A source of cm wave of 400 W produces $10^{20}$ photons per second. Identify the part of the electromagnetic spectrum to which these radiations belongs to:

ultraviolet rays

The correct answer is Option (4) → ultraviolet rays

Given, the source produce $10^{20}$ photons per second and the power of the source is 400W.

Power = Energy per photon × Number of photons per second (N)

⇒ Energy per photon = $\frac{Power}{N}=\frac{400}{10^{20}}$

$E=4×10^{-18}J$

and,

$E=\frac{hc}{λ}$

$⇒λ=\frac{hc}{E}=\frac{6.63×10^{-34}×3×10^8}{4×10^{18}}$

$λ=50nm$

Wavelength of $λ=50nm$ lies in UV region.