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Spectral Harmonics

Interestingly there is also a kind of harmonics in light physcis: If you excite an element it emits light at specific frequency which can be observed as individual lines in a spectrum.

Spectra of Hydogren emission lines in the visible spectrum (Balmer series ). It becomes apparent how the 'intervals' between the emission lines become smaller and smaller (image: Jan Homann / Wikimedia Commons).

At the end of the 19th century Johannes Rydberg discovered that the frequencies at different excitation levels can be described as a fraction of square function. Here the Heaven's Carousel translates for instance the series of light emission of excited Hydrogen in the infrared, visible and ultraviolet light spectrum into audible frequencies. As consequence of the square function the intervals get a microtonal character as the ratios become smaller and smaller.

From the original document detailing the Rydberg formula (image: Wikimedia Commons)