There is plenty of physics in a violin and, no, I’m not talking about string theory. When the bow moves across the strings, there’s friction as the strings of the bow rub against the perpendicularly orientated violin strings. This rubbing gives rise to a chaotic driving force applied to the violin string. This imparts a whole spectrum of frequencies, but there is only one frequency at which the string of the violin is resonant. Only this single frequency is amplified and all the others are dampened.
A violin string amplifies a single frequency of sound like a Fabry–Pérot etalon amplifies a single frequency of light.
Why is this so? It is because the driving force shakes the string of the violin such that waves of different wavelengths run up and down the string. Most waves have wavelengths which are such that when the wave runs up the string, reflects and runs back down again, the wave wants to move a given piece of string against the way in which that piece of string was already going. Because of that, the string will only move very little either way and so cause only an insignificant amount of sound.
There is one wavelength however for which this running and reflecting of waves works out very well. For that one wavelength, when it reflects at the end of the string and runs back down the string in the other way, the wave moves the string in the same way as it was already going (constructive interference). As a result, the string vibrates, or resonates, beautifully at this wavelength only. The resonance is then further amplified by the hollow violin body. As it turns out, the underlying physics is as classical as the violin’s music.
Such a pure tone must be the right one though. And this is when you’ll need to occasionally tune your violin. I’ve found a free app (no strings attached) which helps you do just that. Just open the app, play the string you want to tune and the app will listen and indicate whether the string needs to be tuned up or down. Or, of course, it might say that you’ve got it spot on.