Given that the work function for sodium metal is $2.28 \mathrm{eV}$, what is the threshold frequency $v_0$ for sodium?
We must first convert $\phi$ from electron volts to joules.
$$
\begin{aligned}
\phi & =2.28 \mathrm{eV}=(2.28 \mathrm{eV})\left(1.602 \times 10^{-19} \mathrm{~J} \cdot \mathrm{eV}^{-1}\right) \\
& =3.65 \times 10^{-19} \mathrm{~J}
\end{aligned}
$$
Using Equation 1.11, we have
$$
v_0=\frac{3.65 \times 10^{-19} \mathrm{~J}}{6.626 \times 10^{-34} \mathrm{~J} \cdot \mathrm{s}}=5.51 \times 10^{14} \mathrm{~Hz}$$