A p-n photodiode is fabricated from a semiconductor with a band gap of 2.5 eV. It can detect a signal of wavelength

Only signals having  wavelength  less than  threshold wavelength will be detected.Energy E=hv=hcλ⇒λ=hcESubstituting  the values  of h, c and E in the  above equationλ=6.6×10-34×3×1082.5×1.6×10-19=5000 A0 As 4000 A0 < 5000 A0