When light enters a transparent medium, the ultraviolet photons and infrared photons are absorbed while the frequencies of visible light are transmitted. In the case of UV light, the electrons in the medium to begin to resonate, or vibrate with the influx of energy. Those vibrating electrons that strike neighboring atoms, release or transfer much of their vibrational energy in the form of heat. With IR, entire atoms (not just electrons) begin to vibrate, subsequently generating heat.

 

Physlet Animation
Interaction of Radiation with an Atom 

 

Those electrons which are free to vibrate without striking neighboring atoms complete two quantum mechanical processes: excitation and de-excitation. Excitation occurs when a ground state electron absorbs a photon and jumps up to a higher, unstable energy level. Photons are bundles of radiant energy that represent the particle nature of light. The amount of energy present in a photon is calculated with the equation E = hf where f is the frequency of the light wave and h is Planck's constant, 6.64 x 10^-34 J sec. When the electron falls back to its ground state it releases a photon. The energy of the released photon exactly match the difference in the electron energy states and the energy of the initially absorbed electron. This process is called de-excitation. The photon emitted is then free to travel at 3 x 10⁸ m/sec until it is again absorbed by another electron. There is no energy lost in the process.

 

The closer the energy of the photon is to a difference in the fundamental energy states of the atom, the more interaction takes place. Since the energy of the emitted photon exactly equals the energy of the absorbed photon, the frequency of the photons/light does not change. However, the time delay caused by this absorption/readmission process increases the time required for a photon to travel through the medium and therefore results in a slower average speed of light in that particular medium. The amount of time delay is evidenced by the optically dense medium's index of refraction; the greater the value of n, the more interaction and the greater the amount of refraction.