An experimental setup of verification of photoelectric effect is shown if Fig. The voltage across the electrodes is measured with the help of an ideal voltmeter, and which can be varied by moving jockey J on the potentiometer with wire. The battery used in potentiometer circuit is of 20 V and its internal resistance is `2 Omega`. The resistance of 100 cm long potentiometer wire is `8 Omega`.
The photocurrent is measured with the help of an ideal ammeter. Two plates of potassium oxide of area `50 cm^2` at separation 0.5mm are used in the vacuum tube. Photocurrent in the circuit is very small, so we can treat the potentiometer circuit as an independent circuit. The wavelength of various colors is as follows:
Q. It is found that ammeter current remains unchanged `(2muA)` even when the jockey is moved from the end P to the middle point of the potentiometer wire. Assuming that all the incident photons eject electrons and the power of the light incident is `4xx10^(-6)` W. Then, the color of the incident light is

Options

A. Green
B. Violet
C. Red
D. Orange

Answer

D

Explanation

`P = (nhC)/(lamda)`
where n = no of photons incident per unit time
Also, `I = n e rArr P = (l hC)/(e lamda)`
`lamda = ((2xx 10^(-6))(6.6 xx10^(-34))(3xx10^(8)))/((4 xx10^(-6))(1.6xx10^(-19)))`
`=(9.9)/(1.6)=10^(-7)m=(9900)/(1.6)Å = 6187 Å`
Which came in the range of orange light.