CHAPTER TWO

Emil Wolf and Bristol

Michael V. Berry1 H.H. Wills Laboratory, Bristol, United Kingdom 1Corresponding author: e-mail address: [email protected]

Contents 1. Education and early science in Bristol 9 2. Later interactions with Bristol University 10 References 12

Abstract Emil Wolf’s scientific life as a world-leading optical theorist began at the , England. His first papers were published while in Bristol, and there were several fruitful later interactions with the Physics Department. Keywords: Emil Wolf, Optical theory, Catastrophe

1. Education and early science in Bristol

Emil Wolf was surely the most accomplished and prolific theorist in optics (mostly classical) in the second half of the 20th century. He spent his formative years in Bristol, but despite its importance this period of his life is not well known. My purpose here is to report what I have been able to find out about it, and also describe some later interactions between Emil and Bristol University. After fleeing from Czechoslovakia before World War II, Emil eventually arrived in the United Kingdom in 1940. He attended high school in Wales and enrolled as an undergraduate in Bristol University in 1942 at the age of 20. After 3 years he was awarded his first degree: a BSc in mathematics. He initially registered for an MSc, intending to return to Czechoslovakia after a year. But the Soviet takeover made this impossible, and he moved to physics (across the corridor) to begin a PhD, supervised by E.H. Linfoot. The end of World War II was the start of a heroic period for physics in Bristol. Nevill Mott (Nobel Prize 1977) was leading research into the emerging field of solid state physics. Cecil Powell (Nobel Prize 1950) was

# Progress in Optics, Volume 65 2020 Elsevier B.V. 9 ISSN 0079-6638 All rights reserved. https://doi.org/10.1016/bs.po.2019.03.001 10 Michael V. Berry developing his photographic emulsion technique leading to his discovery of the pion. Charles Frank, newly arrived from his war work in Scientific Intelligence, was conceiving what came to be called muon-catalyzed fusion (in parallel with Andrei Sakharov), and beginning his seminal re-creation and application of dislocation theory. And, most relevant to young Emil in this vibrant milieu, there was the brilliant and eccentric C.R. (Bill) Burch (Allibone, 1984), described to me privately by John Ziman as “The most unusual person I ever met who is not actually mad.” In the third of his several different careers in industry-related physics, Burch was devising radical new designs and grinding techniques for aspheric lenses. It was Burch’s persuasion, immediately before the war, that had transformed Linfoot from a pure mathematician to a theoretical physicist specializing in optics. Perhaps the best known of Emil’s early works were his studies with his supervisor Linfoot, on the effects of on imaging and focusing. But these were published from Cambridge not Bristol. Linfoot had moved to Cambridge during Emil’s PhD years, and after graduating in 1948, Emil followed him there, before moving on to Edinburgh for his famous collaboration with , and then to Manchester. Emil’s first papers were indeed from Bristol, but they concerned geomet- rical rather than wave optics: three contributions (Wasserman & Wolf, 1949; Wolf, 1948a; Wolf & Preddy, 1947) to the theory underlying Burch’s aspheric lenses. These papers formed the basis of his PhD thesis (Wolf, 1948b). He acknowledges Linfoot in all three papers, but, perhaps surpris- ingly, Burch is thanked only in the final one. Burch was undoubtedly the creative genius who inspired Wolf’s research. His ideas, though largely geo- metrical, were not expressed fully mathematically; he wrote that his “see-saw diagram” (Burch, 1942)“ …may prove suggestive to those who, like myself, prefer to hang their mathematical symbols on to easily visualised physical concepts.” Since Emil was systematically formalizing, developing and exten- ding Burch’s ideas, it would be interesting to know how, or indeed whether, the two of them, utterly different in temperament and scientific style, interacted on a day-to-day basis.

