A metal has a work function of 2.5×10−19 J.The threshold frequency beyond which photo electrons will come out is
So, the work function of the metal is 2.5×10−19 J.
This means that we need to give an electron a minimum of 2.5×10−19 J of energy for it to come out of the metal. This energy needs to be supplied by light, in this case a photon.
Energy of a photon is hν.
So hν≥2.5×10−19 J.
ν≥2.5×10−19h
ν≥2.5×10−196.626×10−34
ν≥3.8×1014 Hz
So, the threshold frequency is 3.8×1014 Hz.