Klaus Hentschel studied physics, philosophy and history of science at the University of Hamburg, with a diploma in high-energy physics, a master in philosophy of science and a 1989 Ph.D. thesis on the misinterpretations of Einstein’s theory of relativity by contemporaries. He was assistant professor at the University of Göttingen, fellow of the Dibner Institute at MIT, Ernst Cassirer guest professor in Hamburg, and DFG-researcher in Berne. Since 2006 he has been full professor and director of the section for history of science and technology at the University of Stuttgart. His habilitation thesis deals with the interplay of precision experimentation, instrument making, and theory formation in astrophysics which led to the discovery of gravitational redshift in the solar spectrum. He also published books on the Einstein Tower in Potsdam, on Gauss’s instrument maker, on the mapping of spectra, and on visual cultures in science and technology. He was contributing editor and consultant to the Collected Papers of Albert Einstein, and he is the editor of nearly 20 books on research technologies, on the mentality of physicists, on Kayser, Kirchhoff and Planck, on materials science, and co-editor of the Compendium of Quantum Physics. Concepts, Experiments, History and Philosophy (Springer 2009).For his historiographic work Hentschel was awarded five international prizes. He is a member of the German National Academy of Scientists Leopoldina and of the Académie International d’Histoire des Sciences.About the translator: Ann M. Hentschel earned her Bachelor of Arts in German and French at Tufts University in Massachusetts 1984 and was editorial assistant for the Einstein Papers Project at Boston University 1987-1991. She currently works as a freelance translator in the history of science in Stuttgart. Her publications include multiple volumes of The Collected Papers of Albert Einstein (Princeton Univ. Press, since 1998); The German Physical Society inthe Third Reich edited by Dieter Hoffmann & Mark Walker, (Cambridge University Press 2007); Science and Conscience: The Life of James Franck by Jost Lemmerich (Stanford University Press 2011); and Gustav Robert Kirchhoff's Treatise "On the Theory of Light Rays" (1882) ed. Klaus Hentschel & Ning Yan Zhu (World Scientific 2017).

• Introduction.- Planck’s and Einstein’s pathways to quantization.- Twelve semantic layers of ‘light quantum’ and ‘photon’.- Early mental models.- Early reception of the light quantum.- Light quanta reflected in textbooks and science teaching.- The ‘light quantum’ as a ‘conceptual blend’.- Quantum experiments with photons since 1945.- What is today’s mental model of the photon?.- Summary.

“The book is … doubly interesting, presenting readers with a useful review of key concepts in science along with reflections on the workings of science. It is highly recommended for physicists, physics students, and historians of science. … Nonetheless, researchers interested in science learning will find a rich source of material for further investigation in this book.” (Olival Freire, Isis, Vol. 111 (2), 2020)

Klaus Hentschel studied physics, philosophy and history of science at the University of Hamburg, with a diploma in high-energy physics, a master in philosophy of science and a 1989 Ph.D. thesis on the misinterpretations of Einstein’s theory of relativity by contemporaries. He was assistant professor at the University of Göttingen, fellow of the Dibner Institute at MIT, Ernst Cassirer guest professor in Hamburg, and DFG-researcher in Berne. Since 2006 he has been full professor and director of the section for history of science and technology at the University of Stuttgart. His habilitation thesis deals with the interplay of precision experimentation, instrument making, and theory formation in astrophysics which led to the discovery of gravitational redshift in the solar spectrum. He also published books on the Einstein Tower in Potsdam, on Gauss’s instrument maker, on the mapping of spectra, and on visual cultures in science and technology. He was contributing editor and consultant to the Collected Papers of Albert Einstein, and he is the editor of nearly 20 books on research technologies, on the mentality of physicists, on Kayser, Kirchhoff and Planck, on materials science, and co-editor of the Compendium of Quantum Physics. Concepts, Experiments, History and Philosophy (Springer 2009).For his historiographic work Hentschel was awarded five international prizes. He is a member of the German National Academy of Scientists Leopoldina and of the Académie International d’Histoire des Sciences.About the translator: Ann M. Hentschel earned her Bachelor of Arts in German and French at Tufts University in Massachusetts 1984 and was editorial assistant for the Einstein Papers Project at Boston University 1987-1991. She currently works as a freelance translator in the history of science in Stuttgart. Her publications include multiple volumes of The Collected Papers of Albert Einstein (Princeton Univ. Press, since 1998); The German Physical Society inthe Third Reich edited by Dieter Hoffmann & Mark Walker, (Cambridge University Press 2007); Science and Conscience: The Life of James Franck by Jost Lemmerich (Stanford University Press 2011); and Gustav Robert Kirchhoff's Treatise "On the Theory of Light Rays" (1882) ed. Klaus Hentschel & Ning Yan Zhu (World Scientific 2017).

