Can you see single photons? — What is biophysics? #6

The realization that electromagnetic waves are quantized — that light interacts in discrete packets — marked the beginning of quantum mechanics. Whether this fundamental attribute of the physical world sets the performance limits of human vision is a fascinating biophysical question. It’s so fascinating that Bill Bialek’s excellent graduate-level biophysics textbook begins with it, and I, influenced by Bialek’s book, start my graduate biophysics course with it. Not only is the question itself important, but the way it was answered is brilliant. I ask the students in my class to guess when scientists determined the minimal number of photons that need to hit your retina for you to see. You, too, can take a few minutes, think about this, and guess both the threshold photon number and the decade of its discovery! I’ll write the answers at the bottom of this post (below my name).

This series of “What is Biophysics” posts (one, …, five) is intended to highlight the diversity of questions that drive biophysics, not to elaborate on the answers. In fact, I was thinking of omitting the answer entirely, but I decided this would be too frustrating! Fundamental limits on vision are not, by the way, a topic I cover in my recent pop-science biophysics book. (Links: Amazon and Amazon UK, Publisher, my description). Perhaps if I write a sequel…

Today’s illustration An eye, with a glint of photons.

— Raghuveer Parthasarathy; May 6, 2022

Answers: You’ll perceive a flash of light if just a few photons hit your retina! The threshold is about 3-10 photons. This was deduced in the early 1940s, by groups in New York and the Netherlands, through clever experiments that involved a deep understanding of the statistical signatures of randomness. To learn more, see Bialek’s book (noted above), or this excellent review paper: F. Rieke, D. A. Baylor, “Single-photon detection by rod cells of the retina.” Rev. Mod. Phys.70, 1027-1036 (1998). doi:10.1103/RevModPhys.70.1027; or one of the original papers: S. Hecht, S. Shlaer, M. H. Pirenne, “Energy, Quanta, and Vision.” J Gen Physiol. 25, 819-840 (1942).