Q.A photocell is used to automatically switch on the street lights in the evening when the sunlight is low in intensity. Thus it has to work with visible light. The material of the cathode of the photo cell is (A) zinc (B) aluminum (C) nickel (D) potassium

[!TLDR] A street-light photocell must respond to ordinary VISIBLE light at dusk, so its cathode needs a LOW work function -- among the four options, potassium (an alkali metal, $\phi_0\approx2.3$ eV) is the only one whose work function is low enough for visible-light photons to eject electrons. [!ANSWER] (D) potassium.

For the photocell to switch the street lights on automatically at dusk, it must respond to visible light (photon energy roughly 1.6-3.2 eV across the visible spectrum), not just to ultraviolet radiation. This requires the cathode's work function $\phi_0$ to be LOW enough that $h\nu\geq\phi_0$ is satisfied by visible-light photons.\n\nChecking the work functions from Table 14.1: zinc has $\phi_0=3.6$ eV, nickel has $\phi_0=5.0$ eV, and aluminum has $\phi_0=4.3$ eV -- all of these require ultraviolet radiation (photon energies above about 3.5 eV, corresponding to wavelengths below about 350 nm) to show any photoelectric emission at all, and would NOT respond reliably to ordinary visible light. Potassium, however, has a low work function of only $\phi_0=2.3$ eV, well within the energy range of visible-light photons (violet light at 400 nm carries about 3.1 eV, and even orange-red light around 550-600 nm carries 2.1-2.25 eV, close to potassium's threshold). This is exactly why alkali metals like potassium are used as the photosensitive cathode material in practical photocells meant to operate with ordinary daylight/visible-light intensity changes, such as automatic street-light switches. [!ANSWER] (D) potassium.

Compare each option's work function (Table 14.1) against the photon energy range of visible light; only a low-work-function alkali metal like potassium responds to visible light rather than requiring ultraviolet.

Picking a familiar 'photoelectric metal' like zinc without checking that its higher work function actually requires ultraviolet radiation, not the visible light this device must operate with.