• Introduction.- Planck’s and Einstein’s pathways to quantization.- Twelve semantic layers of ‘light quantum’ and ‘photon’.- Early mental models.- Early reception of the light quantum.- Light quanta reflected in textbooks and science teaching.- The ‘light quantum’ as a ‘conceptual blend’.- Quantum experiments with photons since 1945.- What is today’s mental model of the photon?.- Summary.

“The book is … doubly interesting, presenting readers with a useful review of key concepts in science along with reflections on the workings of science. It is highly recommended for physicists, physics students, and historians of science. … Nonetheless, researchers interested in science learning will find a rich source of material for further investigation in this book.” (Olival Freire, Isis, Vol. 111 (2), 2020)

Klaus Hentschel studied physics, philosophy and history of science at the University of Hamburg, with a diploma in high-energy physics, a master in philosophy of science and a 1989 Ph.D. thesis on the misinterpretations of Einstein’s theory of relativity by contemporaries. He was assistant professor at the University of Göttingen, fellow of the Dibner Institute at MIT, Ernst Cassirer guest professor in Hamburg, and DFG-researcher in Berne. Since 2006 he has been full professor and director of the section for history of science and technology at the University of Stuttgart. His habilitation thesis deals with the interplay of precision experimentation, instrument making, and theory formation in astrophysics which led to the discovery of gravitational redshift in the solar spectrum. He also published books on the Einstein Tower in Potsdam, on Gauss’s instrument maker, on the mapping of spectra, and on visual cultures in science and technology. He was contributing editor and consultant to the Collected Papers of Albert Einstein, and he is the editor of nearly 20 books on research technologies, on the mentality of physicists, on Kayser, Kirchhoff and Planck, on materials science, and co-editor of the Compendium of Quantum Physics. Concepts, Experiments, History and Philosophy (Springer 2009).For his historiographic work Hentschel was awarded five international prizes. He is a member of the German National Academy of Scientists Leopoldina and of the Académie International d’Histoire des Sciences.About the translator: Ann M. Hentschel earned her Bachelor of Arts in German and French at Tufts University in Massachusetts 1984 and was editorial assistant for the Einstein Papers Project at Boston University 1987-1991. She currently works as a freelance translator in the history of science in Stuttgart. Her publications include multiple volumes of The Collected Papers of Albert Einstein (Princeton Univ. Press, since 1998); The German Physical Society inthe Third Reich edited by Dieter Hoffmann & Mark Walker, (Cambridge University Press 2007); Science and Conscience: The Life of James Franck by Jost Lemmerich (Stanford University Press 2011); and Gustav Robert Kirchhoff's Treatise "On the Theory of Light Rays" (1882) ed. Klaus Hentschel & Ning Yan Zhu (World Scientific 2017).

• Introduction.- Planck’s and Einstein’s pathways to quantization.- Twelve semantic layers of ‘light quantum’ and ‘photon’.- Early mental models.- Early reception of the light quantum.- Light quanta reflected in textbooks and science teaching.- The ‘light quantum’ as a ‘conceptual blend’.- Quantum experiments with photons since 1945.- What is today’s mental model of the photon?.- Summary.

“The book is … doubly interesting, presenting readers with a useful review of key concepts in science along with reflections on the workings of science. It is highly recommended for physicists, physics students, and historians of science. … Nonetheless, researchers interested in science learning will find a rich source of material for further investigation in this book.” (Olival Freire, Isis, Vol. 111 (2), 2020)