Three photodiodes D₁, D₂ and D₃ are made of semiconductors having band gaps of 2.5eV, 2eV and 3eV, respectively. How many photodiodes will not be able to detect light of wavelength of $6000\stackrel{0}{\mathrm{A}}$ ?

Energy of incident photon, $E=\frac{hc}{\lambda }=\frac{12400}{6000}eV=2.06eV$

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.