Write down Einstein's equation of photo electricity and show that electrons are emitted only when υ≥υ₀,

Kmax=hυ-hυ₀

1.when υ>V₀     Kmax=+vr

2.when υ=υ₀      Κmax=0

3.whenυ<υ₀      Κmax=-ve,

no. 3 is impossible.
Thus photoelectrons emitted when υ≥υ₀.