Let’s start with light, since every body ”knows” what light is. After all, we can see it – or think we can. Actually, we don’t see light. We only see the surfaces that reflect or emit light. Whatever, light is a stream of photons, yet only those within a narrow part of a near infinite range of electromagnetic radiation. Physicists count them and measure their energy, without ever knowing exactly where the photons are and can’t even agree on what they are!
Some introduction, eh? Don’t let that discourage you. Here is how I’ll deal with photons as they relate to vision.
As many readers already know, photons have a dual personality – or maybe it’s just the physicists. On one hand, photons are massless particles that speed along in straight lines called rays. From this perspective, basic geometry is enough to see how rays can form images. On the other, photons are waves of energy describable by Maxwell’s equations using calculus. Don’t worry, the calculus quickly gets beyond the abilities or interests of general readers. Still their waviness can’t be ignored. The wave perspective is necessary to grasp how light interacts with matter – interactions that make images possible and that enable those images to be sensed.
This “glance” compromises between the particle and wave perspectives. Photons are treated as quantal packets of energy that vibrate transversely to their direction of travel. Taking the average of their vibrations as location enables treating them as rays, while their vibration alters the probability of where they go when interacting with matter.
Avoiding calculus and using the outdated (?) Rutherford-Bohr model of the atom may cause apoplexy amongst some physicists. Yet as Paul Dirac pointed out “All the same the mathematics is only a tool and one should learn to hold physical ideas in ones mind without reference to the mathematical form.” 1
Until someone comes up with a better pictorial analogy, photons as vibrating packets of radiating energy will have to do.