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  • Writer's pictureDale DeBakcsy

Letting the Light Through: Katharine Burr Blodgett and the Physics of Non-Reflective Coating.

Every day, we subject our eyes to a nearly ceaseless barrage of screen-mediated experiences - phones, computers, televisions, tablets, handheld gaming consoles, car dashboard displays, self-checkout touchscreens, fast food menus, and on and on and on until our eyeballs would be entirely within their rights to just leap out of our faces and establish a small nation elsewhere entirely independent of us, where there is nothing to gaze upon but hills, flowers, and ducklings. That they have not yet done so is largely down to how good those modern screens are at transmitting the light we want and rejecting the light we do not, ensuring that when we look at our phones we get a nice crisp picture of what it is attempting to display and a minimum of glare, making life as easy as possible on the long-suffering and overworked Windows of Our Soul.


The physics behind those screens, and behind virtually every piece of glass that our eyes engage with, from movie projector lenses to eyeglasses to airplane dials, is the principle of thin film interference, the practical potential of which was unlocked in the 1930s in a series of landmark experiments by Katharine Burr Blodgett (1898 - 1979) while working at General Electric. Blodgett was not one given to talking extensively about her youth and so we have little of personal significance to report for roughly the first two decades of her life. This period was dominated practically by the absence of her father, a patent attorney for General Electric who was murdered by a burglar before she was born. George Blodgett left behind not only significant financial resources for his family, but also deep connections with the research departments of one of the world’s leading innovation factories, General Electric, which would be crucial for getting his daughter’s foot in the door of an overwhelmingly male dominated environment.


With the money George left behind his widow, Katharine Burr, moved the family from Schenectady, where General Electric was headquartered, to New York City, and thence to France, setting a pattern of time spent between New York and Europe that would define the first fourteen years of the younger Katharine’s life. We have little information about her education during this era, except for a period spent at Rayson School, an exclusive private institution, and another at a school located on New York’s Saranac Lake.



However patchwork Blodgett’s early education was, it must have been a thorough one, augmented by her own natural genius for science and mathematics, for she was accepted at the age of fifteen to not only attend Bryn Mawr College, beginning in 1913, but to receive a scholarship to do so. This was a wonderful time to be a student at Bryn Mawr, coinciding with the tail end of mathematical legend Charlotte Agnas Scott’s (1858 - 1931) career at the college, while in the physics department James Barnes (1879 - 1963) was at the beginning of his storied time there that stretched from 1906 to 1931. We know virtually nothing about her life as a student, other than her graduation in 1917, and her subsequent return to General Electric, where a colleague of her father’s, Irving Langmuir (1881 - 1957) took her on a tour of the facilities, and offered her a slot as a researcher in his lab as soon as she completed a graduate level degree. This she did with characteristic Blodgetty aplomb, received a Master’s degree from the University of Chicago in 1918, where her work with Harvey Lemon (1885 - 1965) centered around the war-related question of how gases of different types are adsorbed by coconut charcoal (of particular importance for designing effective gas masks to be used in the trenches of World War I), whereupon she joined Langmuir at General Electric.


If Langmuir’s name sounds familiar it is because we have met him before in the context of the trailblazing work done by Agnes Pockels (1862 - 1935). Pockels had developed a simple but incredibly powerful device to investigate the chemical properties of impure water surfaces, and Langmuir had taken that concept and refined it to carry out his own investigations into surface chemistry. For her first six years as Langmuir’s lab assistant, Blodgett developed her own methodology for the Pockels - Langmuir trough, to the point that, in his 1919 paper on the transference of mono-layers of molecules from a water surface to a solid surface, he wrote, “The writer is much indebted to Miss Katharine Blodgett for carrying out most of the experimental work.”



In 1924, on the insistence of Langmuir, who felt that with her talents Blodgett could become a first rank scientist, and that to accomplish all that she was capable of she would require a doctorate, Katharine went to Cambridge to pursue her PhD, which she wrapped up in an incredible two years under the loose but benevolent watch of Ernest Rutherford, the father of nuclear physics. By 1926, then, she was back in majestic Schenectady, working with Langmuir at GE again, only now not as a lab assistant, but as a colleague.


Both working with Langmuir, and pursuing her own interests, she investigated a wide array of physical phenomena besides the multi-layer film research most associated with her name. This included the sort of research one might expect from a scientist working at a company most associated with electricity and lighting, including studies of how electrons move in ionized mercury vapor (1926), the effects that the cooling which occurs at the leads of a tungsten filament has on the voltage and candle power of the filament (1929), the use of reduction by hydrogen on glass containing lead, bismuth, or antimony to allow that glass to conduct electricity (1948), and the recovery of rare gases in objects like light bulbs when they become trapped against the surface of the bulb by metal atoms that are ejected from the filament and coat themselves on that surface (1960).


That was all important and practical work, but Blodgett’s most celebrated research centered around her investigations in the 1930s and 1940s of monomolecular layers. Pockels had determined a method of creating single molecule thick layers of substances on the surface of water, and in the 1920s Langmuir and Blodgett had perfected methods of transferring those single layers onto a solid substance such as a pane of glass or metal plate. In the 1930s, Blodgett took this work several steps further, discovering the unique suitableness of fatty acids, with their strong hydrophilic and hydrophobic ends, for building up multiple monomolecular layers on the surface of a plate, creating films of atomically precise thickness. Two large results flowed from this rigorously built up methodology: a gauge of Blodgett’s invention that used color to determine the thickness of a film layer that was some ten thousand times more accurate than existing methods, and the development of non-reflective coatings.


