$A$ monochromatic source of light operating at $15 \text{ kW}$ emits $2.5 \times 10^{22} \text{ photons/s}$. The region of an electromagnetic spectrum to which the emitted electromagnetic radiation belongs to . . . . . . . (Take $h = 6.6 \times 10^{-34} \text{ J}\cdot\text{s}$ and $c = 3 \times 10^8 \text{ m/s}$).

Which of the following is not an electromagnetic wave?

Which of the following waves has the maximum wavelength?

In which one of the following regions of the electromagnetic spectrum will the vibrational motion of molecules give rise to absorption?

Given below are some famous numbers associated with electromagnetic radiations in different contexts in physics. State the part of the electromagnetic spectrum to which each belongs.
$(a)$ $21 \; cm$ (wavelength emitted by atomic hydrogen in interstellar space).
$(b)$ $1057 \; MHz$ (frequency of radiation arising from two close energy levels in hydrogen; known as Lamb shift).
$(c)$ $2.7 \; K$ (temperature associated with the isotropic radiation filling all space; thought to be a relic of the 'big-bang' origin of the universe).
$(d)$ $5890 \; \mathring{A} - 5896 \; \mathring{A}$ (double lines of sodium).
$(e)$ $14.4 \; keV$ (energy of a particular transition in $^{57}Fe$ nucleus associated with a famous high-resolution spectroscopic method, Mossbauer spectroscopy).

Microwaves are used in the following: