Question 11.10:
Light of frequency 7.21 x 1014 Hz is incident on a metal surface. Electrons with a maximum speed of 6.0 x 105 m/s are ejected from the surface. What is the threshold frequency for the photoemission of electrons?

Hint: Use Einstein's photoelectric equation.
Step 1: Find the threshold frequency.
$$\text{As}~~~\frac{1}{2}mv^2=h(\nu-\nu_0)\\ ~~~~~\Rightarrow\nu_0=\nu-\frac{mv^2}{2h}\\ ~~~~~=7.21\times10^{14}-\frac{(9.1\times10^{-31})(6\times10^5)^2}{2\times(6.626\times10^{-34})}\\ ~~~~~=7.21\times10^{14}-2.472\times10^{14}\\ ~~~~~=4.738\times10^{14}~Hz.$$