L–The Physics of Eyesight

Essay Task – Connecting Physics with Everyday Life by U.W. on 24 of June 2024 — as part of our Quantum Hardware Lecture

Sunlit rays scatter from a keyboard, slip through a pupil about 3 mm wide, and—after a single bend in cornea and lens—project a 20 mm miniature 2D image onto the retina. Bright-day optics sharpen this image to about 1 arc-minute resolution, neatly matching the 2 μm spacing of foveal cones. At twilight, iris dilation admits five times more light but inevitably softens each edge—a non-negotiable trade-off.

Visual scenes arrive as a swift stream of roughly ten trillion photons per second. Each photon absorbed in a rod cell can trigger a molecular flip, with rods integrating signals at around 50 Hz to tame Poissonian noise. Faster cones sort incoming photons at 100 Hz into so-called "red", "green", and "blue" wavelength channels. Optimised retinal circuits rapidly subtract backgrounds and accentuate edges, compressing the gigabit-scale data stream down to a manageable trickle of tens of megabits per second along the optic nerve. The brain then imposes its internal model at approximately 20 Hz, using predictive loops to weigh incoming neural spikes against expectations: familiar patterns are suppressed, unexpected events amplified. Quick eye movements reaching angular speeds of up to 600°/s occur several times per second, constantly steering the fovea centralis toward whatever appears informative. More than 90% of the raw data is discarded or re-ranked before it ever reaches conscious perception. Thus, what enters awareness is not a mere reflection of reality, but a strongly filtered hypothesis, shaped by prior knowledge and current goals. Each individual generates distinct space-time viewpoints, biased by their experiences.

Our vision thus exists in duality: on one hand, it generates an internal, imaginary reconstruction of the external world; on the other, it serves as an actively tuned control system that counts quanta, edits sensory information, and continuously tests and recalibrates intuitions more than a hundred times per second. Human eyesight, as a highly adaptable and integrated sensor, grounds us in active feedback within our complex environment.

Special thanks to all my past, current and future environments.