A quantum of electromagnetic radiation is called photon, and the energy of a photon is given by
[tex]E=hf[/tex] (1)
where h is the Planck constant and f is the frequency of the photon.
In this problem, the photon energy is
[tex]E=2.0 keV=2.0 \cdot 10^3 eV=3.2 \cdot 10^{-16} J[/tex]
If we re-arrange equation (1), we can find the energy of the photon:
[tex]f= \frac{E}{h}= \frac{3.2 \cdot 10^{-16} J}{6.6 \cdot 10^{-34} Js}=4.8\cdot 10^{17} Hz [/tex]
