Question

What is the threshold frequency that is required to emit photoelectron from selenium?

Answer 1

$$hf_{\rm th}=\phi$$

$$f_{\rm th}=\phi/h\\ ={\rm(5.11\: eV\times 1.6*10^{-19}\: J/eV)/(6.62*10^{-34}\: J\cdot s)}\\ =1.24*10^{15}\:\rm Hz$$