The electrical conductivity of a semiconductor increases when electromagnetic radiation of wavelength shorter than $2480 \; nm$ is incident on it. The band gap in $eV$ for the semiconductor is

For an $LED$ to emit light in the visible region of the electromagnetic spectrum,it can have an energy band gap in the range of (Planck's constant,$h = 6.6 \times 10^{-34} \ J s$ and speed of light,$c = 3 \times 10^8 \ m s^{-1}$ in vacuum).

$A$ $p-n$ 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 $c = 3 \times 10^8 \ m/s$]

Identify the devices which work on solar energy.
$(a)$ Photodiode
$(b)$ Photovoltaic cell
$(c)$ $LED$
$(d)$ Photo-thermal devices

Which of the following will $NOT$ be observed when a multimeter (operating in resistance measuring mode) probes connected across a component are reversed?

$A$ Light Emitting Diode $(LED)$ has a voltage drop of $2\,V$ across it and passes a current of $10\,mA$ when it operates with a $6\,V$ battery through a limiting resistor $R$. The value of $R$ is.........$k\,\Omega$.