Transparent materials, including glass, diamonds or even water, will bend, or refract, rays of light. If a globule of the transparent material possesses axial symmetry, then it can focus the light, or act as “a lens” (a word inspired by the Latin word for a grain of lentil, evocative of the shape of the double convex lens.) Depending on its shape a simple lens can be positive, causing the rays to converge; or negative, causing the rays to diverge.
However, in the bending of light the longer wavelengths (reds) will bend less than the shorter wavelengths (blues and violet). Accordingly, all the colors will not converge at the same location. This is the cause of “chromatic aberration,” a smearing of the focal point, which is an inherent problem for refracting telescopes. Mirrors, in distinction to lenses, reflect light, and, properly designed, they too can focus light.
Reflection is independent of wavelength, and the point of convergence of colors (different wavelengths) will be the same. Chromatic aberration is not a problem. The physics of lenses and mirrors has been understood for centuries, and is regularly taught in beginning physics classes.
In his book Opticks (1704) Isaac Newton explained precisely how rainbows are created by the phenomenon of refraction of light in individual water droplets. In 2009 I visited the area around the City of Buffalo in New York State, ostensibly to give a talk at the State University of New York in Fredonia, and found time to visit Niagara Falls. In shooting the photograph, I stood near the edge of the storied falls on the American side, where the water pouring into a seemingly bottomless pit produced the spray, and light rays dispersing through the water droplets in the spray created the rainbow. The colors range from the longer wavelength reddish glow in the upper bands, changing gradually to turquoise in the middle bands, and finally the shorter wavelength violet in the lower. (These colors represent just the ‘visible region’ of the electromagnetic spectrum.) For the reader with a technical mind, a rainbow is a composite effect of rays of white light striking numberless spherical droplets of water and becoming dispersed. Light rays are refracted twice (across the outer surface of a droplet as they enter and leave the droplet), and reflected once (at a diametrically opposite area in the inner surface). Ultimately, it is the composite image the eye receives from multiple droplets that reveals the bands of colors.
Lenses have been around only in the past millennium. By the late Middle Ages the glass workshops in Venice, Murano and Treviso in Italy were beginning to produce spectacles as corrective glasses.
Simple lenses can also be used in combination to create compound lenses. In the middle ages the friar Roger Bacon (1214-1292) at Oxford, in writing about the possibility of arranging lenses “… to see the object near or at a distance,” had presaged the refracting telescope. The brilliant monk had garnered a reputation as a sorcerer for his ability to conjure “… suspicious innovations,” and even been incarcerated for his abilities. But then, a sorcerer would have stood no higher on the social ladder than the mathematician or natural philosopher, the scientist.
Galileo Galilei is generally given credit for the invention of the first astronomical telescope in 1609. He had been inspired by the story of a similar apparatus, a spy glass, created in the Netherlands by the German-Dutch optician Hans Lippershey a few months earlier. Galileo created a number of refracting telescopes, arranging a combination of lenses in a cylindrical tube, and pointing them toward the sky. He saw mountains and craters on the moon, sun spots moving across the otherwise pristine surface of the sun, phases of Venus, and four of the moons of Jupiter. For him they confirmed the Copernican sun-centered (heliocentric) theory published in 1543. Galileo’s telescopes are on display in the Museum of the History of Science in Florence.
In the compound microscope the idea is similar. Developed by Robert Hooke in England (a commonly repeated mistake has Leeuwenhoek in Holland inventing the first microscope, but in reality he was inspired by Hooke’s 1665 book Microphraphia).
Mirrors, as reflectors of one’s visage, have been around for at least three millennia. They are seen in carvings on ancient Egyptian and Hittite walls; and as paintings on Greek, Etruscan and Roman craters and frescos. But, as creations of illiterate craftsmen rather than scientists, their provenance is lost. The next installment in this series will ‘focus‘ on Newton’s invention of the reflecting telescope.
For the photo of the cross section of the Nikon Camera, I am grateful to Sam Sun of Kuala Lumpur, Malaysia. And for inspiring the series of blogs on the history of the astronomical telescope I am deeply indebted to Prof. André Buys, nuclear engineer, amateur telescope maker. The series will reveal the first ever inventor of the astronomical telescope, a surprise!