Why do we get hot while lying in the sun

We have already seen how, on the microscopic level, the vibrational energies of bonds are quantized in a similar manner to the way the energies required for electronic excitation are quantized. For this reason, irradiation with an infrared light from the sun or a lamp results in a photon absorption, and the bonds vibrate, which we experience as the sensation of heat. 

Mathematically, the movement of vibrating atoms at either end of a bond can be approximated to simple-harmonic motion (SHM), like two balls separated by a spring. From classical mechanics, the force necessary to shift an atom or group away from its equilibrium position is given by 

Equation (9.19) is a chemical version of Hooke s law, and only applies where the Morse curve is parabolic, i.e. near the bottom of the curve where molecular vibrations are of low energy. 

Photons of infrared light are not sufficiently energetic to excite electrons, but can excite between quantum-mechanical vibrational levels. 

Molecules translate, rotate and vibrate at any temperature (except absolute zero), jumping between the requisite quantum-mechanically allowed energy levels. We call the common pool of energy enabling translation, rotation and vibration the thermal energy . In fact, we can now rephrase the statement on p. 34, and say that temperature is a macroscopic manifestation of these motions. Energy can be readily distributed and redistributed at random between these different modes. 

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