# Convergence of Light on the Retina

So, I've learned about lens ray diagrams-but the problem I'm having is that when ray diagrams are drawn for a point of an image, they converge to another point, but there are two problems that I see with regards to our eyes: 1) In the simplest drawn case it's at least three rays converging to the same point, but our eye would only have once receptive cell at a point, how does it take in all of them? 2) At any given time, isn't there more than one point being sent to the same point on the back on the retina? We draw 2D cases in high school physics, but I feel like there should be more than one point that gets sent to the same point. In any case, part one is the more pressing matter.

## 2 Answers

Geometric optics can be confusing. On your first question: you have to keep in mind that the three rays one usually draws are just example rays highlighting the divergent cone of light emitted from a point and hitting the lens and subsequently being focused via a converging cone onto a unique point in the image plane. The receptive cell at that point receives the total energy of the infinitely many light rays contained in the light cone.

Your second question boild down to "is there a 1:1 mapping between points in the subject plane and points in the image plane. The answer is "yes". This can be seen from the fact that for each point imaged the central ray (out of the three rays commonly drawn) follows a straight line path through the center of the lens, thereby creating a 1:1 correspondence between points in the subject plane and points in the image plane (your retina).

You are a bit confused.

Rays are a geometrical representation of the classical electromagnetic waves and are very good for optics situations.

The cells in the retina are in the quantum mechanical regime, because to react to light they absorb photons at specific molecular energy levels of the molecules in the cells which send signals of detection to the brain .

Photons are individual particles which in their great plethora build up the classical electromagnetic light. They are discrete in space and time, and the cells in the retina have many molecules, of order 10^18 or so ( there are 10^23 molecules in a mole of matter) so a great number of photons can excite a great number of molecules in your optically drawn "ray".

Do not confuse geometric arguments with physics arguments.