State the band gap of the semiconductor used in the making of $LED$.

The figure shows $2$ $LEDs$ that can be used as a polarity detector. Applying a positive source voltage results in a green light,while negative supplies result in a red light. Packages of such combinations are commercially available. Find the resistor $R$ to ensure a current of $20 \ mA$ through the $ON$ diode for the configuration. Both diodes have a reverse breakdown voltage of $3 \ V$ and an average turn-on voltage of $2 \ V$. The source voltage is $8 \ V$. Find $R$ in $\Omega$.

$A$ photodiode is a device

The energy gap of an $LED$ is $2.4 eV$. When the $LED$ is switched $ON$,the momentum of the emitted photons is

An $LED$ is manufactured using zinc selenide; then it emits:

$A$ $p-n$ junction is fabricated from a semiconductor with a band gap of $2.8 \ eV$. What approximate wavelength can it $NOT$ detect (in $nm$)?
[Use $h = 6 \times 10^{-34} \ J \cdot s$ and $c = 3 \times 10^8 \ m/s$]