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What happens to the light which is absorbed by an object?


Light (electromagnetic radiation) is energy. 
Since energy is quantized, each energy range produces only a particular type of response in atoms or molecules, as indicated in the tables below. Therefore each energy range can be used to study that particular type of change happening in the atom or molecule. The technique of using the interaction of light (electromagnetic radiation) to analyse the structure of atoms and molecules is known as spectroscopy or spectrophotometry.

Radio waves

Microwaves

IR

Visible

UV

X-rays

g-rays

nuclear spin orientation in   magnetic field

electron spin orientation

 

molecular vibration and   rotation

excitation of valence electrons

excitation of middle and valence shell electrons

excitation of K and L shell electrons

nuclear fission and fusion

Application in NMR spectroscopy

Application in ESR spectroscopy

Application in IR spectroscopy

Application in colourimetry

Application in UV spectroscopy

Application in X-ray crystallography

nuclear reactors

Energy falling on a molecule is used for executing various kinds of motion by the molecule:

1.  Translation: The molecule as a whole moves from one location to another.

2.  Rotation: The molecule rotates about its centre of gravity around various axes.

3.  Vibration: Atoms in the molecule can vibrate ie. they can change their position relative to other atoms in the molecule. There are different kinds of vibrations like symmetrical stretching, unsymmetrical stretching, scissoring, rocking, wagging and twisting.

4.  Electronic transitions: Electrons move from one energy level to another inside the atoms.

5.  Electron spin changes.

6.  Nuclear spin changes.

The energy required for the various transitions are different. The total energy possessed by a molecule is the sum of all the different energies required for the various kinds motions described above.

 Etotal = Etrans + Erot + Evib + Eelec + Enucl

 Read following related sections:

The Born-Oppenheimer approximation

Beer-Lambert Law


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