For detecting the light ____.
A) The photodiode has to be forward biased.
B) The photodiode has to be reversed biased.
C) The LED has to be connected in forward biased.
D) The LED has to be connected in reverse biased.

Hint: Light consists of photons as a packet of energy. Current in photodiode changes when the light falling on photodiode with frequency is greater than threshold frequency of photodiode. An LED is a device used to produce light when current is passed through it. Light is detected when there is change in current in the circuit.

Complete step by step answer:
We know that LEDs produce light when current is passed through it and it just produces light energy. Then LED is a type of diode which can never be used to detect light.
Photodiode is a diode, which shows some changes when light falls on it.
In the reversed biased condition of photodiode, the width of the depletion layer increases as the applied reverse biased voltage increases across the diode when light falls on it. So, by applying a large voltage, more of the incident photons are converted into electric current by photodiode and increasing the efficiency.
In forward bias condition of photodiode, the width of depletion layer decreases due to only some part of incident photons being converted into electric current by photodiode and efficiency is less in comparison to reverse biased.
Then, for detecting the light the photodiode has to be reversed biased.
Hence correct answer of option B.

Note: Each photodiode has its fixed threshold frequency for the incident light. If the frequency of photons is less than threshold frequency then the photons of light do not convert into current. Because when light falls on photodiodes, the electrons jump from lower potential to higher potential due to which current produces an electron requiring a photon of minimum energy which is equal to threshold energy. The energy of a photon is directly proportional to frequency of light.