Philosophical Society of Washington

Minutes of the 2350th Meeting

President Larry Millstein called the 2350 th meeting of the Society to order at the Cosmos Club in Washington, D.C. on September 11, 2015 at 8:05 p.m.  He announced the order of business and welcomed new members.  The minutes of the previous meeting were read and approved.  President Millstein then introduced the speaker for the evening, Charles W. Clark of the Joint Quantum Institute, the US National Institute of Standards and Technology, and the University of Maryland.  His lecture was titled “Over the Rainbow: Other Worlds Seen by Animals.”


Dr. Clark began by noting that the rainbow defines the limits of ordinary sight and by inviting the audience to look beyond the colors of visible light.

The discovery that white light is actually a mixture of colors occurs rather late in the human understanding of light. Prisms were known to the ancients, but the prevailing idea was that the glass impressed something onto the white light, changing it into the myriad colors of the rainbow. Sir Isaac Newton’s elegant double prism experiment in 1672 demonstrated that white light was a whole, composed of individual colors.

It was not until more than a century later that new experiments hinted at the existence of something very much like light but just beyond the reach of our vision.

In 1800, Sir Frederick William Herschel empirically placed the sun’s radiant heat alongside its visible light on what would come to be known as the electromagnetic spectrum. Herschel’s apparatus was simple: a prism and thermometers. The prism directed the different “colors” of light onto different thermometers, some placed beyond the edges of the visible spectrum. The temperature of the thermometers rose as the color moved from blue to red. Fascinatingly, the temperature continued to rise even as the thermometer moved into the dark area, beyond the edge of the visible red light. Something was coming from the sun and being duly split by the prism just like visible light, but it was invisible to humans. As it carried heat but not light, Herschel called it “calorific rays.” We know it today as infrared radiation.

In 1810, Joseph von Fraunhofer applied Herschel’s technique to solve a problem he encountered while developing a highly pure “optical glass.” Fraunhofer detected systematic errors in the sunlight received through the glass. Spreading the light through a prism and examining it at high resolution, he plotted several conspicuous gaps in the rainbow, which remained consistent regardless of the prism. Finally, he considered the possibility that the “problem” wasn’t with the glass, but with the sun.

To eliminate atmospheric influences he thought might confound accurate observation, Fraunhofer observed the spectra of stars, such as Sirius. There he observed the same holes in the spectrum and new ones. The holes, now known as “Fraunhofer lines,” are due to a shroud of gas around the sun that captures particular narrow wavelengths of light. This discovery led to characterization by Johann Blamer in 1885 of the precise wavelengths of light produced by excited hydrogen atoms and these in turn led Niels Bohr to his atomic theory, launching the field of quantum mechanics.

Fraunhofer’s demonstration of the infrared rays raised the obvious question . . . what’s beyond blue light?

That question was answered by Johann Wilhelm Ritter in 1801. Ritter noticed that different colors of light decomposed silver chloride at different rates, with a markedly increased rate beyond the blue edge of the spectrum. Ritter’s laconic one paragraph report to the academy regarding his “chemical rays” remains a landmark of the scientific literature not only for its scientific contribution but also for its brevity.

Dr. Clark then turned from recounting history to a demonstration. He showed how a relatively weak white light flashlight can provoke a faint green photoluminescent reaction from a glowing exit sign, whereas neither powerful red nor green lasers could excite a reaction. A 405 nm blue laser, however, being higher on the energy scale, caused an intense reaction.

These demonstrations help to visualize the foundational effect of quantum mechanics, that E = hν. Energy is proportional to frequency times the Planck constant. Explained in 1905, this relationship earned Einstein his Nobel prize for service to theoretical physics and in particular the photoelectric effect.

Finally, turning to animals and animal vision, Dr. Clark noted that male and female birds of the same species generally have distinctly different plumage, but there are a few oddities. For example, even trained ornithologists could not identify any evidence of sexual dimorphism in Malaysian thrush until 1996 when visible coloration differences were shown occurring exclusively in the ultraviolet range. Birds, it seems, have different photoreceptors than humans, including a more prominent blue receptor and the addition of an ultraviolet receptor that has no human analogue.

Mantis shrimp, which are neither mantis nor shrimp, contain no fewer than twelve distinct receptors extending from an unbelievable 300 nm (near ultraviolet) to 700 nm (just shy of infrared).

These observations have practical significance. A 2013 discovery showed that sea turtles are sensitive to UV to a degree that fish are not. Accordingly, scientists were able to develop string fishing nets embedded with UV LEDs, which turtles were able to avoid without deterring fish.

Dr. Clark concluded by noting that, both under and over the rainbow, we still have much to learn about light.

After the question and answer period, President Millstein thanked the speaker, presented him with a plaque signed by the General Committee, made the usual housekeeping announcements, and invited guests to join the Society. At 9:18 p.m., President Millstein adjourned the 2350th meeting of the Society to the social hour.


Attendance: 103
The weather: Overcast
The temperature: 20°C

Respectfully submitted,

Preston Thomas
Member of the Society


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