconductor photodiode can detect a photon with a maximum wavelength of $400\, nm$, then its band gap energy is (Given: Planck's constant $h = 6.63 \times 10^{-34}\, J \cdot s$, Speed of light $c = 3 \times 10^{8}\, m/s$) (in $ eV$)

Describe the construction, working, and use of a solar cell.

The semiconductor used for the fabrication of visible LEDs must at least have a band gap of (in $eV$)

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

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