In front of IISER Pune’s guest house, there is a small, artificial pond which is filled with rainwater. In there are tiny aquatic creatures and some beautiful lotus flowers. Recently, I happened to capture a high speed video (920fps) of the water surface fluctuating in this pond using the reflection of sun’s image (see video above). You will also notice a nice flower in the foreground which adds to the aesthetics.
What is interesting about this oscillation is the way the reflected image of the sunlight fluctuates as a function of time. In physics, there is a wonderful connection between fluctuating surfaces and the light reflected from such a surface. In principle, one can find out a lot about the nature of the fluctuation of the surface, including its topography, spatial frequency etc., by studying the amplitude and phase of the light that is reflected from such a moving surface.
One such example is the way atomic force microscope (AFM) works. In an essence, the topography of the surface an AFM reads, is by recording the fluctuation of light that is reflected from a tiny cantilever close to the surface.
Another fascinating concept related to probing fluctuations using light is the field of cavity optomechanics. The radiation pressure of the optical field couples to a tiny mechanical oscillator, and this interaction leads to a change in the the spectral characteristics of the light in a cavity. By studying this spectrum, one will be able to extract meaningful information about the tiny fluctuations in a cavity. This concept also applies to quantum fluctuations, and is one of the happening subfields in quantum optics and photonics.
Of course there are many such applications of using fluctuation of light to study oscillations in matter.
The model of simple harmonic oscillator that we study in physics is not only of basic relevance to understand any kind of fluctuation, but also applies to a variety of scientific processes in spatial, temporal and spectral domains. Added to this, if we learn about Fourier series and Fourier transforms, then we can go deeper in understanding fluctuations of any kind.
This reminds me of a quote attributed to Sidney Coleman:
“The career of a young theoretical physicist consists of treating the harmonic oscillator in ever-increasing levels of abstraction.”
This is also true of experimental physics or for that matter most of the aspects of measurement science and technology. After all, fluctuations are ubiquitous, and harmonic oscillators are the windows into this beautiful world.