Having developed a procedure for producing films of any desired molecular thickness, Blodgett realized that this was a perfect means for eliminating the waste in energy and light transmission that occurs when light reflects off a piece of glass instead of moving through it. The principle is a relatively simple one - by introducing an extra layer on top of the glass you introduce an extra surface that light can bounce off of, as in the following expert drawing:




Light has wave-like properties, meaning that it has a frequency and wavelength that we perceive as the “color” of the light. Two photons of light can cancel each other out if they have the same frequency, but are out of sync with each other, a phenomenon similar to that behind noise-cancelling headphones. If you ensure that the high part of one wave always corresponds to the lowest part of another wave, then when they meet and you add them together, they will sum to a big ole zero. For a piece of glass covered by a film, there will be two reflections, one coming off the film, and one coming off the glass, so if you want them to cancel each other out, you’ll need them to be out of phase with each other, corresponding to one lagging the other by a half a cycle (this ensures that crests meet troughs and vice versa). The way to do THAT is to have the film be one quarter of a wavelength in thickness, so that by the time the light goes through it, bounces off the glass, and then moves through the film again, it will have travelled through an extra distance of a half of a wavelength compared to the wave that bounced off the surface, putting it at a half wavelength lag perfect for cancellation.


Sounds simple - all you need to do to cancel out light reflecting off a surface is to coat it with a layer of something that is the thickness of one quarter of your average photon of visible light. Visible light varies from 3800 to 7400 Angstroms, so around 5600 Angstroms would put you right in the middle, with a quarter of that wavelength being around 1400 Angstroms. Here’s the thing, though - that is VERY THIN, as in .00000014 meters in thickness, which is a technical challenge to anybody who didn’t happen to be Katharine Burr Blodgett, who had spent a decade of her life working with how to precisely control molecular-scale film thicknesses. By using 44 layers of cadmium arachidate, she was able to create a layer of 1388 Angstroms, which was able to eliminate over 90% of reflection from the surface of the glass.



The implications of her paper, published in December of 1938, were far-reaching. Not only would this all but eliminate the glare coming off of glass surfaces like the navigational dials of airplanes, or the multitudes of screens that humans would eventually cram into their lives, but it also meant that, by eliminating the reflecting waves, the energy that they would have carried away could be reclaimed, allowing more light to pass through the glass than before, which would permit the creation of projection, camera, and eyeglass lenses of a theretofore impossible level of transmission and clarity.


This was an important discovery which made Blodgett for some time something of a celebrity in the United States, though practically the method used for creating non-reflective coatings which was eventually adopted as an industry standard wasn’t hers (her early films were essentially layers of soap that were highly fragile and easily wiped away) but those published a few weeks later by scientists at MIT who used vacuum evaporation methods to create tightly controlled layers of calcium fluoride on glass surfaces. The beauty of her methods and results, however, could not be denied, and resulted in a string of honorary degrees from Elmira College, Brown University, Western College, and Russell Sage College, and awards including the Garvan Medal of the American Chemical Society, and the Annual Achievement Award from the American Association of University Women.


With the arrival of World War II, Blodgett put away her work on monolayer films and worked with Langmuir on projects for the United States war effort, including methods for de-icing planes that, as far as I have been able to ascertain, remain classified (if anyone out there has found a way to access these papers, let me know!) as well as new techniques for creating smoke screens (controlled layers of smoke to mask the movements of an army and complicate the targeting of that army’s units by enemy fire) that were employed by the Allies.


After the war, Blodgett’s research interest continued to be divided between extending her work on films, and the studies of glass conductivity and gas reclamation and impurity cleaning in filament-based tubes that we noted above. She never married, but lived instead in “Boston Marriages” with other women which allowed her to pursue her researches while not being bogged down with too many domestic duties. She retired in 1963, and filled her retirement years with amateur dramatic productions, fiercely competitive bridge matches, and inspiring her niece, Katharine Blodgett Gebbie (1932 - 2016), with a love of science and scientific experiments that lead that young woman to one day become an astrophysicist and the founding director of the National Institute for Standards and Technology.



Katharine Burr Blodgett, a physicist’s physicist who regularly crossed boundaries between the realms of the theoretical and practical, performed her own final crossing at home on October 12, 1979.


FURTHER READING:


There is no single book about Blodgett, and in fact, in a write-up of the important scientists of General Electric written in 1953 to celebrate the organization’s 75th anniversary, her name is not mentioned once, though by that point she had published her definitive non-reflective coating paper, and carried out important work for the war effort. That we continue not to have a single volume about her is largely due to the dearth of personal information about her - she did not talk about her personal life, did not write about it, and to all accounts subsumed herself entirely in her work from 1918 to her retirement in 1963, to the extent that, for us trying to piece her story back together now, she has simply become that work, relegated to five-page biographies in summaries of women physicists, like those of Byers (2004), Grinstein (1993), and Shearer (1997).


This has been the 254th feature article in the Women in Science series.

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