$$E = hf$$
where:
* E is the energy in joules (J)
* h is Planck's constant (6.626 x 10^-34 J s)
* f is the frequency in hertz (Hz)
We are given that the energy of the electromagnetic wave is 5.0 x 10^-20 J. We can rearrange the equation above to solve for the frequency:
$$f = E/h$$
$$f = (5.0 \times 10^{-20} J)/(6.626 \times 10^{-34} J s)$$
$$f = 7.53 \times 10^{13} Hz$$
Therefore, the frequency of the electromagnetic wave is 7.53 x 10^13 Hz.
