(a) Write Einstein's photoelectric equation. How did Millikan prove the validity of this equation? (b) Explain the existence of threshold frequency…

$\displaystyle h \nu=h \nu_{0}+K_{\max }=h \nu_{0}+e V_{0}$

By finding the value of Planck's constant using the $\displaystyle V_{0}$ versus $\displaystyle \nu$ straight line plot for sodium.

Since $\displaystyle K_{\max }$ must be non-negative, photoelectric emission is possible only when $\displaystyle h \nu>h \nu_{0}$, which implies the existence of $\displaystyle \nu_{0}$.