Brief History of the Astronomical Telescope II: Lenses and Mirrors

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.

Earliest example of optical quality lenses, 10th Century Viking, Island of Visby, Gotland. Source: André Buys, Pretoria, South Africa

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.

Chromatic aberration caused by different wavelengths, corresponding to different wavelengths converging at different distances.

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.

LEFT. "... the Sun shining upon these Drops certainly causes the Bow to appear to a Spectator standing in a due position to the Rain and Sun... this Bow is made by refraction of the Sun's Light in Drops of falling Rain." From Newton's personal annotated copy of 'Optics.' RIGHT. Parallel rays strike a pair of droplets of water. The refractions and internal reflections manifest themselves with red in the upper band, blue and violet in the lower.
A Footbridge Between Cliffs. The refraction of light entering and exiting a water droplet explains why the upper bands are reddish and the lower bands bluish. (Photo by the author)

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.

Fresco by Tomaso de Modena (1352) with the first artistic depiction of eyeglasses. Church of St. Nicholas, Treviso, Italy. (Photo by author)

Compound Lenses

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.

Galileo's and Kepler's designs, respectively, for the refracting telescope. There is no evidence that Kepler's progressed beyond a mental invention.

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).

 

 

The cross section of a digital SLR Nikon camera, replete with its compound, zoom lens. The "SLR" ("See Through the Lens") feature of the camera is produced by the small mirror, reflecting the light at 45° into a second mirror (located in the bulbous area, and not visible). The second mirror in turn transmits the image to the view finder in the top right. At the instant the photo is shot, the first mirror pulls out of the way, allowing the incident light to fall on the image sensor directly behind the mirror. (Photo by Sam Sun, Kuala Lumpur, Malaysia)

 

Mirrors

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!

 

 

 

 

 

 

Changing Planet

Bulent Atalay, a scientist, artist and author, has been described by NPR, PBS and the Washington Post as a “Modern Renaissance Man.” He is the author of two successful books on the intersection of art, science and mathematics, with Leonardo, the pre-eminent Renaissance man, serving as the foil. His best selling book, "Math and the Mona Lisa," (Smithsonian Books, 2004) has appeared in 13 languages. Professor Atalay's academic background is in theoretical physics. He travels around the world lecturing at academic institutions and on cruise ships on the "A-subjects," art, archaeology, astrophysics, atomic physics and Ataturk, confessing that he knows much less about the "B-subjects," business, banking, biology and botany... He is the President of the Ataturk Society of America (ASA), dedicated to promoting Ataturk's ideals of science and reason over dogma and superstition, of a secular state with full equality of genders. For more details click on Bulent Atalay