Milk fat has fundamental vibrations in the mid-infrared range (around 5um) but has noticeable overtones upto 700nm and has combination frequency around 2.4um. What do we exactly mean by overtones and combination frequency? What is the physical significance?
Please explain in simpler terms.
According to quantum mechanics a vibration cannot have arbitrary energy but only certain levels. The energie difference between two neighbored levels is a constant (for a harmonic oscillator at least) and relates to a photon of a certain frequency and wavelength. Quantum mechanics also tells us that it is only possible to excite the vibration from one to the next level if it is a harmonic oscillator. However, in reality, oscillators are not strictly harmonic. This means that it is also possible to jump from the ground level 0 to level 2 or 3 if a phonon has the corresponding energy (or frequency/wavelength). These are the overtones at multiples of the fundamental frequency (actually it is a bit less, as anharmonicity is accompanied by increasingly smaller energy differences the higher the energy levels). A combination band is related to a photon which has exactly the energy of the sum of two fundamentals or sometimes also a difference. Hope this helps!
Thank you Sir for the detailed explanation. As an analogy, overtones are integral multiples of the fundamental frequency of resonance in a closed ends pipe?
And I guess that's photon mistyped as phonon?
Yes, this is a good analogy! Concerning the photon: No, it is not mistyped, the photon (the light with the corresponding frequency) excites two different fundamental vibrations/phonons.
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