Final answer:
The reading on the voltmeter cannot be calculated without knowing the resistance of the circuit, as Ohm's Law (V = I × R) requires both current and resistance to determine voltage.
Step-by-step explanation:
To calculate the reading on the voltmeter when the light intensity is 20 lux and the current through the circuit is 0.0002 A, we can use Ohm's Law, which is stated as V = I × R, where V is voltage, I is current, and R is resistance. However, the student’s question does not provide the resistance of the circuit, which is necessary to calculate the voltage. Without the resistance value or additional context, we cannot determine the voltmeter reading. Therefore, I'm unable to provide an answer to this question without additional information about the circuit’s resistance.