Question

Match the list –I (Phenomenon associated with electromagnetic radiation) with List –II (Part of electromagnetic spectrum) and select the correct code from the choices given below

List I	List II
I. Doublet of sodium	A. Visible Radiation
II. Wavelength corresponding to temperature associated with the microwave	B. Isotropic radiation filling all space
III. Wavelength emitted by atomic hydrogen in interstellar space	C. Short Radio Wave
IV. Wavelength of radiation arising from two close energy levels in hydrogen	D. X-rays

(a) (I)-(A), (II)-(B), (III)-(B), (IV)-(C)
(b) (I)-(D), (II)-(C), (III)-(A), (IV)-(B)
(c) (I)-(B), (II)-(A), (III)-(D), (IV)-(A)
(d) (I)-(A), (II)-(B), (III)-(C), (IV)-(D)

Answer 1

Hint: The wavelength of the doublet structure of sodium is around $588.9nm$ and $589.5nm$. This falls in the range of the wavelength of X-rays. And the wavelength emitted by atomic hydrogen in interstellar space falls in the wavelength end of radio waves.

Complete step by step solution:
The sodium spectrum is dominated by the bright doublet, which are known as the sodium D lines. The wavelength of these lines are around the range $588.9nm$ and $589.5nm$. Now the wavelength of these sodium D lines falls under the wavelength range of the X rays in the spectrum. So we can say that the doublet of sodium belongs to the X rays.
Most of the hydrogen gas in the interstellar medium is in cold atomic form or in the molecular form. They emit a wavelength due to a slight energy change in the hydrogen atoms. This wavelength is $21.1cm$ and so it is called the 21 cm line radiation. This wavelength lies in the spectrum of the visible light. Hence the wavelength emitted by atomic hydrogen in interstellar space belongs to the visible light spectrum.
The difference in the energy of two close energy levels in a hydrogen atom is called lamb shift. The wavelength corresponding to this difference in energy is around 30 cm. This is in the range of the microwaves.
Therefore we see that the I matches with D; II matches with C; III matches with A; and IV matches with B.
Hence, (I)-(D), (II)-(C), (III)-(A), (IV)-(B) is correct.
So, option (B) is right.

Note:
The 21 centimeter radiation is that which is emitted by cold, neutral and interstellar hydrogen. In very cold space the hydrogen atoms are at their lowest possible state of energy. However, sometimes the collision between the particles can excite the atoms. The 21 cm is the wavelength corresponding to that energy.