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:
Answer & explanation
Correct answer: option 4
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.