Monochromatic light of frequency $6.0\times {10}^{14} \mathrm{Hz}$ is produced by a laser. The power emitted is $2.0\times {10}^{-3} \mathrm{W}.$ How many photons per second, on average, are emitted by the source?

$$\begin{gathered} 1. 6\times {10}^{15} \\ 2. 5\times {10}^{14} \\ 3. 6\times {10}^{14} \\ 4. 5\times {10}^{15} \end{gathered}$$

The work function of caesium is 2.14 eV. The threshold frequency for caesium is:

$$\begin{gathered} 1. 5.03\times {10}^{12} \mathrm{Hz} \\ 2. 6.12\times {10}^{14} \mathrm{Hz} \\ 3. 5.16\times {10}^{14} \mathrm{Hz} \\ 4. 6.51\times {10}^{12} \mathrm{Hz} \end{gathered}$$