X-rays of wavelength λ = 1 Å have frequency

3 × 108 Hz 3 × 1018 Hz 3 × 1010 Hz 3 × 1015 Hz

3 × 1018 Hz

Frequency is $ f = \frac{c}{\lambda} = \frac{3\times 10^8}{10^{-10}} = 3\times 10^{18}$ Hz