Question

Assertion:A photocell is called an electric eye.
Reason:When light is incident on some semiconductor its electrical resistance is reduced
A. Both assertion and reason are correct and reason is the correct explanation of assertion
B. Both assertion and reason are correct but reason is not the correct explanation of assertion
C. Assertion is correct but reason is incorrect
D. Both assertion and reason are incorrect

Answer 1

Hint: In case of photoelectric effect, the cathode is hit by a beam of photons which causes electrons in the cathode to come out of the cathode and go to anode. Potential difference is maintained between cathode and anode. Photocells work on this principle.

Complete answer:
Various photocells are designed in such a way that respond to particular frequency of electromagnetic waves like some respond to x rays and some for visible light and so on.
When radiation is hit on the cathode some of the energy of that radiation will be used by the electron to come out of that electrode which is called a work function and the remaining energy is used as a kinetic energy to reach the anode. When an electron reaches anode automatically current is generated. Hence in case of photocell light is used to generate electricity. The amount of current produced depends on the intensity of the radiation. Now if potential difference is maintained between cathode and anode and if we use photocell in dark then no electricity is generated because electrons are not emitted from the electrode at all. Hence we state that its resistance reduces when light is incident on it. These are used as electrical alarms to avoid robbery. During night when the thief focuses light on the money photo cells in the bank sense that light and automatically rings the alarm or siren. Hence these are called as an electrical eye. Hence assertion and reason both are correct but reason is not the correct explanation as the correct explanation would be intensity of that light determining current flow to ring the alarm.

So option B is correct.

Note:
The amount of current generated depends on the intensity of the incident light and doesn’t depend on the energy of the incident light and as distance between the light source and the photocell decreases its intensity will increase.