There are three types of transitions occurred in molecules which are helpful in spectroscopy i.e. electronic, vibrational and rotational transitions. All these three types of transitions are quantized. Electrons are held by the nucleus through strong electrostatic forces. So, high energy is required to excite an electron from lower electronic energy level to higher ones. This high energy is provided in uvvisible region. If photons with the right energy strike molecules, they can cause the molecule to vibrate more violently with higher amplitude. As in case of electrons, the vibrations of molecules are quantized. However, this energy is weaker because the force of chemical bond between the atoms of molecules is weaker than the nuclear forces upon electrons. Therefore, this energy is absorbed in infrared region. The rotation of molecules also has quantized energy levels. As these levels do not experience some type of force of attraction, the excitation between these energy levels require much less energy. This energy is absorbed in microwave region. The gaps in the energy levels is in the order of electronic > vibrational > rotational energies.