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Asked in NEET 2009 · Photodiode and solar cell
Idea: a photodiode responds only to photons carrying at least the gap energy, so there is a longest wavelength it can see.
λₘₐₓ=(hc)/(E_g)=(12400 eV A)/(2.5 eV)=4960 A.
Anything longer than 4960 A carries too little energy per photon and is not absorbed.
Of the four wavelengths offered only 4000 A is shorter than this limit.
So the diode can detect 4000 A.
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