2. Later interactions with Bristol University

Emil and I published only one paper together, a largely pedagogical work (Berry, Foley, Gbur, & Wolf, 1998). Together with John Foley and Greg Gbur, we showed that it is possible to drive an infinite string with forces in a finite region in such a way that no waves propagate outside. Emil Wolf and Bristol 11

But he influenced my scientific life positively in a different way. When we first met in Rochester in 1978, he heard me talk about “catastrophe optics”: the theory and applications of focusing and diffraction near caustics, based on the recently-developed mathematics of singularity theory. He immediately invited me to write a review of the subject for Progress in Optics. The resulting article (Berry & Upstill, 1980), written with Colin Upstill, became a standard reference. Much later, after I developed and reviewed the diffraction theory of Hamilton’s conical refraction, Emil again invited a detailed description of this work for Progress in Optics, leading to an article (Berry & Jeffrey, 2007) written with Michael Jeffrey. Emil’s contributions to the theory of aspheric optics can be regarded as prefiguring the current developments in freeform optics, and therefore (as I realize only now) my recent “Magic windows” (Berry, 2017). During each of our meetings in 1978 and several times afterward, Emil mentioned the problem of defining boundary conditions for waves encoun- tering a perfectly black object, i.e., one that totally absorbs all the light that falls on it. This question was one of the influences that led my Bristol colleagues John Hannay and John Nye, together with W. Liang, to develop a consistent theory for black screens, supported by experiments (Hannay, 1995; Nye, Hannay, & Liang, 1995; Nye & Liang, 1997). Emil’s contributions to optics were recognized in Bristol in 1997, by the award of the Honorary Degree of Doctor of Science during a Conference celebrating the 70-year Jubilee of the H.H. Wills Physics Laboratory. He is pictured in Fig. 1, together with the other distinguished graduands.

Fig. 1 Bristol University Degree Congregation 1997. Emil Wolf is at the bottom right, and his Public Orator Balazs Gyorffy€ is at the top right. The other graduands (bottom left to right) are Christopher Llewellyn-Smith, Yakir Aharonov, and Peter Higgs. 12 Michael V. Berry

I welcome this opportunity to describe, albeit briefly, Emil Wolf’s early years at his Alma Mater. In Bristol University we are proud to recognize him as one of our most distinguished alumni.

References Allibone, T. E. (1984). Cecil Reginald Burch, 12 may 1901-19 July 1983. Biographical Memoirs of Fellows of the Royal Society, 30,3–42. Berry, M. V. (2017). Laplacian magic windows. Journal of Optics, 19, 5. 06LT01. Berry, M. V., Foley, J. T., Gbur, G., & Wolf, E. (1998). Non-propagating string excitations. American Journal of Physics, 66, 121–123. Berry, M. V., & Jeffrey, M. R. (2007). Conical diffraction: Hamilton’s diabolical point at the heart of crystal optics. Progress in Optics, 50,13–50. Berry, M. V., & Upstill, C. (1980). Catastrophe optics: Morphologies of caustics and their diffraction patterns. Progress in Optics, 18, 257–346. Burch, C. R. (1942). On the optical see-saw diagram. Monthly Notices of the Royal Astronomical Society, 102, 159–165. Hannay, J. H. (1995). Path linking interpretation of Kirchhoff diffraction. Proceeding of the Royal Society of London, 450,51–65. Nye, J. F., Hannay, J. H., & Liang, W. (1995). Diffraction by a black half-plane: Theory and observations. Proceedings of The Royal Society A, 449, 515–535. Nye, J. F., & Liang, W. (1997). Near-field diffraction by two slits in a black screen. Proceedings of the Royal Society A, 454, 1635–1658. Wasserman, G. D., & Wolf, E. (1949). On the theory of aplanatic aspheric systems. Proceedings of the Royal Society B, 62,2–8. Wolf, E. (1948a). On the designing of aspheric profiles. Proceedings of the Physical Society, 61, 494–503. Wolf, E. (1948b). A contribution to the theory of aspheric optical systems. PhD Thesis. In Physics. Bristol University. Wolf, E., & Preddy, W. S. (1947). On the determination of aspheric profiles. Proceedings of the Physical Society, 59, 704–711.