Optics Informed by History and Nature

Joseph A. Shaw

Montana State University, USA

In optics, we stay current by following the newest ideas and accomplishments. However, historical optical systems offer many valuable lessons, something that drove John Greivenkamp to collect and display historical cameras, telescopes, and microscopes. I developed a similar love of classic optical systems by being exposed to them by my physicist father, who loaned me a 1950s Leica camera to use in my high school photography class. I was initially embarrassed to be the only one in the class with such an old camera, but I soon learned to focus instead on the incredible capabilities of that classic camera.

Throughout my career, I have turned to that camera and other historic cameras, lenses, and telescopes to teach about optical system design. This kind of teaching is particularly effective if our students see real historic items up close and, ideally, hold them to experience their magnificent craftsmanship. There is nothing quite like the feel of a classic camera with its mechanical shutter and metal body! It is also impressive to use classic lenses and observe first-hand how incredibly sharp some of those old lenses were (but also to see some of the limitations being removed with newer technology).

The 1950s Leica camera I used to learn photography—a beautifully designed classic camera and lens.

To me, the only source of optical ideas and stories better than history is nature. A seemingly simple rainbow, for example, can be used to teach principles of refraction, transmission, reflection, dispersion, and polarization. Plotting rays into a spherical water drop from different angles, one finds most of the rays clustering near the minimum-deviation angle, which defines the angular size of a rainbow. The same phenomenon occurs with ray tracing in hexagonal ice crystals, leading to the 22-degree halo.

The refractions and reflections that give rise to halos and rainbows also produce partial polarization. For example, two refractions and a partial reflection at the back of a water drop produce the readily identifiable color distribution in the primary rainbow, but they also polarize the light. The refractions and reflection each favor orthogonal polarization states, with the net polarization dominated by the strongly polarized reflection near the Brewster angle. By considering how the plane of refraction changes orientation for different parts of the bow, we can deduce that polarization of rainbow light is oriented tangentially to the bow.

Rainbow polarization illustrated with primary rainbow photographs recorded with (left) a linear polarizer oriented tangentially to the bow and (right) a polarizer oriented perpendicular to the bow.

By teaching optics from history and from nature, we broaden students’ perspectives and help them better understand not only the optical principles we are teaching, but also how to learn from those who came before—and from the continual array of optical examples that nature parades before our eyes if we only learn to see them and learn to question what we see.