The symbolic representation of a photodiode is:

Which one of the following statements is $WRONG$ regarding $LED$?

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

The material used in the fabrication of infrared $LED$'s is

The following graph represents:

To measure light intensity,we use: