Question

light quanta of energy 3.5*10^-19J falls on cathode of phto cell the current through cell is reduced to zero by applying potential difference of 0.25Vwhat is work function of cathode

Answer 1

The energy of quanta will be enough to perform work of ejecting electrons from the surface of the metal and to stop the current:

$$h\nu=A+E_{p.d},\\ A=h\nu-E_{p.d.}=h\nu-e\phi=\\ =3.5\cdot10^{-19}-1.6\cdot10^{-19}\cdot0.25=\\ =3.099\cdot10^{-19}\text{ J}.$$

Seems like caesium was used as a material for the cathode.