Question

A photocell is a device that operates using the photoelectric effect.  What metal would provide the best results if it is desired that the photocell operate using visible light? 

manganese (φ = 4.1 eV)

gadolinium (φ = 2.90 eV)

beryllium (φ = 4.98 eV)

cesium (φ = 2.14 eV)

Answer 1

Frequency range of visible light is 430 THz to 770 THz.

Photoelectric equation,   

is work function of metal , is the kinetic energy ejected electron.

, the energy of photon is not sufficient to emerge an electron from any of the given metals since for all the given metals.

for the metal Gadolinium with and for the  metal Cesium with

So only for those two metals , electrons will be ejected.

For Gadolinium with ,

For Cesium with ,

The ejected electron has more kinetic energy for the metal Cesium.

Hence the metal Cesium provides the best results.