JEE Main · 2010 · 11 Apr 2010 · Offline · Modern Physics

The radiated power is shared among the photons sent out each second, so one photon carries $E=P/N=\frac{4\times 10^3}{10^{20}}=4\times 10^{-17}$ J. Its frequency follows from $E=h\nu$: $\nu =\frac{4\times 10^{-17}}{6.63\times 10^{-34}}\approx 6\times 10^{16}$ Hz, a wavelength of about $5$ nm. Radiation of this very short wavelength and high frequency lies in the X-ray part of the spectrum.