# (a) Three photo diodes $D_{1}, D_{2}$ and $D_{3}$ are made of semiconductors having band gaps of 2.5 eV, 2 eV and 3 eV respectively. Which of them will not be able to detect light of wavelength 600 nm? (b) Why photodiodes are required to operate in reverse bias? Explain.

(a) $E=h{c/\lambda }$ is the energy of a photon of wavelength $\lambda$. Where, h is  the plank's constant and c is the speed of light in vacuum.

equating the values in the given equation:-

$E=h{c/\lambda }$

$E=\frac{6.626\times 10^{-34/s}\times 3\times 10^{8m/s}}{600\times 10^{-9m}}$

$= 3.313\times 10^{-19}$

$= 3.313\times 10^{-19}\times \frac{1eV}{1.6\times 10^{-19}}$

$E=2.07 ev$

The correct answer is $D_{1}$ and $D_{3}$,  Since the bandgap of diode D2 is less than the energy of the photon

(b) As it is easier to observe the change in current with the change in light intensity in reverse bias.

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