Key Idea Only signals having wavelength less than threshold wavelength will be detected. Energy E=hv=hλcλ=Ehc ⇒λ=Ehc Substituting the values of and in the above equation λ=2.5×1.6×10−196.6×10−34×3×108=5000A˚ As 4000A˚<5000A˚ Signal of wavelength 4000A˚ can be detected by the photodiode.