Question

In which one of the following regions of the electromagnetic spectrum will the vibrational motion of molecules give rise to absorption?
A. Ultraviolet
B. Microwaves
C. Infrared
D. Radio waves

Answer 1

The electromagnetic spectrum is defined as the spectrum or range of frequencies of electromagnetic radiations and their respective wavelengths. Molecular vibration is defined as the process in which atoms of the molecules vibrate periodically relative to each other, such that the center of mass of the molecules will remain unchanged. Here, the absorption of these vibrated molecules can be understood on the basis of Kirchhoff’s law in spectroscopy.

Complete answer:
The electromagnetic spectrum is defined as the spectrum or range of frequencies of electromagnetic radiations and their respective wavelengths. Molecular vibration is defined as the process in which atoms of the molecules vibrate periodically relative to each other, such that the center of mass of the molecules will remain unchanged.
The absorption of these vibrated molecules can be understood on the basis of Kirchhoff’s law in spectroscopy. The molecular spectra due to the vibrational motion of the molecules will lie in the microwave region. Hence, in the microwave region of the electromagnetic spectrum the vibrational motion of the molecules will give rise to absorption.

Therefore, option B is the correct option.

Additional information :
Here, we have used the term Kirchhoff’s law of spectroscopy. Kirchhoff formulated the three laws in spectroscopy that tells the continuity, emission and absorption in the spectrum.

Note: When a molecule will absorb infra-red radiations, they will not remain in their fixed position. The molecules will start vibrating about their mean position which is the result of change in the vibrational energy of the molecules. The vibrational energy will change due to the change in energy levels of molecules.