Three photodiodes D₁, D₂, D₃ are made of semiconductors having band gaps of 2.5ev, 2ev and 3ev which can detect light of wave length 6000A^o

Energy of incident light photon,

$\mathrm{E}=\mathrm{h\vartheta }=\frac{\mathrm{hc}}{\mathrm{\lambda }} =\frac{6.6\times 10-34\times 3\times {10}^8}{6\times {10}^{-7}\times 1.6\times {10}^{-19}}=2.06\mathrm{eV}$

For the incident radiation to be detected by the photodiode , energy of incident radiation photon should be greater than the band gap. This is true only for D2​. Therefore , only D2​ will detect this radiation.