Molecular lines.

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Polar molecules like water are influenced by any incident electromagnetic radiation, and such molecules can be set into vibrations or rotation by the external, oscillating electric field. The water molecules try to align their dipole movement with the external electric field which is oscillating. This sets the molecule into rotation. The rotational energy levels are quantized, and the molecules have distinct frequencies at which there are resonances where the molecule emits or absorbs maximum energy. The Microwave ovens utilize a $2.45 {\rm GHz}$ rotational transition of water. The water molecules absorb the incident electromagnetic radiation and start rotating. The rotational energy of the molecules is converted to random motions or heat.

Most of the rotational and vibrational transitions of molecules lie in the microwave and Infrared (IR) bands. The frequency range $50 \, {\rm GHz}$ to $10 \, {\rm THz}$ is often called Tera hertz radiation or T-rays. The water vapour in the atmosphere is opaque at much of these frequencies. Dry substances like paper, plastic which do not have water molecules are transparent to T- rays, whereas it is absorbed by substances with water and is reflected by metals which makes it suitable for various imaging applications.